Question 52:
The enzyme reverse transcriptase:
The correct answer is (B) Makes a copy of DNA from an RNA molecule.
Reverse transcriptase (RT) is an RNA-dependent DNA polymerase that synthesizes complementary DNA (cDNA) from an RNA template, a process central to retroviral replication and molecular techniques like RT-PCR.
Option Breakdown
(A) Joins RNA-RNA fragments
This describes RNA ligase, which seals RNA nicks, not RT’s DNA synthesis role.
(B) Makes a copy of DNA from an RNA molecule
RT catalyzes reverse transcription using RNA as template, adding dNTPs to a primer’s 3′ end, often with RNase H activity to degrade RNA in RNA:DNA hybrids.
(C) Removes terminal phosphates from 5’ end
This is phosphatase activity (e.g., alkaline phosphatase), unrelated to RT’s polymerization.
(D) Cleaves RNA at random positions
RNase A performs random RNA cleavage; RT’s RNase H is specific to RNA in RNA:DNA hybrids.
Introduction to Reverse Transcriptase DNA from RNA
Reverse transcriptase DNA from RNA defines the enzyme’s core action in retroviruses and biotech, converting viral RNA genomes to DNA for integration. Key for GATE Life Sciences exam questions on molecular enzymology.
Mechanism of Reverse Transcription
RT binds primer-annealed RNA, extends 3′ end with dNTPs via RNA-dependent polymerase activity, then RNase H clears RNA for second-strand DNA synthesis. This yields double-stranded cDNA for PCR or cloning.
Discovered in 1970, it’s vital in HIV lifecycle and gene expression studies.
Enzyme Functions Comparison Table
| Option | Function Description | Matches RT? | Biological Role |
|---|---|---|---|
| Joins RNA-RNA | Seals RNA fragments | No | RNA processing |
| DNA from RNA | Synthesizes cDNA | Yes | Retroviral replication |
| Removes 5′ Phosphates | Dephosphorylates ends | No | Ligation prep |
| Cleaves RNA Randomly | Nonspecific RNA hydrolysis | No | RNA degradation |
This table highlights why reverse transcriptase DNA from RNA is the precise match.
Relevance for Life Sciences Students
Links to prior E. coli DNA pol I query on replication enzymes; mastering RT boosts genetics and virology scores in competitive exams.
