Commas and introductory elements

WEBVTT
Kind: captions
Language: en

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- [Voiceover] Hello grammarians!

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Paige and I are here to teach you

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about introductory elements in sentences

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and how commas relate to them.

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Paige, how should we define
what an introductory element is?

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- [Voiceover] So, it's
pretty much something

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that happens at the
beginning of a sentence.

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It can be a dependent clause or an adverb.

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But as we will see soon, it is something

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that is separated off
with, of course, a comma.

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- [Voiceover] Because
that's what commas do.

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They are separators.

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So, let's talk about
dependent clauses first.

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And let me just write out a sentence

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that begins with a dependent clause.

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"When you come in, please
take off your shoes."

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And I've made the difference
between the dependent clause

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and the independent clause pretty clear.

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So, this the dependent clause is purple,

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the independent clause is green.

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Is there a need for a comma here?

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- [Voiceover] Of course.

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- [Voiceover] Okay.
(Paige laughs)

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Because we're leading
with a dependent clause,

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and that means that this thing

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can't stand on it's own, right?

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It's like the ladder up against the tree.

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Because an independent clause,

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in green, can stand on its
own; a dependent clause cannot.

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So, we need to differentiate it

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from the rest of the sentence
by putting the comma there.

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So, this is a dependent clause.

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And this is an independent clause.

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So, if you start a sentence
with a dependent clause,

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you're gonna need to put
the comma in the middle

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before you proceed to
the independent clause,

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which is the part that makes it

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an actual functioning sentence.

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- [Voiceover] Right.

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- [Voiceover] If you have it
the other way around though,

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if it's just, "Please take off
your shoes when you come in,"

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no need for a comma.

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I'll show you.

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So, I'm not sure why this is.

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I think it may just sort
of be a style relic.

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I'm not entirely certain.

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I mean, if you go back in American history

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and you look at the Federalist Papers

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and you look at the way
that people used commas

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in the 18th century, it doesn't
make a whole lot of sense,

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relative to how we use commas today.

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So, a lot of this is cultural.

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But I can tell you that
when an independent clause

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comes before a dependent
clause, you don't use a comma.

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And if you are uniting
two independent clauses,

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in the following sentence,
"I rode an elephant

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"and then I ate a mango," these two things

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are both independent clauses, right?

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I rode an elephant.

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Then I ate a mango, right?

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These two things need to be connected

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by this conjunction, and.

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But that's not all.

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They also need to be joined by a comma.

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Now, you could also sub
out, if you wanted to,

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get rid of this comma and this
and and put in a semicolon,

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but that's a story for another time.

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So, if you're uniting
two independent clauses,

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just do comma and then a conjunction.

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So, that's one way to think about

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how to use commas for
introductory elements

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like dependent and independent clauses.

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But there's also another thing
I wanna introduce you to,

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and that's sentence adverbs.

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Follow us over to the next screen.

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So, Paige, what is a sentence adverb?

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How does it work?

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- [Voiceover] So, we've been talking about

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starting sentences with clauses,

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but that doesn't always
have to be the case.

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You can start a sentence with an adverb.

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Like, let's say,
"Initially, I was afraid."

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- [Voiceover] So, what is initially doing

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in this sentence here, in this expression?

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- [Voiceover] Basically, it's modifying

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the whole rest of the sentence.

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It's modifying the "I was afraid."

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- [Voiceover] So, we're
gonna put a comma here

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to separate it from the
rest of that expression.

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That's why we call it a sentence adverb,

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'cause it's not, this is
not the same as saying,

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"I was initially afraid."

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This is kind of, like you said,

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modifying the entire expression.

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Let's look at another example.

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- [Voiceover] "Basically,
you're the greatest."

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- [Voiceover] Aww, thanks, Paige!

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So, we've got this word, basically,

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and basically is modifying
the entire expression.

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It's kind of qualifying the whole thing.

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- [Voiceover] Right.

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- [Voiceover] So, we're gonna put a comma

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between this sentence adverb
and the sentence itself.

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- [Voiceover] Exactly.

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- [Voiceover] Cool, so, initially, Paige,

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this seemed pretty complicated to me.

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- [Voiceover] Right, but,
basically, I think we got it down.

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- [Voiceover] All right,
we think that, essentially,

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you can learn anything.

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- [Voiceover] David out.
- [Voiceover] Paige out.


Punctuating a list

WEBVTT
Kind: captions
Language: en

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- [Voiceover] Hey Paige!

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- [Voiceover] What's up David?

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- [Voiceover] Is this right?

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Okay, so I'm about to
go to the grocery store,

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and it looks like it says,
"I need to get squid pickles

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and chocolate at the grocery store."

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- [Voiceover] Yeah.

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- [Voiceover] Did you want squid pickles?

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- [Voiceover] No, I
wanted squid and pickles.

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- [Voiceover] I must have
written it down wrong, okay.

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So I think what we need to
do in order to fix this list

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and avoid confusion like this

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in the future is using
commas to punctuate a list.

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'Cause right now this just
looks like squid pickles,

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which, I mean probably
delicious, pickled squid.

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- [Voiceover] Yeah.

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- [Voiceover] Probably delicious.

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- [Voiceover] Sure.

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- [Voiceover] But not what
we were looking to get today.

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- [Voiceover] Right.

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- [Voiceover] If we don't want
to get pickled squid today,

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then we have to put commas

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in between the elements of the list.

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- [Voiceover] Right.

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- [Voiceover] Right, because
this is what commas do.

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The separate elements of every everything.

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So let's put in those dividers.

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I need to get squid,
comma, pickles, comma,

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and chocolate at the grocery store.

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- [Voiceover] Exactly.

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- [Voiceover] Okay, so
we can punctuate a list

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by separating out nouns, and
I see from the second sentence

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here, Paige could you
give me a read for that?

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- [Voiceover] I'm going to
go for a run, read a chapter

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of my book and go see the
new Colonel Justice movie.

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- [Voiceover] Oh I hear that's good.

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So right now it says that,
but it could also just

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be "I'm going to go for
a run read a chapter

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of my book, and go see..."

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you know there's like no--

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- [Voiceover] It's a little confusing.

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- [Voiceover] It's a
little confusing, right?

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There could be such a thing as a run read.

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- [Voiceover] There probably is.

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- [Voiceover] You know, like where you go

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for a jog while holding a book.

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- [Voiceover] Sounds difficult.

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- [Voiceover] And so we can
also use commas in lists

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to separate not just nouns
like in this first one,

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but also verb phrases.

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So I'm going for a run, comma,
read a chapter of my book,

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comma, and go see the new
Colonel Justice movie.

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- [Voiceover] Perfect.

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- [Voiceover] Cool.

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So that's how you punctuate
a list with commas.

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- [Voiceover] Yeah, you got it.

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- [Voiceover] You can learn anything.

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David out.

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- [Voiceover] Paige out.


Darwinism vs. Social Darwinism part 2

WEBVTT
Kind: captions
Language: en

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- [Voiceover] So Emily
and I have been talking

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about how natural selection,

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Darwin's theory of evolution,

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has differed from some of the ways

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that people have interpreted
evolution over time,

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and specifically, was interested in

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this group known as the Social Darwinists,

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who were mostly a group of sociologists

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and other sort of public policy-makers

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who were trying to apply Darwin's theories

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to how the world should be ordered,

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and how races should relate to each other,

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classes should relate to each other,

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basically how this idea of evolution

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could be used to explain
the world around them

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in a social way.

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So, Emily, you were explaining to me

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how natural selection actually works.

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- [Voiceover] So just
as like a quick recap

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on whether that natural selection works

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in biological populations, is that

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there's some variation in the
population that's heritable,

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meaning that it's in the organism's genes

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and it can be passed on to offspring,

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and if that variation affects how well

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the organism can survive and reproduce.

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So, if it affects how
many offspring it's able

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to leave behind in the next generation,

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then it's possible for certain
traits that are well-suited

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to the organism's immediate environment

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to become more and more common

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in the population over generations.

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So that's kind of the
idea of natural selection

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in a nutshell.

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- [Voiceover] And we also
talked about the fact that

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there's no such thing as one race

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or one organism being more
evolved than another organism

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because we've all been
evolving this whole time.

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So what I'd like to do
now is just to talk about

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some of the ways that the
sociologists of this time

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kind of mis-applied Darwin's theories,

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and just get your opinions on how

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what they were saying actually relates

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with how Biology really works.

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So, Herbert Spencer was a
sociologist from England.

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He's actually the person
who coined the term

00:02:04.142 --> 00:02:07.369
"survival of the fittest",
which I think is interesting.

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So it wasn't Darwin who coined this term

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but rather, Spencer.

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And I think what was animating people

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like Spencer and other Social
Darwinists in this time

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was just to say, "Why are some people

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"in a better social position than others?"

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So, why are some people poor versus rich,

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why do African-Americans or
people from colonized nations

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have a worse situation in the world

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than people in civilized nations,

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or "civilized," kind of in air quotes,

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from that time period?

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And what Spencer said was,
"Oh, this must be Darwinism."

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Right, it must be that
people who are wealthy

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are better adapted to their environment;

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they're more evolved
than people who are poor.

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So I think this idea of
difference between the classes

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was one of the first animations behind

00:03:05.016 --> 00:03:07.663
the idea of Social Darwinism.

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So, is there any way that

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that actually relates to how Biology

00:03:12.281 --> 00:03:13.814
explains Darwinism?

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- [Voiceover] I mean, I
think that that's really

00:03:15.123 --> 00:03:17.623
a case of an idea from Biology

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being not very accurately
applied to society.

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You know, I think that there is probably

00:03:25.598 --> 00:03:28.747
a strong motivation for
someone in Spencer's position

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to want to explain things in that way

00:03:30.976 --> 00:03:34.761
because that would justify
inaction on his part

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and sort of say that this is acceptable

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because it's "natural".

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And, you know, I think
that it's really not

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a very scientific or even an
investigative perspective.

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I think that it was sort of an effort

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to fit something, sort of a round peg

00:03:56.944 --> 00:03:59.684
in a square hole, so to speak.

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- [Voiceover] Yeah, it's interesting.

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As I've been learning more about this,

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I've noticed that people in general

00:04:05.118 --> 00:04:07.231
seem to take up one of these explanations

00:04:07.231 --> 00:04:08.879
for like how the universe works,

00:04:08.879 --> 00:04:10.876
and then try to apply it to everything,

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even if it might not necessarily apply.

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So I think your word "inaction" here

00:04:15.869 --> 00:04:18.446
is really accurate
because one of the things

00:04:18.446 --> 00:04:20.675
that Spencer is just wondering about

00:04:20.675 --> 00:04:23.763
is like, "Should the
government do something

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"for people who are poor,

00:04:25.319 --> 00:04:27.269
"for people who are in a bad position?"

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And he basically says,
"No," 'cause his idea

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is that if the government helps them,

00:04:32.796 --> 00:04:36.279
then you're interfering with
this survival of the fittest,

00:04:36.279 --> 00:04:38.346
and that those sorts of people should

00:04:38.346 --> 00:04:41.248
naturally be bred out of the populace

00:04:41.248 --> 00:04:44.359
to make our other race more evolved.

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So is there any truth into this idea

00:04:46.496 --> 00:04:49.584
that you could make the
human race more evolved

00:04:49.584 --> 00:04:52.069
by breeding out certain parts of it?

00:04:52.069 --> 00:04:54.925
- [Voiceover] My tendency would be to say

00:04:54.925 --> 00:04:59.174
that any program designed
to reduce variation

00:04:59.174 --> 00:05:00.822
in a biological population

00:05:00.822 --> 00:05:03.121
is probably not going to be beneficial

00:05:03.121 --> 00:05:04.788
for that population.

