topic of this presentation is DNA replication
please watch our video structure od DNA berfore watching this video to understand DNA replication in a better way
first of all we will discuss about Enzymes which play important role in DNA replication
and catalyze this process
DNA polymerase is the main enzyme
It catalyses the polymerization of deoxynucleotides on the parental DNA template.
that's why this enzyme is called DNA-dependent DNA polymerase
as it uses  parental DNA template to synthesize new DNA strand
second enzyme is called Helicase
It unwinds the DNA strand to form the replication fork.
it breaks hydrogen bonds between base pairs to unwind double strand of DNA
third enzyme is DNA ligase
okazaki fragments are formed during this process
DNA ligase joins the Okazaki fragments
here we have used some terms which our students might not have heard about yet like replication fork, okazaki fragments and lagging strand
we will discuss all these terms in detail ahead in this presentation
we will now discuss some Rules which are followed during DNA Replication
first rule is that DNA replication is semi-conservative
we will discuss this point ahead in this presentation
second rule is - DNA replication does not start randomly form anywhere on DNA strand
DNA strand has some already defined points - unique points - from where replication starts
those points are called origin of replication
or in short we call them ‘ori
third important rule is that DNA polymerase  shows directional activity
it always catalyses polymerisation only in one direction (5’   to  3’)
knowing these rules ease the understanding of DNA replication
we will discuss semi-conservative replication with the help of this diagram
we have shown a parental DNA here in the diagram
double strand of parental DNA is shown
by now we have learnt that double strand unwinds during replication
after unwinding, a new strand will be polymerized on the old strand
it means that during replication when two DNA strands are forming from one strand
then in new DNA double strand one strand is parental while the other is the new one
such phenomenon is called semi-conservative synthesis
the synthesis in which old strand is conserved- preserved
and a new strand is formed
this phenomenon is called semi-conservative DNA replication
now we will learn what are Replication Bubbles
by now we have learnt that DNA replication does not starts randomly
there are some defined points on a DNA strand
which are called origin of replication
and one DNA strand can have more than one  origin of replication
replication always starts from these points
unwinding of DNA starts from origin of replication
we will see these bubble like structures after unwinding of DNA strand at these points of origin
these are called Replication Bubbles
when a new DNA is formed, these bubbles proceed in both the directions
as a result we will get two daughter DNA strands from one parental  DNA strand
so we have made our student understand the concept of replication bubble with the help of this diagram
we will learn about Replication Fork with the help of this diagram
by now we have understood what replication bubbles are
now visualize that this bubble is broken in the middle like into two parts like this
and now we have rotated one half of this bubble by 90 degree
so we get this kind of structure
this is the diagram you see in most the text book
and this is called Replication Fork
I just want to make my students visualize that what the replication fork is and it is formed
replication fork is never formed at the ends of a DNA strand
DNA replication starts somewhere from the interior points
we represent replication bubble in the form of fork just for the sake of convenience and simplicity
we have learnt about replication fork with this diagram
we have learnt enough concepts to understand the process of DNA replication
now we can easily understand the process of DNA replication
by now we have understood what the replication bubble is
what the replication fork is
parental DNA has underwent unwinding
whatever we have discussed by now, we will apply all those concepts in this diagram
semi-conservative DNA Replication is shown
as this is the parental strand on which new strand is formed
but I am observing some unique thing in the diagram
that one strand is synthesized in this direction whereas the other strand many small fragments are proceeding in the opposite direction
why is this happening
this is happening because -  I want to recall one rule that
that DNA replication always proceeds from 5' to 3' direction
we have to follow this rule
we also have to follow the rule of anti-parallel orientation
what is  anti-parallel orientation
if one strand runs fro 5' to 3' direction then the other strands runs from 3' to 5' direction
as you can see here that this is a parental strand which runs fro 3' to 5'
now, the new strand that will be formed on this parental strand will run from 5' to 3'
it means that we have follow two rules - anti-parallel orientation and 5' to 3' proceeding
on this strand the parental strand which runs from 3' to 5' direction
polymerization is occurring in usual manner on this strand
but the parental DNA which runs from 5' to 3' direction, we will see some unusual process here
now visualize that replication bubble is proceeding slowly in both the direction
and DNA polymerase which is working on this strand catalyze the process in opposite direction in order to follow the rules
that's why numerous small fragments are formed in the opposite direction on 5' to 3' parental strand
that's why these small fragments are formed in the opposite direction
whereas a continuous strand is synthesized on 3' to 5' parental strand
and this strand is called a leading strand
whereas the  small fragments which are formed in the opposite direction on 5' to 3' parental strand
these are called okazaki fragments
so continuous polymerization occurring on 3' to 5' strand, it is called continuous synthesis
whereas polymerization occurring in small fragments on 5' to 3' strand, it is called dis-continuous synthesis
now DNA ligase about which we have already discussed
DNA ligase joins these okazaki fragments
it means that DNA ligase plays no role in continuous synthesis
DNA ligase plays its role in discontinuous synthesis only
joining the okazaki fragments
these fragments are called lagging strands
we have also shown helicase in the diagram
Helicase keeps unwinding the DNA strand to form the replication fork during DNA replication
exposing the base pairs
some small bead like structures are also shown in the diagram
these are single strand stabilizing proteins
the exposed bases on the single strands of DNA have tendency to form pairs again as these these are complimentary base pairs
therefore these base have tendency to form hydrogen bonds again
DNA replication cannot proceed if this happens
these stabilizing proteins mask the expose bases on single strands of DNA
and prevent formation of hydrogen bonds between them
so we have discussed DNA replication
we have summarized differences between leading and lagging strands in this table
this table is very important for students who are preparing for board exams
