RNA makes DNA by the use of which enzyme?
ANSWER: (A) Reverse TranscriptaseREF: Lippincott s biochemistry 4th Ed Page 408, Harper 29th ed Table 39-3CLASSES OF PROTEINS INVOLVED IN REPLICATIONProteinFunction-DNA polymerasesDeoxynudeotide polymerizationHeli casesProcessive unwinding of DNATopoisomerasesRelieve torsional strain that results from helicase-induced unwindingDNA primaseInitiates synthesis of RNA primersSingle-strand binding proteinsPrevent premature reannealing of dsDNADNA ligaseSeals the single strand nick between the nascent chain and Okazaki fragments on lagging strand SOME OP THE ENZYMES USED IN RECOMBINANT DNA RESEARCHEnzymeReactionPrimary useAlkaline phosphataseDephosphorylates 5' ends of RNA and DNARemoval of 5 -PO4 groups prior to kinase labeling; also used to prevent seif-ligationBAL31 nudeaseDegrades both the 3' and 5' ends of DNAProgressive shortening of DNA moleculesDNA ligaseCatalyzes bonds between DNA moleculesJoining of DNA moleculesDNA polymerase ISynthesizes double-stranded DNA from single-stranded DNASynthesis of double-stranded cDNA; nick translation; generation of blunt ends from sticky endsDNase IUnder appropriate conditions, produces single-stranded nicks in DNANick translation; mapping of hypersensitive sites; mapping protein- DNA interactionsExonuclease IIIRemoves nucleotides from 3' ends of DNADNA sequencing; mapping of DNA- protein interactionsX ExonucleaseRemoves nucleotides from 5' ends of DNADNA sequencingPolynucleotide kinaseTransfers terminal phosphate (g--position) from ATP to 5'-OH groups of DNA or RNA32P end-labeling of DNA or RNAReverse transcriptaseSynthesizes DNA from RNA templateSynthesis of cDNA from mRNA; RNA (5' end) mapping studiesSI nucleaseDegrades single-stranded DNARemoval of "hairpin" in synthesis of cDNA; RNA mapping studies (both 5' and 3' ends)Terminal transferaseAdds nucleotides to the 3' ends of DNAHomopolymer tailingA reverse transcriptase is involved in the replication of retroviruses, such as human immunodeficiency virus (HIV). These viruses carry their genome in the form of ssRNA molecules. Following infection of a host cell, the viral enzyme, reverse transcriptase, uses the viral RNA as a template for the 5'- 3' synthesis of viral DMA, which then becomes integrated into host chromosomes. Reverse transcriptase activity is also seen with transposons, DNA elements that can move about the genome. In eukaryotes, such elements are transcribed to RNA, the RNA is used as a template for DNA synthesis by a reverse transcriptase encoded by the transposon, and the DNA is randomly inserted into the genome. Reverse transcriptases, then, are RNA-directed DNA polymerases.cDNA libraries: If a gene of interest is expressed at a very high level in a particular tissue, it is likely that the messenger RNA (mRNA) corresponding to that gene is also present at high concentrations in the cell. For example, reticulocyte mRNA is composed largely of molecules encoding the a-globin and b-globin chains of hemoglobin. This mRNA can be used as a template to make a complementary dsDNA (cDNA) molecule using the enzyme reverse transcriptase. The resulting cDNA is thus a double-stranded copy of mRNA. cDNA can be amplified by cloning or by the polymerase chain reaction. It can be used as a probe to locate the gene that coded for the original mRNA (or fragments of the gene) in mixtures containing many unrelated DNA fragments. If the mRNA used as a template is a mixture of many different size species, the resulting cDNA are heterogeneous. These mixtures can be cloned to form a cDNA library. Because cDNA has no interveningsequences, it can be cloned into an expression vector for the synthesis of eukaryotic proteins by bacteria . These special plasmids contain a bacterial promoter for transcription of the cDNA, and a Shine-Dalgamo sequence that allows the bacterial ribosome to initiate translation of the resulting mRNA molecule.
Reverse Transcriptase
DNA Polymerase Synthase
RNA Polymerase synthase
DNA Topoisomerase
DNA Replication 