00:05:05.759 --> 00:05:08.271
So that's kind of a broad statement

00:05:08.271 --> 00:05:10.438
but if you look at factors

00:05:11.661 --> 00:05:14.285
that make endangered species endangered,

00:05:14.285 --> 00:05:16.282
one of the big ones is that they have

00:05:16.282 --> 00:05:19.708
very low genetic variation
in their gene pool.

00:05:19.708 --> 00:05:23.377
And that means that harmful gene versions

00:05:23.377 --> 00:05:26.465
are more likely to come together
in the same individuals,

00:05:26.465 --> 00:05:28.185
so it's more likely for there to be

00:05:28.185 --> 00:05:31.222
genetic defects or genetic disorders.

00:05:31.222 --> 00:05:33.737
In general, a high level of variation

00:05:33.737 --> 00:05:36.198
is an indicator of a healthy population

00:05:36.198 --> 00:05:38.149
and a low level of variation,

00:05:38.149 --> 00:05:39.937
or any population where you have

00:05:39.937 --> 00:05:43.303
intensive inbreeding
of similar individuals,

00:05:43.303 --> 00:05:46.154
that's gonna be a population
that's probably less healthy.

00:05:46.154 --> 00:05:47.571
- [Voiceover] Interesting,
because there's this

00:05:47.571 --> 00:05:49.661
very popular movement in the United States

00:05:49.661 --> 00:05:51.843
and then moving on to places like Germany,

00:05:51.843 --> 00:05:54.862
where we see this very
strongly in the Holocaust

00:05:54.862 --> 00:05:56.604
of eugenics, right?

00:05:56.604 --> 00:05:59.604
The idea that you can make your race

00:06:02.079 --> 00:06:05.283
and your country more evolved, more fit,

00:06:05.283 --> 00:06:07.303
this survival of the fittest,

00:06:07.303 --> 00:06:09.248
by breeding out certain things

00:06:09.248 --> 00:06:11.222
that are termed "undesirable."

00:06:11.222 --> 00:06:12.987
- [Voiceover] I mean,
in terms of long-term

00:06:12.987 --> 00:06:16.354
population survival from
a biological standpoint,

00:06:16.354 --> 00:06:18.142
I don't see how that could be beneficial.

00:06:18.142 --> 00:06:19.883
I mean, you never know
what's down the road.

00:06:19.883 --> 00:06:22.414
You never know if there's
a new infectious disease

00:06:22.414 --> 00:06:25.131
that's gonna show up and who's
gonna be resistant to it.

00:06:25.131 --> 00:06:26.983
Somebody who has genetic variation,

00:06:26.983 --> 00:06:28.725
that doesn't matter right now,

00:06:28.725 --> 00:06:30.420
but you don't know who that's gonna be.

00:06:30.420 --> 00:06:33.206
So it's that sort of
general principle applies

00:06:33.206 --> 00:06:35.226
that the reason variation is great,

00:06:35.226 --> 00:06:36.735
is it prepares a population

00:06:36.735 --> 00:06:38.569
to deal with an uncertain future.

00:06:38.569 --> 00:06:40.311
- [Voiceover] This is so interesting to me

00:06:40.311 --> 00:06:43.306
because I think you see
this a lot in history,

00:06:43.306 --> 00:06:45.071
because people in some ways here,

00:06:45.071 --> 00:06:48.902
there are very much confusing
culture for Biology,

00:06:48.902 --> 00:06:52.269
because in many cases, they're saying that

00:06:52.269 --> 00:06:55.497
traits that they see as
culturally undesirable,

00:06:55.497 --> 00:06:59.235
you know, looking through
the eyes of white supremacy

00:06:59.235 --> 00:07:02.022
and sort of this racial
and cultural supremacy

00:07:02.022 --> 00:07:05.690
of people from England,
from the United States,

00:07:05.690 --> 00:07:07.455
from Western Europe, you know,

00:07:07.455 --> 00:07:09.731
they're looking around
them and they see people

00:07:09.731 --> 00:07:11.147
who are different from them,

00:07:11.147 --> 00:07:15.536
and they ascribe that to a
kind of biological inferiority

00:07:15.536 --> 00:07:18.113
when, in fact, it's just
a cultural difference.

00:07:18.113 --> 00:07:19.572
- [Voiceover] I think that
that's a very good way

00:07:19.572 --> 00:07:21.508
to state it; I think
that there's just a big

00:07:21.508 --> 00:07:24.587
conflation here of
cultural and biological,

00:07:24.587 --> 00:07:27.025
and applying ideas that
might be great for one

00:07:27.025 --> 00:07:29.905
in a place where they're
not actually as well-suited.

00:07:29.905 --> 00:07:30.964
- [Voiceover] Yeah, and
I think that one thing

00:07:30.964 --> 00:07:32.570
that also interests me about this

00:07:32.570 --> 00:07:35.128
is the way that things like eugenics

00:07:35.128 --> 00:07:37.357
are used as justifications,

00:07:37.357 --> 00:07:40.097
and using Darwinism as a justification

00:07:40.097 --> 00:07:41.908
for not helping certain people,

00:07:41.908 --> 00:07:44.300
for saying, "Oh, you're less evolved;

00:07:44.300 --> 00:07:46.829
"you belong at the bottom
of the social scale,

00:07:46.829 --> 00:07:49.011
"and if I help you,
then I'm not helping us

00:07:49.011 --> 00:07:50.451
"evolve as a race,"

00:07:50.451 --> 00:07:52.727
with also kind of a flip-side of saying,

00:07:52.727 --> 00:07:54.816
"Oh, we must help certain cultures

00:07:54.816 --> 00:07:56.465
"become more civilized."

00:07:56.465 --> 00:08:00.691
And you see that a lot in
the era of colonialism,

00:08:00.691 --> 00:08:02.409
which is also big in the Gilded Age.

00:08:02.409 --> 00:08:07.193
It's kind of becoming a
world event in the Gilded Age

00:08:07.193 --> 00:08:10.860
as England begins to
take colonies in India,

00:08:11.744 --> 00:08:15.180
and Africa, and other nations do the same.

00:08:15.180 --> 00:08:17.526
One of the justifications
that they're giving is

00:08:17.526 --> 00:08:20.126
"These people are less civilized than us.

00:08:20.126 --> 00:08:21.450
"They're less evolved than us,

00:08:21.450 --> 00:08:23.284
"and so we have to help them.

00:08:23.284 --> 00:08:25.002
"It's the white man's burden,"

00:08:25.002 --> 00:08:27.208
a phrase by Rudyard Kipling,

00:08:27.208 --> 00:08:30.111
to help these other races evolve.

00:08:30.111 --> 00:08:32.944
- [Voiceover] Yeah, that's
a very definite conflation

00:08:32.944 --> 00:08:36.519
of biological and social,
I mean, teaching people

00:08:36.519 --> 00:08:39.538
to behave in a different way
does not affect their biology.

00:08:39.538 --> 00:08:42.742
That was actually one of
Spencer's misconceptions.

00:08:42.742 --> 00:08:45.204
He was an adherent of Lamarckian evolution

00:08:45.204 --> 00:08:47.851
which would suggest
that if you gain a trait

00:08:47.851 --> 00:08:50.730
during your lifetime,
you will biologically

00:08:50.730 --> 00:08:53.029
and genetically pass
it on to your offspring

00:08:53.029 --> 00:08:54.979
and you may pass it on to your offspring

00:08:54.979 --> 00:08:56.628
by teaching them something,

00:08:56.628 --> 00:08:58.996
but genetic traits that are
acquired during your life,

00:08:58.996 --> 00:09:00.575
you don't pass those on.

00:09:00.575 --> 00:09:02.340
You actually don't generally inherit--

00:09:02.340 --> 00:09:04.314
- [Voiceover] They are
acquired genetic traits.

00:09:04.314 --> 00:09:05.521
- [Voiceover] Yeah, exactly.

00:09:05.521 --> 00:09:07.356
So I think that that definitely reflects

00:09:07.356 --> 00:09:10.421
the confusion in terms of
what biological evolution is

00:09:10.421 --> 00:09:12.789
and how it works mechanistically.

00:09:12.789 --> 00:09:14.554
- [Voiceover] I think if there's anything

00:09:14.554 --> 00:09:16.667
for us to take away from this, it's that

00:09:16.667 --> 00:09:19.453
one, there's no such thing as one person

00:09:19.453 --> 00:09:22.356
being more evolved than another person,

00:09:22.356 --> 00:09:25.049
or one race being more
evolved than another race.

00:09:25.049 --> 00:09:28.904
I think there's actually not
much of a biological basis

00:09:28.904 --> 00:09:31.179
for the concept of race, to start with,

00:09:31.179 --> 00:09:36.125
since human beings are
biologically 99.9% the same.

00:09:36.125 --> 00:09:38.656
- [Voiceover] Yeah, I mean,
from everything that I've seen,

00:09:38.656 --> 00:09:42.487
race is really something
that people have come up with

00:09:42.487 --> 00:09:45.593
in an effort to categorize
the world around them,

00:09:45.593 --> 00:09:48.264
but it's not actually
reflected very meaningfully

00:09:48.264 --> 00:09:49.993
in people's genetics.

00:09:49.993 --> 00:09:52.319
You know, there are
certain very superficial

00:09:52.319 --> 00:09:54.548
physical traits that are inherited,

00:09:54.548 --> 00:09:57.659
that we define as "race",
but if you actually

00:09:57.659 --> 00:10:00.283
look at the genomes of the people who belong

00:10:00.283 --> 00:10:03.975
to a particular racial group,
that is socially defined,

00:10:03.975 --> 00:10:05.995
there's a huge amount of variation there,

00:10:05.995 --> 00:10:08.039
and there's much more variation within

00:10:08.039 --> 00:10:09.780
what we would consider a race,

00:10:09.780 --> 00:10:13.356
than there are differences
that separate races.

00:10:13.356 --> 00:10:14.981
Basically, it's a pretty arbitrary way

00:10:14.981 --> 00:10:16.731
to categorize people.


Darwinism vs. Social Darwinism part 1

WEBVTT
Kind: captions
Language: en

00:00:00.316 --> 00:00:02.108
- [Voiceover] Hey, this
is Kim from Khan Academy.

00:00:02.108 --> 00:00:06.207
I am the History Fellow here,
and I am here with Emily.

00:00:06.207 --> 00:00:08.317
- [Voiceover] Hi, I'm the Biology Fellow.

00:00:08.317 --> 00:00:09.484
- [Voiceover] So, Emily and I are here

00:00:09.484 --> 00:00:11.860
talking about Darwinism.

00:00:11.860 --> 00:00:13.260
I'm interested in Darwinism,

00:00:13.260 --> 00:00:16.299
because in the late 19th century,

00:00:16.299 --> 00:00:18.522
usually called the Gilded Age,

00:00:18.522 --> 00:00:21.929
there is a very prominent interpretation

00:00:21.929 --> 00:00:26.242
of Darwinism that is
called, Social Darwinism.

00:00:26.242 --> 00:00:30.706
Social Darwinism wasn't so
much an actual form of biology,

00:00:30.706 --> 00:00:33.802
as it was, kind of a misinterpretation

00:00:33.802 --> 00:00:38.065
of how natural selection and
the theory of evolution worked,

00:00:38.065 --> 00:00:41.011
that was used to justify or explain

00:00:41.011 --> 00:00:44.807
a lot of the social inequalities
of this time period.

00:00:44.807 --> 00:00:46.693
The way people often thought about it

00:00:46.693 --> 00:00:49.864
was that white Anglo-Saxon people,

00:00:49.864 --> 00:00:52.897
so Europeans, Northern Europeans,

00:00:52.897 --> 00:00:55.480
were kind of the most evolved.

00:00:56.688 --> 00:00:59.992
This is our timeline of evolution,

00:00:59.992 --> 00:01:02.909
from least evolved to most evolved,

00:01:04.033 --> 00:01:06.866
and people like African Americans,

00:01:08.800 --> 00:01:11.383
or Asians, or Native Americans,

00:01:13.960 --> 00:01:16.127
or even Eastern Europeans,

00:01:18.234 --> 00:01:20.409
were less evolved.

00:01:20.409 --> 00:01:23.576
That they were on a scale of evolution

00:01:26.192 --> 00:01:30.592
where they hadn't come
as far as Anglo-Saxons.

00:01:30.592 --> 00:01:33.735
So, Emily, you're a biologist,

00:01:33.735 --> 00:01:35.761
and I would love to get your take

00:01:35.761 --> 00:01:39.184
on how it is that natural
selection actually works,

00:01:39.184 --> 00:01:41.344
and how this doesn't
quite describe what was

00:01:41.344 --> 00:01:42.618
really going on.

00:01:42.618 --> 00:01:44.514
- [Voiceover] Yeah, definitely.

00:01:44.514 --> 00:01:48.159
So, maybe I can speak first
to that specific graph

00:01:48.159 --> 00:01:50.048
that you've drawn on.

00:01:50.048 --> 00:01:52.042
I think that this is
actually a common point

00:01:52.042 --> 00:01:55.025
of confusion when it comes to evolution,

00:01:55.025 --> 00:01:57.163
that there's not really such a thing,

00:01:57.163 --> 00:02:00.178
as more or less evolved in evolution.

00:02:00.178 --> 00:02:02.403
So--
- [Voiceover] Okay, so,

00:02:02.403 --> 00:02:05.631
this gradient really doesn't exist.

00:02:05.631 --> 00:02:09.823
- [Voiceover] No, I mean,
there's sort of the,

00:02:09.823 --> 00:02:11.506
I think that people
sometimes see the pictures

00:02:11.506 --> 00:02:14.488
of like, the ape standing up,
and turning into a person,

00:02:14.488 --> 00:02:18.087
and they think, "Oh, this
is sort of a linear path

00:02:18.087 --> 00:02:20.409
"from one thing to another."

00:02:20.409 --> 00:02:24.576
What you really get is different
types of organisms evolving

00:02:25.542 --> 00:02:28.363
from a shared ancestor, and branching off.

00:02:28.363 --> 00:02:29.295
So--

00:02:29.295 --> 00:02:30.128
- [Voiceover] All right so--

00:02:30.128 --> 00:02:32.711
- [Voiceover] Nobody
who's alive on Earth today

00:02:32.711 --> 00:02:35.690
has been evolving for more or less time,

00:02:35.690 --> 00:02:37.793
since their last common ancestor,

00:02:37.793 --> 00:02:39.964
than anybody else.

00:02:39.964 --> 00:02:40.989
- [Voiceover] So would you,

00:02:40.989 --> 00:02:44.508
I'm gonna draw what I think
is how you're explaining this,

00:02:44.508 --> 00:02:47.181
and please correct me, as I go along.

00:02:47.181 --> 00:02:49.867
Say this is my common ancestor,

00:02:49.867 --> 00:02:54.461
and then would there possibly
be branches like this?

00:02:54.461 --> 00:02:58.311
- [Voiceover] Yeah, that's
a great way to draw it.

00:02:58.311 --> 00:03:01.845
Certainly, humans are all
extremely closely related

00:03:01.845 --> 00:03:02.678
to each other,

00:03:02.678 --> 00:03:04.881
but we could even say this for us,

00:03:04.881 --> 00:03:07.048
versus a dog, a bacterium.

00:03:08.305 --> 00:03:11.228
Pretty much anything, all life on Earth,

00:03:11.228 --> 00:03:13.193
shares a common ancestor.

00:03:13.193 --> 00:03:16.987
That bacteria is just as
evolved as you are, actually,

00:03:16.987 --> 00:03:19.713
in this sense of absolute time,

00:03:19.713 --> 00:03:21.191
since those two split apart.

00:03:21.191 --> 00:03:23.525
- [Voiceover] Right, so
from the moment that life

00:03:23.525 --> 00:03:27.175
first appeared on Earth,
there has been so much time,

00:03:27.175 --> 00:03:31.635
and all of us have been
evolving from that point.

00:03:31.635 --> 00:03:33.989
Even whether you're a piece of bacteria,

00:03:33.989 --> 00:03:34.822
piece a good word?

00:03:34.822 --> 00:03:35.852
A bacterium?

00:03:35.852 --> 00:03:37.204
- [Voiceover] There you go!
- [Voiceover] Excellent.

00:03:37.204 --> 00:03:38.405
- [Voiceover] Beautiful.
(laughs)

00:03:38.405 --> 00:03:39.292
- [Voiceover] So bacterium.

00:03:39.292 --> 00:03:40.959
I took biology once,

00:03:42.403 --> 00:03:44.509
or you are Albert Einstein.

00:03:44.509 --> 00:03:47.453
You have been evolving for
precisely the same amount

00:03:47.453 --> 00:03:48.286
of time.

00:03:48.286 --> 00:03:49.119
- [Voiceover] Yep.
- [Voiceover] Awesome.

00:03:49.119 --> 00:03:52.651
Okay, wow, I'm really relieved
to find that I could describe

00:03:52.651 --> 00:03:54.192
that as well as I could.

00:03:54.192 --> 00:03:55.267
(laughs)

00:03:55.267 --> 00:03:56.100
- [Voiceover] You did a great job.

00:03:56.100 --> 00:03:57.183
Beautiful.

00:03:57.183 --> 00:03:59.348
- [Voiceover] Okay, so you've done this,

00:03:59.348 --> 00:04:03.077
this distinction between
evolving from a common ancestor.

00:04:03.077 --> 00:04:06.880
How is it that the actual theory
of natural selection works?

00:04:06.880 --> 00:04:08.698
- [Voiceover] Yeah,
that's a great question.

00:04:08.698 --> 00:04:11.716
So, natural selection,
often people talk about it

00:04:11.716 --> 00:04:15.613
as sort of, having three key ingredients.

00:04:15.613 --> 00:04:18.495
To see how it works, let's
imagine that we're just looking

00:04:18.495 --> 00:04:20.775
at a population of beetles.

00:04:20.775 --> 00:04:24.192
So picture you're beetles, to start with.

00:04:25.768 --> 00:04:29.275
What would we need to have,
in order for these beetles,

00:04:29.275 --> 00:04:32.580
as a group, to evolve
by natural selection?

00:04:32.580 --> 00:04:33.908
One thing that we would need is,

00:04:33.908 --> 00:04:36.827
we would need some
variation among the beetles.

00:04:36.827 --> 00:04:37.660
- [Voiceover] Okay.

00:04:37.660 --> 00:04:40.179
- [Voiceover] So, if you
have identical beetles,

00:04:40.179 --> 00:04:43.410
you're not going to have any
that are better at surviving

00:04:43.410 --> 00:04:46.364
or reproducing, than any others,

00:04:46.364 --> 00:04:48.441
which is kind of a key ingredient,

00:04:48.441 --> 00:04:50.010
for what we're gonna talk about.

00:04:50.010 --> 00:04:50.843
- [Voiceover] Okay.

00:04:50.843 --> 00:04:52.586
So, I've got two different beetles here.

00:04:52.586 --> 00:04:55.045
They're slightly
different from each other.

00:04:55.045 --> 00:04:56.099
- [Voiceover] Awesome.

00:04:56.099 --> 00:04:59.080
You've made them different
colors, which is perfect.

00:04:59.080 --> 00:05:00.663
We have variation,

00:05:00.663 --> 00:05:03.040
and the next ingredient
that we're going to need,

00:05:03.040 --> 00:05:05.447
is we're gonna need that
variation to be heritable.

00:05:05.447 --> 00:05:07.304
We're gonna say that those beetles,

00:05:07.304 --> 00:05:10.113
one of them is green
and one of them is blue,

00:05:10.113 --> 00:05:12.602
and that's because of
something in their DNA.

00:05:12.602 --> 00:05:15.043
They have differences in their DNA,

00:05:15.043 --> 00:05:17.659
that create the variation in colors.

00:05:17.659 --> 00:05:19.242
- [Voiceover] Okay.

00:05:21.973 --> 00:05:24.588
Alright, so, when you say
heritable, you mean that this

00:05:24.588 --> 00:05:28.192
is something that their
descendants could inherit?

00:05:28.192 --> 00:05:29.400
- [Voiceover] Exactly.

00:05:29.400 --> 00:05:31.386
That is exactly it.

00:05:31.386 --> 00:05:35.204
So the final ingredient
is that, the differences,

00:05:35.204 --> 00:05:36.898
these heritable differences,

00:05:36.898 --> 00:05:39.914
need to affect how good the beetles are

00:05:39.914 --> 00:05:42.950
at leaving offspring
in the next generation.

00:05:42.950 --> 00:05:44.650
- [Voiceover] Alright, okay.

00:05:44.650 --> 00:05:46.335
- [Voiceover] For example, let's say,

00:05:46.335 --> 00:05:48.302
we would probably have
more than two beetles,

00:05:48.302 --> 00:05:49.968
in our actual population --

00:05:49.968 --> 00:05:50.801
- [Voiceover] Okay.

00:05:50.801 --> 00:05:52.290
- [Voiceover] But, let's say
this population has just moved

00:05:52.290 --> 00:05:56.394
into a new area that is very blue colored.

00:05:56.394 --> 00:05:57.227
- [Voiceover] Okay.

00:05:57.227 --> 00:06:00.617
- [Voiceover] So blue rocks,
blue flowers, whatever.

00:06:00.617 --> 00:06:02.499
There's also a bunch of
birds in that environment

00:06:02.499 --> 00:06:05.355
that really like to eat beetles.

00:06:05.355 --> 00:06:06.188
- [Voiceover] Oh.

00:06:06.188 --> 00:06:07.021
- [Voiceover] I'm guessing,

00:06:07.021 --> 00:06:09.598
that if they have blue
beetles and green beetles,

00:06:09.598 --> 00:06:12.181
that one of those is
gonna show up a lot better

00:06:12.181 --> 00:06:15.081
against the blue
environment, than the other.

00:06:15.081 --> 00:06:18.454
Probably a lot of our
green beetles are gonna get

00:06:18.454 --> 00:06:21.462
picked off by birds, and
they're not gonna be able

00:06:21.462 --> 00:06:25.502
to leave offspring, because
gosh, they kinda got ate.

00:06:25.502 --> 00:06:26.335
(laughs)

00:06:26.335 --> 00:06:29.357
So, when you look at the
next generation of beetles,

00:06:29.357 --> 00:06:32.024
if we know that the
colors get passed on --

00:06:32.024 --> 00:06:32.857
- [Voiceover] Mm-hmm.

00:06:32.857 --> 00:06:36.311
- [Voiceover] We're gonna
probably see a bigger group of,

00:06:36.311 --> 00:06:38.244
a bigger proportion of that group,

00:06:38.244 --> 00:06:40.447
being made up of blue beetles,

00:06:40.447 --> 00:06:42.737
and less being made up of green.

00:06:42.737 --> 00:06:46.705
That is an example of
natural selection in action,

00:06:46.705 --> 00:06:50.217
where you can see that
organisms that survive and,

00:06:50.217 --> 00:06:54.663
specifically, reproduce better,
in a certain environment,

00:06:54.663 --> 00:06:58.403
are going to increase in
frequency in a population.

00:06:58.403 --> 00:07:00.858
You're gonna get more and
more of these blue guys,

00:07:00.858 --> 00:07:02.779
less and less of the green guys.

00:07:02.779 --> 00:07:04.331
- [Voiceover] Is there
a word you would use

00:07:04.331 --> 00:07:07.779
to describe this, sort
of, this adaptation?

00:07:07.779 --> 00:07:09.478
Is that the natural selection part,

00:07:09.478 --> 00:07:12.101
as from variation, inheritable?

00:07:12.101 --> 00:07:16.196
Like, the natural selection
is just like a good situation?

00:07:16.196 --> 00:07:19.868
- [Voiceover] Yeah, I mean,
natural selection is really,

00:07:19.868 --> 00:07:23.839
just the differential
survival and reproduction.

00:07:23.839 --> 00:07:26.580
As you mentioned a great word
there, which is adaptation.

00:07:26.580 --> 00:07:28.038
- [Voiceover] Mm-hmm.

00:07:28.038 --> 00:07:30.418
- [Voiceover] Adaptation
is the word that biologists

00:07:30.418 --> 00:07:34.501
often give to the process
of a population getting

00:07:36.329 --> 00:07:38.865
better and better suited
to its environment.

00:07:38.865 --> 00:07:42.274
You would say that the
population was adapting to being

00:07:42.274 --> 00:07:43.280
in a blue space --

00:07:43.280 --> 00:07:44.863
- [Voiceover] Mm-hmm.

00:07:44.863 --> 00:07:46.974
- [Voiceover] As it gradually
started to have more and more

00:07:46.974 --> 00:07:49.724
blue beetles, across generations.

00:07:51.150 --> 00:07:53.766
- [Voiceover] What I find
really interesting about this,

00:07:53.766 --> 00:07:57.494
is that it's entirely by chance, right?

00:07:57.494 --> 00:08:00.959
More or less, you've
got a genetic mutation,

00:08:00.959 --> 00:08:04.526
and then that mutation happens
to suit the environment

00:08:04.526 --> 00:08:07.742
that you're in, which allows
you to thrive and your genes,

00:08:07.742 --> 00:08:11.239
and your adaptations to
be passed on, over time.

00:08:11.239 --> 00:08:12.072
- [Voiceover] Yes!

00:08:12.072 --> 00:08:15.110
I mean, the variation would
have occurred randomly

00:08:15.110 --> 00:08:15.967
to start with.

00:08:15.967 --> 00:08:19.018
Like you say, it would've been a mutation,

00:08:19.018 --> 00:08:21.215
it didn't happen 'cause the
beetles went to a blue place,

00:08:21.215 --> 00:08:24.759
and said, "Gosh, I should be
blue, that would be awesome."

00:08:24.759 --> 00:08:26.118
That was not what happened.

00:08:26.118 --> 00:08:29.078
It was already there and it
just happened to be successful

00:08:29.078 --> 00:08:30.583
in that environment.

00:08:30.583 --> 00:08:32.895
If they'd gone to a green
place, well, guess what?

00:08:32.895 --> 00:08:35.646
You would've gotten exactly
the opposite effect.

00:08:35.646 --> 00:08:38.882
Suddenly, the green guys
would've been more successful,

00:08:38.882 --> 00:08:40.670
more able to leave offspring,

00:08:40.670 --> 00:08:44.431
and you would've seen
green be increasing or,

00:08:44.431 --> 00:08:46.067
"favored by natural selection."


Application of the fundamental laws (solve)

WEBVTT
Kind: captions
Language: en

00:00:00.373 --> 00:00:02.792
- So in the last video, we
did our circuit analysis.

00:00:02.792 --> 00:00:05.587
We set up the four
equations that we needed

00:00:05.587 --> 00:00:08.109
to solve in order to
figure out all the voltages

00:00:08.109 --> 00:00:11.263
and currents in our example circuit.

00:00:11.263 --> 00:00:12.892
And so now, we're gonna solve it.

00:00:12.892 --> 00:00:14.741
This is a matter of doing the algebra

00:00:14.741 --> 00:00:18.630
to solve a set of four
simultaneous equations.

00:00:18.630 --> 00:00:21.620
What I wanna do now is just
write my equations down here

00:00:21.620 --> 00:00:23.753
and tidy them up a little bit.

00:00:23.753 --> 00:00:25.335
So the first one we're gonna bring down

00:00:25.335 --> 00:00:27.976
is the voltage statement here,

00:00:27.976 --> 00:00:30.395
which I can write as

00:00:30.395 --> 00:00:31.228
V1 plus V2

00:00:35.305 --> 00:00:36.138
equals Vs.

00:00:39.285 --> 00:00:43.548
I'm gonna bring down the
i equation over here.

00:00:43.548 --> 00:00:44.631
It's gonna be

00:00:47.200 --> 00:00:48.117
i1 minus i2

00:00:52.241 --> 00:00:53.074
equals Is.

00:00:55.869 --> 00:00:57.495
And now I have to pick.

00:00:57.495 --> 00:00:58.989
I have four variables here.

00:00:58.989 --> 00:01:01.514
I'm gonna start doing substitutions

00:01:01.514 --> 00:01:04.402
based on these expressions up here.

00:01:04.402 --> 00:01:05.999
These are my Ohm's law expressions.

00:01:05.999 --> 00:01:09.148
So we're gonna use those to
substitute for the voltages,

00:01:09.148 --> 00:01:12.849
and we're gonna solve the whole
thing in terms of currents.

00:01:12.849 --> 00:01:15.766
So the first equation becomes V1 is

00:01:19.147 --> 00:01:19.980
R1 i1 plus

00:01:23.777 --> 00:01:24.610
R2 i2

00:01:27.652 --> 00:01:28.485
equals Vs.

00:01:29.683 --> 00:01:31.352
Second equation stays the same.

00:01:31.352 --> 00:01:32.269
i1 minus i2

00:01:35.796 --> 00:01:36.629
equals Is.

00:01:39.334 --> 00:01:40.872
The next step in the solution is gonna be

00:01:40.872 --> 00:01:44.271
to figure out how to eliminate
one of these currents.

00:01:44.271 --> 00:01:47.471
Now I think what I'll do is I'll eliminate

00:01:47.471 --> 00:01:51.554
i2, and I can do that if
I multiply this equation

00:01:53.948 --> 00:01:57.198
by R2 and add these equations together.

00:01:59.788 --> 00:02:02.612
So let's do the
multiplication by R2 first.

00:02:02.612 --> 00:02:04.876
We'll rewrite the first equation up here.

00:02:04.876 --> 00:02:06.209
R1 i1 plus R2 i2

00:02:11.653 --> 00:02:15.403
equals Vs, and then
down, the bottom equation

00:02:17.166 --> 00:02:18.583
is gonna be R2 i1

00:02:23.191 --> 00:02:24.108
minus R2 i2

00:02:28.372 --> 00:02:29.455
equals R2 Is.

00:02:34.235 --> 00:02:37.131
We're gonna add these equations together.

00:02:37.131 --> 00:02:40.881
And we can see these
terms nicely cancel out,

00:02:40.881 --> 00:02:43.214
and so I'm gonna end up with

00:02:44.292 --> 00:02:45.125
R1 plus R2

00:02:49.763 --> 00:02:51.013
times i1 equals

00:02:55.336 --> 00:02:56.169
Vs plus R2

00:03:01.010 --> 00:03:01.843
Is.

00:03:04.900 --> 00:03:08.179
And I can get the solution
in symbolic terms for i1.

00:03:08.179 --> 00:03:09.012
I1 equals

00:03:12.238 --> 00:03:13.321
Vs plus R2 Is

00:03:18.004 --> 00:03:19.337
over R1 plus R2.

00:03:27.051 --> 00:03:28.729
What I wanna do now is actually put in

00:03:28.729 --> 00:03:31.660
the original real values
and solve this circuit

00:03:31.660 --> 00:03:33.327
all the way through.

00:03:34.448 --> 00:03:37.115
So we'll move up and keep going.

00:03:39.904 --> 00:03:42.414
So let me sketch the circuit again,

00:03:42.414 --> 00:03:43.822
the real circuit again.

00:03:43.822 --> 00:03:48.219
We had a voltage source
with fifteen volts.

00:03:48.219 --> 00:03:51.386
It went to a 4k resistor, that was R1.

00:03:57.864 --> 00:03:59.947
We had a 2k resistor here

00:04:02.891 --> 00:04:04.641
and a current source,

00:04:09.451 --> 00:04:11.534
and that was 3 milliamps.

00:04:15.067 --> 00:04:17.234
And we want to discover...

00:04:18.565 --> 00:04:20.982
This was i1, and this was i2.

00:04:24.776 --> 00:04:26.402
Those are the two unknown currents.

00:04:26.402 --> 00:04:27.954
Let's see if we can find those.

00:04:27.954 --> 00:04:28.787
So i1

00:04:30.973 --> 00:04:32.056
equals Vs, 15

00:04:36.264 --> 00:04:39.514
plus R2 is 2k times Is, three milliamps

00:04:46.906 --> 00:04:48.739
divided by 4k plus 2k.

00:04:55.472 --> 00:04:56.738
So let's solve this.

00:04:56.738 --> 00:04:57.905
That equals 15

00:05:00.203 --> 00:05:03.786
plus 2k times three
milliamps is six volts.

00:05:07.473 --> 00:05:10.473
This is volts, and that's divided by

00:05:11.990 --> 00:05:14.157
6k equals 21 volts over 6k

00:05:22.566 --> 00:05:24.316
equals 3.5 milliamps.

00:05:29.344 --> 00:05:31.956
So that's this value right here,

00:05:31.956 --> 00:05:33.039
3.5 milliamps

00:05:36.353 --> 00:05:37.186
is i1.

00:05:40.605 --> 00:05:42.498
So we have one of our unknown currents,

00:05:42.498 --> 00:05:44.402
and we wanna find the other one.

00:05:44.402 --> 00:05:46.976
That's sitting right here.

00:05:46.976 --> 00:05:48.726
We can figure out i2.

00:05:51.281 --> 00:05:53.531
Minus i2 equals Is minus i1

00:06:01.986 --> 00:06:04.319
equals three milliamps minus

00:06:11.212 --> 00:06:12.295
3.5 milliamps

00:06:17.625 --> 00:06:18.958
equals minus .5.

00:06:24.593 --> 00:06:26.305
Let's get rid of the minus signs.

00:06:26.305 --> 00:06:28.305
i2 equals 0.5 milliamps.

00:06:33.503 --> 00:06:36.003
That's that value right there.

00:06:39.836 --> 00:06:43.462
Let's put some boxes around
our answers we have so far.

00:06:43.462 --> 00:06:44.379
Here's one.

00:06:46.875 --> 00:06:50.375
And here's another one, there's i2 and i1.

00:06:51.395 --> 00:06:55.948
And now let's finish up by
figuring out the voltages.

00:06:55.948 --> 00:06:58.813
We can use Ohm's law to
figure out the voltage

00:06:58.813 --> 00:07:02.200
across the 4k resistor, and that's V1.

00:07:02.200 --> 00:07:04.533
So V1 equals i, which is 3.5

00:07:09.068 --> 00:07:10.651
milliamps times 4k.

00:07:15.154 --> 00:07:20.059
4k ohms and that equals,
three and a half times four

00:07:20.059 --> 00:07:21.059
is 14 volts.

00:07:24.137 --> 00:07:27.606
Whenever you multiply
milliamps times k ohms,

00:07:27.606 --> 00:07:30.773
those units cancel, and you get volts.

00:07:32.409 --> 00:07:34.242
So V1 equals 14 volts.

00:07:45.552 --> 00:07:48.968
And our last current,
sorry, our last voltage

00:07:48.968 --> 00:07:51.718
is V2, is the voltage right here.

00:07:53.656 --> 00:07:54.489
And V2

00:07:57.081 --> 00:08:00.581
equals i2 which we decided was .5 milliamp

00:08:05.019 --> 00:08:07.352
times two thousand ohms, 2k.

00:08:10.679 --> 00:08:12.596
And V2 equals one volt.

00:08:32.727 --> 00:08:34.465
And now we've solved our circuit.

00:08:34.465 --> 00:08:36.308
That's a complete circuit analysis

00:08:36.308 --> 00:08:38.558
using the fundamental laws.


Transcription and mRNA processing

WEBVTT
Kind: captions
Language: en

00:00:00.274 --> 00:00:01.451
- [Voiceover] What we're
going to do in this video

00:00:01.451 --> 00:00:05.828
is a little bit of a deep
dive on transcription.

00:00:05.828 --> 00:00:07.968
And just as a bit of a review,

00:00:07.968 --> 00:00:09.648
we touch on it on the video on

00:00:09.648 --> 00:00:12.951
replication, transcription
and translation.

00:00:12.951 --> 00:00:15.269
Transcription in everyday
language just means to

00:00:15.269 --> 00:00:17.873
rewrite something or to
rewrite some information

00:00:17.873 --> 00:00:18.958
in another form.

00:00:18.958 --> 00:00:20.783
And that's essentially
what's happening here.

00:00:20.783 --> 00:00:23.975
Transcription is when we
take the information encoded

00:00:23.975 --> 00:00:26.831
in the gene in DNA and encode essentially

00:00:26.831 --> 00:00:30.104
that same information in mRNA.

00:00:30.104 --> 00:00:33.271
So transcription we are going from DNA

00:00:35.475 --> 00:00:36.892
to messenger RNA,

00:00:39.135 --> 00:00:40.333
and we're gonna, in this video,

00:00:40.333 --> 00:00:43.549
focus on genes that code for proteins.

00:00:43.549 --> 00:00:45.755
So this first step is the transcription,

00:00:45.755 --> 00:00:48.351
the DNA to messenger RNA,

00:00:48.351 --> 00:00:49.780
and then in a future video

00:00:49.780 --> 00:00:52.169
we'll dig a little bit
deeper into translation.

00:00:52.169 --> 00:00:56.336
We will translate that information
into an actual protein.

00:00:57.485 --> 00:01:00.408
But these diagrams give a
little bit of an overview of it.

00:01:00.408 --> 00:01:02.517
It's a little bit simpler in bacteria.

00:01:02.517 --> 00:01:05.759
You have the DNA just floating
around in the cytosol,

00:01:05.759 --> 00:01:07.762
and so the transcription takes place.

00:01:07.762 --> 00:01:09.549
You start with that DNA,

00:01:09.549 --> 00:01:12.477
that protein coding gene in the DNA,

00:01:12.477 --> 00:01:15.081
and from that you code the messenger RNA,

00:01:15.081 --> 00:01:17.467
you see that in that purple
color right over here,

00:01:17.467 --> 00:01:18.608
and then that messenger RNA

00:01:18.608 --> 00:01:20.300
can be involved with the ribosome,

00:01:20.300 --> 00:01:22.214
and that's the translation process

00:01:22.214 --> 00:01:23.985
to actually produce the polypeptide,

00:01:23.985 --> 00:01:25.996
to produce the protein.

00:01:25.996 --> 00:01:27.381
In eukaryotic cells,

00:01:27.381 --> 00:01:28.214
and we're going to get into

00:01:28.214 --> 00:01:29.787
a little bit more depth in this video,

00:01:29.787 --> 00:01:32.704
the transcription, the DNA to mRNA,

00:01:34.112 --> 00:01:36.608
that happens inside of the nucleus.

00:01:36.608 --> 00:01:38.511
There's essentially two steps here.

00:01:38.511 --> 00:01:42.428
You go from DNA to what
we would call pre-mRNA,

00:01:44.366 --> 00:01:45.513
let me write that down,

00:01:45.513 --> 00:01:46.346
pre-mRNA,

00:01:49.798 --> 00:01:52.426
which is depicted right over there,

00:01:52.426 --> 00:01:54.702
and then it needs to be
processed to turn into

00:01:54.702 --> 00:01:56.688
what we would call mRNA,

00:01:56.688 --> 00:01:58.871
which then can leave the nucleus

00:01:58.871 --> 00:02:02.120
to be translated into a protein.

00:02:02.120 --> 00:02:03.693
So now that we have that overview,

00:02:03.693 --> 00:02:05.297
let's dig a little bit deeper into this

00:02:05.297 --> 00:02:07.217
and understand the different actors

00:02:07.217 --> 00:02:09.307
and understand if we're
talking about a eukaryotic cell

00:02:09.307 --> 00:02:13.057
what type of processing
might actually go on.

00:02:14.252 --> 00:02:15.875
So right over here,

00:02:15.875 --> 00:02:20.258
we are going to start with
the protein coding gene

00:02:20.258 --> 00:02:23.175
inside of the DNA, right over here,

00:02:25.394 --> 00:02:29.239
and the primary actor that's
not the DNA or the mRNA here

00:02:29.239 --> 00:02:31.474
is going to be RNA polymerase.

00:02:31.474 --> 00:02:35.058
It's used to create a
sequence that will become

00:02:35.058 --> 00:02:39.225
a nucleotide sequence, that
will become the messenger RNA.

00:02:40.193 --> 00:02:43.956
So this RNA polymerase, it
needs to know where to start.

00:02:43.956 --> 00:02:47.454
The way it knows where to start
is it attaches to a sequence

00:02:47.454 --> 00:02:50.037
of the DNA known as a promoter.

00:02:51.201 --> 00:02:53.035
And every gene is going to have

00:02:53.035 --> 00:02:55.142
a promoter associated with it,

00:02:55.142 --> 00:02:57.862
especially if we're talking
about eukaryotic cells.

00:02:57.862 --> 00:03:00.623
Sometimes you might have
a promoter associated

00:03:00.623 --> 00:03:02.989
with a collection of genes as well.

00:03:02.989 --> 00:03:04.385
But in general, if you've got a gene,

00:03:04.385 --> 00:03:05.897
you're gonna have a promoter.

00:03:05.897 --> 00:03:08.632
That's how the RNA polymerase

00:03:08.632 --> 00:03:11.937
knows to attach right over there.

00:03:11.937 --> 00:03:13.903
Once it attaches,

00:03:13.903 --> 00:03:18.252
well then, it is able
to separate the strands.

00:03:18.252 --> 00:03:19.480
It separates the strands,

00:03:19.480 --> 00:03:20.342
and it's pretty interesting,

00:03:20.342 --> 00:03:22.316
because when we went in
deep into replication,

00:03:22.316 --> 00:03:25.381
you saw all of these actors,
the helicase and whatever else,

00:03:25.381 --> 00:03:29.113
but this RNA polymerase complex
is actually quite capable.

00:03:29.113 --> 00:03:31.036
Not only it separates the strand

00:03:31.036 --> 00:03:35.036
and then it's actually
able to code for the RNA.

00:03:35.990 --> 00:03:37.030
It does that the same way

00:03:37.030 --> 00:03:38.682
that when we studied DNA polymerase,

00:03:38.682 --> 00:03:40.355
it does it in only one direction.

00:03:40.355 --> 00:03:44.036
It can only add more nucleotides
on the three prime end.

00:03:44.036 --> 00:03:45.738
So it encodes from the five prime

00:03:45.738 --> 00:03:47.440
to the three prime direction.

00:03:47.440 --> 00:03:48.273
Notice this arrow here,

00:03:48.273 --> 00:03:52.153
we're extending it on the
three prime end of the RNA.

00:03:52.153 --> 00:03:53.838
So as you can see here, when it does this,

00:03:53.838 --> 00:03:56.588
it's only encoding one side of...

00:03:58.398 --> 00:04:00.849
Or it's only interacting,
I guess you could say,

00:04:00.849 --> 00:04:05.011
or coding complementary
information to one side.

00:04:05.011 --> 00:04:06.585
But let's think about this a little bit.

00:04:06.585 --> 00:04:09.335
We could call the side that it is

00:04:11.693 --> 00:04:12.947
interacting with,

00:04:12.947 --> 00:04:14.485
you can call that the template strand

00:04:14.485 --> 00:04:15.318
because

00:04:16.605 --> 00:04:18.110
that side of the DNA

00:04:18.110 --> 00:04:21.388
is acting as the template
for forming that RNA.

00:04:21.388 --> 00:04:23.073
But if you think about the information

00:04:23.073 --> 00:04:25.598
that that RNA is actually going to encode,

00:04:25.598 --> 00:04:27.738
well it's gonna contain
the same information

00:04:27.738 --> 00:04:31.734
as the coding strand of DNA,
as the other stand of DNA,

00:04:31.734 --> 00:04:34.532
because these nucleotides right over here,

00:04:34.532 --> 00:04:37.452
this nucleotide is going
to be complementary

00:04:37.452 --> 00:04:38.694
to this one over here,

00:04:38.694 --> 00:04:41.452
just as this nucleotide was complementary

00:04:41.452 --> 00:04:42.819
to that one over there.

00:04:42.819 --> 00:04:44.664
And you can see it in
a little bit more depth

00:04:44.664 --> 00:04:46.969
if we actually were to
add the nucleotides.

00:04:46.969 --> 00:04:48.788
So this is the template strand.

00:04:48.788 --> 00:04:51.956
If you have a thymine, well on the RNA,

00:04:51.956 --> 00:04:53.406
you'd have the adenine.

00:04:53.406 --> 00:04:56.588
Look, on the coding strand
of DNA, the one up here,

00:04:56.588 --> 00:04:58.561
you would also have an adenine.

00:04:58.561 --> 00:05:01.467
Essentially the coding strand and the RNA,

00:05:01.467 --> 00:05:03.188
essentially end up
being the same sequence,

00:05:03.188 --> 00:05:04.206
but the one difference

00:05:04.206 --> 00:05:07.181
is that you won't find
the thymine in the RNA,

00:05:07.181 --> 00:05:11.098
instead you'll find a
similar nitrogenous base,

00:05:12.977 --> 00:05:14.705
and that is uracil.

00:05:14.705 --> 00:05:16.593
But uracil plays the role of thymine,

00:05:16.593 --> 00:05:19.344
so you're essentially
coding the same information.

00:05:19.344 --> 00:05:22.575
So once again, this bottom
strand is acting as a template,

00:05:22.575 --> 00:05:26.673
but it's going to be the
resulting RNA that gets coded,

00:05:26.673 --> 00:05:28.825
is essentially going to
have the same information

00:05:28.825 --> 00:05:32.105
that we had in the coding strand.

00:05:32.105 --> 00:05:35.096
Just to get an appreciation
for what this looks like,

00:05:35.096 --> 00:05:38.065
I would even write, I'd
put looks in quotations,

00:05:38.065 --> 00:05:38.910
I even did little quote things

00:05:38.910 --> 00:05:40.501
with my fingers when I said that,

00:05:40.501 --> 00:05:42.819
is that it's hard to really visualize

00:05:42.819 --> 00:05:44.113
what these things look like,

00:05:44.113 --> 00:05:47.140
but you can see here that
the RNA polymerase complex,

00:05:47.140 --> 00:05:48.864
and this is for a specific organism,

00:05:48.864 --> 00:05:51.421
can be very, very complex and involved,

00:05:51.421 --> 00:05:54.513
and it's fascinating how
these things interact.

00:05:54.513 --> 00:05:57.937
Every time you're studying
biology and someone like me

00:05:57.937 --> 00:06:00.134
is going to give you these
nice clean narratives

00:06:00.134 --> 00:06:01.801
of how these enzymes

00:06:02.962 --> 00:06:05.051
interact with the
different macromolecules,

00:06:05.051 --> 00:06:06.642
like the DNA or the RNA,

00:06:06.642 --> 00:06:09.079
you should always
remember this is amazing.

00:06:09.079 --> 00:06:11.445
These are these molecules
interacting with each other,

00:06:11.445 --> 00:06:13.036
bouncing into each other.

00:06:13.036 --> 00:06:16.168
It's happening incredibly
fast inside of the cell.

00:06:16.168 --> 00:06:18.383
You should be in awe of this.

00:06:18.383 --> 00:06:21.300
It's happening in all of your cells

00:06:22.574 --> 00:06:24.479
or as we speak.

00:06:24.479 --> 00:06:27.480
This is pretty incredible stuff.

00:06:27.480 --> 00:06:29.684
So the next thing you have to think about,

00:06:29.684 --> 00:06:32.780
this right over here, we
are extending the RNA,

00:06:32.780 --> 00:06:35.161
well when does this thing actually stop?

00:06:35.161 --> 00:06:37.157
It stops once we...

00:06:37.157 --> 00:06:39.959
So this RNA polymerase
is going to keep going on

00:06:39.959 --> 00:06:44.690
and then this blue, we've
labeled this a terminator.

00:06:44.690 --> 00:06:46.043
So let me write.

00:06:46.043 --> 00:06:47.790
So this area

00:06:47.790 --> 00:06:49.680
is a terminator,

00:06:49.680 --> 00:06:51.772
and there's multiple ways that

00:06:51.772 --> 00:06:54.340
that signals to the RNA polymerase

00:06:54.340 --> 00:06:56.379
that "Hey, it's time to stop."

00:06:56.379 --> 00:07:00.073
More particularly, it somehow
creates something structurally

00:07:00.073 --> 00:07:02.963
that the polymerase just lets go.

00:07:02.963 --> 00:07:05.770
One mechanism, that's
depicted right over here

00:07:05.770 --> 00:07:08.270
is that the mRNA that's coded,

00:07:11.038 --> 00:07:14.065
this could happen in bacteria,

00:07:14.065 --> 00:07:17.656
is that the mRNA that's
coded forms a hairpin.

00:07:17.656 --> 00:07:21.191
So it has to have the right
complementary base pairs,

00:07:21.191 --> 00:07:22.870
base pairs right over here,

00:07:22.870 --> 00:07:23.993
to form this hairpin.

00:07:23.993 --> 00:07:26.559
This hairpin, along with the
things around the hairpin,

00:07:26.559 --> 00:07:30.726
essentially make it, impair the
polymerase to keep on going.

00:07:31.655 --> 00:07:34.079
So, the complex kind of
changes a little bit.

00:07:34.079 --> 00:07:35.539
So, it let's go,

00:07:35.539 --> 00:07:36.866
or at least that's how people believe it.

00:07:36.866 --> 00:07:40.010
There's other forms of how
the terminator can act.

00:07:40.010 --> 00:07:41.998
It might be sequences

00:07:41.998 --> 00:07:45.704
that parts of the
polymerase complex recognize

00:07:45.704 --> 00:07:48.807
and it makes a conformation change

00:07:48.807 --> 00:07:51.608
so that the RNA polymerase lets go.

00:07:51.608 --> 00:07:55.888
If we're talking about a
prokaryote, we're done.

00:07:55.888 --> 00:07:58.296
This would be our messenger RNA

00:07:58.296 --> 00:07:59.951
which then can go to a ribosome

00:07:59.951 --> 00:08:02.588
and then be translated into a protein.

00:08:02.588 --> 00:08:04.297
But if we're talking about a eukaryote,

00:08:04.297 --> 00:08:06.412
then we have to do a
little bit of processing.

00:08:06.412 --> 00:08:07.728
If we're talking about a eukaryote,

00:08:07.728 --> 00:08:09.697
if this is a prokaryote right over here,

00:08:09.697 --> 00:08:12.110
this would be our mRNA.

00:08:12.110 --> 00:08:16.528
If this is s eukaryote,
then this is our pre-mRNA,

00:08:16.528 --> 00:08:18.927
which now has to be processed.

00:08:18.927 --> 00:08:21.840
And you might say, "Well how
is that going to be processed?"

00:08:21.840 --> 00:08:24.110
Well, there's a couple of things
that are going to be done.

00:08:24.110 --> 00:08:26.167
Some things are going to
be added at the beginning

00:08:26.167 --> 00:08:28.837
and the end of the mRNA.

00:08:28.837 --> 00:08:32.780
The five prime cap, this
is a modified guanine,

00:08:32.780 --> 00:08:33.613
modified

00:08:36.431 --> 00:08:37.814
guanine

00:08:37.814 --> 00:08:38.647
right over here,

00:08:38.647 --> 00:08:41.605
which is going to help in
the translation process

00:08:41.605 --> 00:08:44.731
as the ribosomes attach onto it.

00:08:44.731 --> 00:08:46.521
And then you have this poly-A tail,

00:08:46.521 --> 00:08:47.928
and it's called a poly-A tail

00:08:47.928 --> 00:08:50.735
because it has a bunch
of adenines at the end,

00:08:50.735 --> 00:08:52.340
right over here.

00:08:52.340 --> 00:08:55.805
These not only help in
the translation process,

00:08:55.805 --> 00:08:59.248
it helps make sure that the
information is more robust,

00:08:59.248 --> 00:09:03.110
that the ends of the mRNA
don't in some way become,

00:09:03.110 --> 00:09:04.850
or makes it less likely

00:09:04.850 --> 00:09:06.724
that they're going to become damaged.

00:09:06.724 --> 00:09:08.571
Now the other thing that
needs to be processed,

00:09:08.571 --> 00:09:09.883
and this is one of
those fascinating things

00:09:09.883 --> 00:09:11.404
in evolutionary biology,

00:09:11.404 --> 00:09:14.987
is that we will have
in this mRNA sequence,

00:09:16.087 --> 00:09:17.672
you're going to have
parts of the sequence,

00:09:17.672 --> 00:09:20.886
which we currently consider
to be nonsense sequence.

00:09:20.886 --> 00:09:22.088
Nonsense sequences,

00:09:22.088 --> 00:09:24.251
and we call them introns.

00:09:24.251 --> 00:09:25.999
I'm gonna put it in quotes

00:09:25.999 --> 00:09:29.106
because in general in evolution

00:09:29.106 --> 00:09:31.886
it's seldom that things
have absolutely no purpose,

00:09:31.886 --> 00:09:35.224
but these are not coding for the protein

00:09:35.224 --> 00:09:38.329
that is going to be coded
by our initial gene.

00:09:38.329 --> 00:09:41.328
And so, these are actually processed out,

00:09:41.328 --> 00:09:42.617
they are spliced out.

00:09:42.617 --> 00:09:44.396
I'm not going to go into
the details of the actors

00:09:44.396 --> 00:09:45.990
that cause the splicing,

00:09:45.990 --> 00:09:48.924
but as part of this eukaryotic processing,

00:09:48.924 --> 00:09:52.318
you add the cap, you add the tail,

00:09:52.318 --> 00:09:54.870
and then you splice out the introns,

00:09:54.870 --> 00:09:56.618
and once you've spliced out the introns

00:09:56.618 --> 00:09:59.303
all you have left are the exons.

00:09:59.303 --> 00:10:01.087
So you have that.

00:10:01.087 --> 00:10:02.378
It's going to be connected to that.

00:10:02.378 --> 00:10:03.792
It's going to be connected to that.

00:10:03.792 --> 00:10:05.743
And so this is what you have resulted.

00:10:05.743 --> 00:10:09.524
This is in a eukaryote, you
will have this mature mRNA.

00:10:09.524 --> 00:10:13.066
And that's what we saw right over here

00:10:13.066 --> 00:10:14.229
that can then,

00:10:14.229 --> 00:10:16.485
let me underline that
in a color you can see,

00:10:16.485 --> 00:10:17.722
right over here,

00:10:17.722 --> 00:10:19.678
which then migrates out of the nucleus

00:10:19.678 --> 00:10:23.095
to a ribosome where it can be translated.


Ideal sources

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:02.813
- There's two kinds of ideal
sources we're gonna talk about.

00:00:02.813 --> 00:00:04.652
One is an ideal voltage source,

00:00:04.652 --> 00:00:07.768
and the other is an ideal current source.

00:00:07.768 --> 00:00:10.681
A ideal voltage source, the symbol is,

00:00:10.681 --> 00:00:12.264
looks like a circle

00:00:14.457 --> 00:00:15.290
like that.

00:00:17.240 --> 00:00:20.906
We put a voltage indication
right inside there

00:00:20.906 --> 00:00:23.262
and that's called V.

00:00:23.262 --> 00:00:25.540
And this is a constant voltage,

00:00:25.540 --> 00:00:29.123
what I've shown here
is a constant voltage.

00:00:35.091 --> 00:00:37.556
And that come from a power supply

00:00:37.556 --> 00:00:39.479
or it could come from a battery.

00:00:39.479 --> 00:00:40.564
When it comes from a battery,

00:00:40.564 --> 00:00:41.986
we have a special symbol for that,

00:00:41.986 --> 00:00:44.327
the battery symbol looks like this

00:00:44.327 --> 00:00:48.193
and the convention for
this, we also label it V.

00:00:48.193 --> 00:00:52.038
The convention for the
polarity of a battery symbol

00:00:52.038 --> 00:00:55.654
is the long line there
is the plus terminal

00:00:55.654 --> 00:00:58.224
and the short line right there,

00:00:58.224 --> 00:01:00.711
that guy is the minus terminal.

00:01:00.711 --> 00:01:03.093
So that's the convention for a battery.

00:01:03.093 --> 00:01:06.019
The other type of ideal source
is called a current source

00:01:06.019 --> 00:01:09.008
and it also has a symbol with a circle

00:01:09.008 --> 00:01:10.930
and this one we put an arrow

00:01:10.930 --> 00:01:13.919
and it goes in the
direction of the current.

00:01:13.919 --> 00:01:15.695
The current symbol is I.

00:01:15.695 --> 00:01:17.158
It can look like that

00:01:17.158 --> 00:01:19.143
or we could point the arrow
the other way, like that,

00:01:19.143 --> 00:01:22.173
depending on how the application goes.

00:01:22.173 --> 00:01:25.015
And that's an ideal current source.

00:01:25.015 --> 00:01:29.182
And those are the two
symbols for constant current.

00:01:38.087 --> 00:01:39.769
One of the things we can do,

00:01:39.769 --> 00:01:43.761
we can plot these, we can plot
theses two voltage sources,

00:01:43.761 --> 00:01:46.185
the voltage source and the current source.

00:01:46.185 --> 00:01:48.065
We can plot them on a curve

00:01:48.065 --> 00:01:51.368
that has coordinates
as voltage and current.

00:01:51.368 --> 00:01:53.520
So this would be called an IV curve

00:01:53.520 --> 00:01:55.401
or an IV plot that we're about to do here.

00:01:55.401 --> 00:01:57.365
For a constant voltage source,

00:01:57.365 --> 00:01:59.246
the voltage doesn't change.

00:01:59.246 --> 00:02:00.584
The current goes up and down,

00:02:00.584 --> 00:02:03.739
depending on what the rest
of the circuit demands,

00:02:03.739 --> 00:02:06.958
but the voltage is the same everywhere,

00:02:06.958 --> 00:02:09.298
so it plots something like this.

00:02:09.298 --> 00:02:13.298
That would be the IV plot
of a constant voltage,

00:02:14.655 --> 00:02:17.238
where V equals some constant V.

00:02:18.768 --> 00:02:21.900
And if we wanna plot our
constant current source

00:02:21.900 --> 00:02:25.243
on this kind of IV plot,
this would be something where

00:02:25.243 --> 00:02:27.020
the current is always the same,

00:02:27.020 --> 00:02:29.360
the current is the same,
independent of the voltage.

00:02:29.360 --> 00:02:30.969
And so a plot of that

00:02:30.969 --> 00:02:33.268
for a positive current
would look like this

00:02:33.268 --> 00:02:37.435
and we would say something
like I equals a constant I.

00:02:38.725 --> 00:02:41.397
So that's the IV plot of a current source

00:02:41.397 --> 00:02:44.397
and the IV plot of a voltage source.

00:02:45.956 --> 00:02:47.708
Alright, these are the two basic ways

00:02:47.708 --> 00:02:50.049
we deliver power or signals into circuits.

00:02:50.049 --> 00:02:52.285
Now we have a complete set of elements

00:02:52.285 --> 00:02:54.192
that we can build things with.


Polymerase chain reaction (PCR)

WEBVTT
Kind: captions
Language: en

00:00:00.415 --> 00:00:01.765
- [Voiceover] I'm here with Emily,

00:00:01.765 --> 00:00:03.248
our biology content fellow,

00:00:03.248 --> 00:00:05.099
to talk about PCR,

00:00:05.099 --> 00:00:07.410
or Polymerase Chain Reaction,

00:00:07.410 --> 00:00:08.682
which you've actually done a lot of.

00:00:08.682 --> 00:00:10.494
Why have you done PCR?

00:00:10.494 --> 00:00:12.925
- [Voiceover] PCR was kind of the mainstay

00:00:12.925 --> 00:00:15.343
of my graduate project,

00:00:15.343 --> 00:00:17.262
where I built all sorts

00:00:17.262 --> 00:00:19.686
of different recombinant DNA molecules,

00:00:19.686 --> 00:00:21.901
and used them to learn
things about plants.

00:00:21.901 --> 00:00:24.569
- [Voiceover] And so what
does PCR in particular do?

00:00:24.569 --> 00:00:27.025
- [Voiceover] PCR basically
makes you a lot of copies

00:00:27.025 --> 00:00:30.241
of a particular fragment of
DNA that you're interested in.

00:00:30.241 --> 00:00:32.609
- [Voiceover] And so how does that...

00:00:32.609 --> 00:00:34.066
Why would you need to make a lot of copies

00:00:34.066 --> 00:00:36.201
of a particular fragment of DNA?

00:00:36.201 --> 00:00:39.438
- [Voiceover] So you might want
to be making lots of copies

00:00:39.438 --> 00:00:41.725
so that you can clone it into a plasmid,

00:00:41.725 --> 00:00:43.469
and then do some other
experiments with it,

00:00:43.469 --> 00:00:44.795
that's a big use.

00:00:44.795 --> 00:00:46.044
- [Voiceover] So when
we talked about cloning,

00:00:46.044 --> 00:00:47.158
and we're talking about sticking

00:00:47.158 --> 00:00:49.099
a fragment of DNA inside of a plasmid,

00:00:49.099 --> 00:00:51.430
it's not like you're just
sticking one fragment

00:00:51.430 --> 00:00:52.692
into one plasmid,

00:00:52.692 --> 00:00:54.274
you're doing that with many,

00:00:54.274 --> 00:00:55.951
so you need a lot of fragments of DNA.

00:00:55.951 --> 00:00:58.082
- [Voiceover] Exactly. That is exactly it.

00:00:58.082 --> 00:00:59.390
- [Voiceover] And you might start

00:00:59.390 --> 00:01:01.733
with a very small sample of DNA.

00:01:01.733 --> 00:01:05.164
Where else would you have to do PCR?

00:01:05.164 --> 00:01:08.064
- [Voiceover] PCR is
used a lot in forensics,

00:01:08.064 --> 00:01:10.552
it's also used a lot
in medical diagnostics,

00:01:10.552 --> 00:01:12.704
so this could actually be your DNA

00:01:12.704 --> 00:01:15.913
that was being checked
to see if you have a gene

00:01:15.913 --> 00:01:18.330
that would predispose you
to a particular condition,

00:01:18.330 --> 00:01:20.754
all sorts of really
practical applications.

00:01:20.754 --> 00:01:22.627
- [Voiceover] Because
it's hard to identify

00:01:22.627 --> 00:01:25.541
just one fragment of that gene.

00:01:25.541 --> 00:01:27.555
So you want to make copies,

00:01:27.555 --> 00:01:29.349
or as they say amplify it,

00:01:29.349 --> 00:01:31.034
so that you could run it in gels and stuff

00:01:31.034 --> 00:01:32.556
and see how all of those molecules,

00:01:32.556 --> 00:01:34.291
how big they are or something like that.

00:01:34.291 --> 00:01:35.828
- [Voiceover] Exactly,
if you were just looking

00:01:35.828 --> 00:01:38.146
in your DNA pulled out of your cell,

00:01:38.146 --> 00:01:39.802
that would be a needle in a haystack.

00:01:39.802 --> 00:01:41.762
So this is how you can really zoom in

00:01:41.762 --> 00:01:43.641
and look at just the
thing you need to see.

00:01:43.641 --> 00:01:44.610
- [Voiceover] Okay.

00:01:44.610 --> 00:01:46.153
So, you've drawn some diagrams here,

00:01:46.153 --> 00:01:48.245
and I actually have never done PCR,

00:01:48.245 --> 00:01:49.139
but you have,

00:01:49.139 --> 00:01:51.452
so I'm going to tell you how
I understand it happening,

00:01:51.452 --> 00:01:53.993
and then you tell me if this makes sense.

00:01:53.993 --> 00:01:56.738
So, what you drew over here,

00:01:56.738 --> 00:01:58.997
this is double-stranded DNA,

00:01:58.997 --> 00:02:02.405
and this could have been from
a sample of someone's hair,

00:02:02.405 --> 00:02:04.452
or whatever else,

00:02:04.452 --> 00:02:07.318
and let's say we want to replicate

00:02:07.318 --> 00:02:10.839
or make many, many copies
of a fragment of this.

00:02:10.839 --> 00:02:13.226
So let's say the fragment
that we really care about

00:02:13.226 --> 00:02:16.731
is the fragment roughly from there,

00:02:16.731 --> 00:02:17.600
to...

00:02:17.600 --> 00:02:19.014
There.

00:02:19.014 --> 00:02:23.149
This part is what we want
to make multiple copies of.

00:02:23.149 --> 00:02:25.068
And so this first step,

00:02:25.068 --> 00:02:27.076
denaturation...

00:02:27.076 --> 00:02:29.941
I have trouble pronouncing things.

00:02:29.941 --> 00:02:30.774
- [Voiceover] It's a weird word.

00:02:30.774 --> 00:02:32.384
- [Voiceover] It's a weird word.

00:02:32.384 --> 00:02:33.813
You have 96 degrees Celsius,

00:02:33.813 --> 00:02:35.389
so this is almost at the boiling point.

00:02:35.389 --> 00:02:36.645
So it's quite hot,

00:02:36.645 --> 00:02:40.213
and that separates the two strands.

00:02:40.213 --> 00:02:41.506
- [Voiceover] Precisely.

00:02:41.506 --> 00:02:43.349
- [Voiceover] And so
once they're separated,

00:02:43.349 --> 00:02:45.425
then you can cool things down,

00:02:45.425 --> 00:02:47.429
although this still isn't that cool.

00:02:47.429 --> 00:02:50.189
55 degrees Celsius would
be very uncomfortable.

00:02:50.189 --> 00:02:52.453
But you would cool it down to this,

00:02:52.453 --> 00:02:54.711
and then these primers show up.

00:02:54.711 --> 00:02:56.901
And so one thing to remind ourselves is,

00:02:56.901 --> 00:02:59.645
this process is happening
inside of a test tube,

00:02:59.645 --> 00:03:01.395
or in a big solution,

00:03:02.517 --> 00:03:04.632
So you heat it up, the DNA,

00:03:04.632 --> 00:03:06.057
the two strands separate,

00:03:06.057 --> 00:03:09.664
and do you just have
this primer lying around?

00:03:09.664 --> 00:03:11.482
- [Voiceover] So the primer is something

00:03:11.482 --> 00:03:13.353
that you've ordered from a company

00:03:13.353 --> 00:03:14.808
and you've ordered a lot of it,

00:03:14.808 --> 00:03:17.760
so you put in a ton of
primer in your reaction,

00:03:17.760 --> 00:03:19.874
so that there's a really good chance

00:03:19.874 --> 00:03:22.894
that when you get to this
step here called annealing,

00:03:22.894 --> 00:03:24.856
that a primer is going to bind

00:03:24.856 --> 00:03:27.011
to many of your pieces of DNA.

00:03:27.011 --> 00:03:28.968
- [Voiceover] So if this is our solution,

00:03:28.968 --> 00:03:30.818
is this all happening in water?

00:03:30.818 --> 00:03:32.342
- [Voiceover] Water with some salts

00:03:32.342 --> 00:03:34.184
and stuff floating around, yeah.

00:03:34.184 --> 00:03:37.653
- [Voiceover] Okay. So we have
our solution right over here.

00:03:37.653 --> 00:03:41.408
You'd put whatever your
initial DNA sample is in there,

00:03:41.408 --> 00:03:43.792
and once again it's a very small amount,

00:03:43.792 --> 00:03:45.306
you'd put a lot of that primer,

00:03:45.306 --> 00:03:47.155
so you'd want to put
that in a lot of surplus,

00:03:47.155 --> 00:03:49.832
so let me do that in this magenta color.

00:03:49.832 --> 00:03:51.401
You obviously wouldn't
see it in real life,

00:03:51.401 --> 00:03:52.538
it would just all dissolve.

00:03:52.538 --> 00:03:55.256
- [Voiceover] It would just
look like a drop of liquid.

00:03:55.256 --> 00:03:56.195
- [Voiceover] It would look like...

00:03:56.195 --> 00:03:58.012
But for visualization,
you'd put a lot of primer,

00:03:58.012 --> 00:04:01.251
and so you heat it up,
the DNA strands separate,

00:04:01.251 --> 00:04:03.136
and then when you cool it back down,

00:04:03.136 --> 00:04:04.626
this primer is going to be specific

00:04:04.626 --> 00:04:07.554
to the ends of the region
that you want to copy.

00:04:07.554 --> 00:04:08.592
- [Voiceover] Exactly.

00:04:08.592 --> 00:04:10.840
- [Voiceover] And so when you order online

00:04:10.840 --> 00:04:12.776
or wherever that you
want a certain primer,

00:04:12.776 --> 00:04:15.024
you're going to pick the
sequence of that primer

00:04:15.024 --> 00:04:16.955
to be specific to the
regions you want to copy.

00:04:16.955 --> 00:04:18.843
- [Voiceover] Exactly.
That's super important.

00:04:18.843 --> 00:04:21.099
- [Voiceover] Okay. And so
when you cool it back down,

00:04:21.099 --> 00:04:22.842
the primer attaches,

00:04:22.842 --> 00:04:25.456
and then you heat it back up,

00:04:25.456 --> 00:04:27.667
not quite to the 96 degrees Celsius,

00:04:27.667 --> 00:04:29.384
but to the 72 degrees Celsius,

00:04:29.384 --> 00:04:31.435
where you extend those.

00:04:31.435 --> 00:04:32.738
And I'm assuming since it's called

00:04:32.738 --> 00:04:34.618
Polymerase Chain Reaction,

00:04:34.618 --> 00:04:36.819
that this is where the
polymerase is involved.

00:04:36.819 --> 00:04:39.042
- [Voiceover] That is exactly
where the polymerase comes in.

00:04:39.042 --> 00:04:41.140
- [Voiceover] So the polymerase

00:04:41.140 --> 00:04:44.194
is what is actually extending this.

00:04:44.194 --> 00:04:47.928
So I'll just draw a polymerase
enzyme right over there,

00:04:47.928 --> 00:04:49.594
doing the extending,

00:04:49.594 --> 00:04:52.122
and is it any type of polymerase enzyme?

00:04:52.122 --> 00:04:54.374
Can I just take the
polymerase from my cells

00:04:54.374 --> 00:04:56.074
and throw it in there?

00:04:56.074 --> 00:04:58.684
- [Voiceover] So you actually
need a special polymerase,

00:04:58.684 --> 00:05:00.074
because you need one that is going

00:05:00.074 --> 00:05:01.269
to be pretty heat-resistant,

00:05:01.269 --> 00:05:02.442
so as you were mentioning,

00:05:02.442 --> 00:05:04.170
even the cool step of this process

00:05:04.170 --> 00:05:05.650
is not something that your body

00:05:05.650 --> 00:05:07.658
would want to be hanging out in.

00:05:07.658 --> 00:05:09.654
So the polymerase is actually from

00:05:09.654 --> 00:05:12.402
a really heat-tolerant micro-organism.

00:05:12.402 --> 00:05:14.765
- [Voiceover] And what is that?

00:05:14.765 --> 00:05:16.244
It's called a taq polymerase?

00:05:16.244 --> 00:05:18.826
- [Voiceover] It's thermophilus
aquaticus, I think?

00:05:18.826 --> 00:05:20.164
Makes quite a mouthful.

00:05:20.164 --> 00:05:23.215
- [Voiceover] And they
found it at heated vents,

00:05:23.215 --> 00:05:26.586
this organism that is able to
stand these high temperatures.

00:05:26.586 --> 00:05:30.010
But that I guess leads
to another question,

00:05:30.010 --> 00:05:33.069
which is why do you have to
heat it up to begin with?

00:05:33.069 --> 00:05:35.778
I guess just to separate the two strands?

00:05:35.778 --> 00:05:37.578
- [Voiceover] That's
really the key reason.

00:05:37.578 --> 00:05:38.890
You just have to get them apart,

00:05:38.890 --> 00:05:40.244
you don't have an enzyme to do it

00:05:40.244 --> 00:05:41.578
the way you might in a cell,

00:05:41.578 --> 00:05:43.250
so heat does the trick.

00:05:43.250 --> 00:05:44.305
- [Voiceover] Okay, so I get it.

00:05:44.305 --> 00:05:45.564
So this is one step,

00:05:45.564 --> 00:05:49.658
I'm getting at least the
polymerase part of the PCR,

00:05:49.658 --> 00:05:51.463
where you heat it up,

00:05:51.463 --> 00:05:53.434
the strands separate,

00:05:53.434 --> 00:05:56.780
then you have all of
this extra primer there.

00:05:56.780 --> 00:05:59.002
Because there's so much primer,

00:05:59.002 --> 00:06:02.138
the primer is much more likely to bind to,

00:06:02.138 --> 00:06:04.019
at least at this part of the sequence,

00:06:04.019 --> 00:06:05.994
then for these two strands
to get back together

00:06:05.994 --> 00:06:07.178
at this point,

00:06:07.178 --> 00:06:09.306
and then you have the polymerase,

00:06:09.306 --> 00:06:11.083
the taq polymerase in particular,

00:06:11.083 --> 00:06:12.808
and you would have added
that at the beginning,

00:06:12.808 --> 00:06:14.044
the taq polymerase,

00:06:14.044 --> 00:06:17.267
I guess I'll put it in a yellow color.

00:06:17.267 --> 00:06:21.016
So you would also put all
that taq polymerase in there.

00:06:21.016 --> 00:06:23.731
And once again, these
things aren't robots,

00:06:23.731 --> 00:06:25.096
they don't know exactly
what they need to do,

00:06:25.096 --> 00:06:26.576
they just bump into
things in the right way

00:06:26.576 --> 00:06:27.944
and react in the right way,

00:06:27.944 --> 00:06:29.195
and then you would also have to add

00:06:29.195 --> 00:06:30.176
a bunch of nucleotides.

00:06:30.176 --> 00:06:31.266
- [Voiceover] Yes, absolutely.

00:06:31.266 --> 00:06:32.673
Your reaction is not going to work

00:06:32.673 --> 00:06:34.587
if you forget the nucleotides.

00:06:34.587 --> 00:06:36.573
- [Voiceover] So, the taq polymerase,

00:06:36.573 --> 00:06:38.722
when you heat it back up again

00:06:38.722 --> 00:06:40.728
after the primers have been attached,

00:06:40.728 --> 00:06:43.209
is going to start adding
all of these nucleotides.

00:06:43.209 --> 00:06:45.066
And what, do you just wait
a certain amount of time?

00:06:45.066 --> 00:06:47.093
Will it just keep going on forever?

00:06:47.093 --> 00:06:49.897
- [Voiceover] It'll keep
going on for a while,

00:06:49.897 --> 00:06:52.130
usually you do pick
the length of that step

00:06:52.130 --> 00:06:54.809
to match how much time
you expect the polymerase

00:06:54.809 --> 00:06:57.633
to need to complete your fragment.

00:06:57.633 --> 00:06:59.801
But it kind of will stop.

00:06:59.801 --> 00:07:01.945
Either it will fall off or it'll stop

00:07:01.945 --> 00:07:03.777
when you go on to the next step.

00:07:03.777 --> 00:07:06.039
- [Voiceover] Okay, so
this, I get this so far.

00:07:06.039 --> 00:07:08.755
So, so far we have after one cycle,

00:07:08.755 --> 00:07:10.665
what you've written here,

00:07:10.665 --> 00:07:13.264
after one cycle we would have doubled

00:07:13.264 --> 00:07:17.144
at least that part of the
sequence that we care about.

00:07:17.144 --> 00:07:20.929
Although we might have copied
even beyond that sequence.

00:07:20.929 --> 00:07:23.780
So, where does the chain
reaction come into this?

00:07:23.780 --> 00:07:26.371
- [Voiceover] So I guess you
can interpret chain reaction

00:07:26.371 --> 00:07:27.553
in two ways,

00:07:27.553 --> 00:07:30.217
and one is that's sort of
what the polymerase does,

00:07:30.217 --> 00:07:32.513
is you know, add things to make a chain,

00:07:32.513 --> 00:07:35.204
but there's actually even
more of a chain reaction

00:07:35.204 --> 00:07:36.432
to mention here,

00:07:36.432 --> 00:07:38.608
and that's that we're actually getting

00:07:38.608 --> 00:07:41.525
this kind of exponential process going on.

00:07:41.525 --> 00:07:43.200
- [Voiceover] So you do it one cycle,

00:07:43.200 --> 00:07:44.658
you get to this situation right here,

00:07:44.658 --> 00:07:46.353
you heat it up, the strands separate,

00:07:46.353 --> 00:07:48.553
you cool it down, the primers attach,

00:07:48.553 --> 00:07:52.625
you heat it up again, the
taq polymerase does its job,

00:07:52.625 --> 00:07:54.016
and like all polymerase,

00:07:54.016 --> 00:07:56.289
it goes from the 5 prime
to the 3 prime direction,

00:07:56.289 --> 00:07:58.066
we talked about that in the application.

00:07:58.066 --> 00:07:58.899
- [Voiceover] Precisely.

00:07:58.899 --> 00:08:00.277
- [Voiceover] So now you have two strands,

00:08:00.277 --> 00:08:01.834
but now, since all of that
stuff is in that solution,

00:08:01.834 --> 00:08:03.822
you can heat it up again,

00:08:03.822 --> 00:08:07.396
now these two strands can turn into...

00:08:07.396 --> 00:08:10.367
Or these two double strands can now

00:08:10.367 --> 00:08:12.715
turn into four single strands,

00:08:12.715 --> 00:08:14.949
then you can cool it down again.

00:08:14.949 --> 00:08:17.055
Now, they get primers attached to them,

00:08:17.055 --> 00:08:18.308
and they're still the same primer,

00:08:18.308 --> 00:08:20.836
because we still care
about the same sequence.

00:08:20.836 --> 00:08:24.157
And so now you go from one to two to four,

00:08:24.157 --> 00:08:25.980
and so you keep repeating this.

00:08:25.980 --> 00:08:27.276
And so how many times would it be typical

00:08:27.276 --> 00:08:29.044
for you to repeat this cycle?

00:08:29.044 --> 00:08:32.115
- [Voiceover] So like, 35 might
be a pretty typical number

00:08:32.115 --> 00:08:33.284
of cycles to do.

00:08:33.284 --> 00:08:34.562
It depends a little what you're doing.

00:08:34.562 --> 00:08:36.333
But you're going to do it a lot of times.

00:08:36.333 --> 00:08:38.804
- [Voiceover] And so if
you do this 35 times,

00:08:38.804 --> 00:08:40.547
I mean each time you're multiplying by 2.

00:08:40.547 --> 00:08:43.171
So it would be 2 to the 35th power,

00:08:43.171 --> 00:08:46.444
which is well over a billion times,

00:08:46.444 --> 00:08:48.267
so how long would that take?

00:08:48.267 --> 00:08:49.563
You've done this before.

00:08:49.563 --> 00:08:51.611
- [Voiceover] It depends on
the length of your fragment,

00:08:51.611 --> 00:08:53.629
but usually like two to three hours.

00:08:53.629 --> 00:08:54.883
- [Voiceover] So in two to three hours,

00:08:54.883 --> 00:08:56.434
you can start with one fragment,

00:08:56.434 --> 00:08:58.260
and get into the billions.

00:08:58.260 --> 00:09:00.493
- [Voiceover] If it's perfectly efficient,

00:09:00.493 --> 00:09:01.755
which I wish it always were,

00:09:01.755 --> 00:09:04.839
but you usually get
quite a few pieces made.

00:09:04.839 --> 00:09:06.782
- [Voiceover] And one thing
that I've always wondered

00:09:06.782 --> 00:09:08.710
when I first learned about this,

00:09:08.710 --> 00:09:11.191
and I'd like to go into a
lab and do this with you,

00:09:11.191 --> 00:09:14.862
is okay, I get that you have your primer,

00:09:14.862 --> 00:09:19.029
and then the polymerase is
just going to extend it,

00:09:20.029 --> 00:09:20.862
like that,

00:09:20.862 --> 00:09:22.852
but I was like well, it's going to be...

00:09:22.852 --> 00:09:24.375
How does it know where to stop?

00:09:24.375 --> 00:09:25.622
And you explained, well,

00:09:25.622 --> 00:09:28.831
on that first pass, it might
not know where to stop,

00:09:28.831 --> 00:09:31.375
but then when you start
going in the other direction,

00:09:31.375 --> 00:09:32.787
it's going to, so over here,

00:09:32.787 --> 00:09:34.104
when it goes in the other direction,

00:09:34.104 --> 00:09:35.640
it's going to hit a...

00:09:35.640 --> 00:09:38.167
It's not going to have
anything else to copy.

00:09:38.167 --> 00:09:39.527
- [Voiceover] Exactly.

00:09:39.527 --> 00:09:42.804
- [Voiceover] So most of
the billions of molecules

00:09:42.804 --> 00:09:43.852
that you produce,

00:09:43.852 --> 00:09:47.795
are going to be both ends
kind of a nice clean cut.

00:09:47.795 --> 00:09:51.023
- [Voiceover] The vast,
vast majority, exactly.

00:09:51.023 --> 00:09:52.871
- [Voiceover] Fascinating.