Questions: Reverse Transcription and Retroviral Replication

Question 1

HIV reverse transcriptase makes roughly one error per 10,000 nucleotides copied, while human DNA polymerase makes roughly one error per 10⁹ nucleotides. What is the primary biochemical reason for this difference, and what is its clinical consequence?

• A. Reverse transcriptase copies RNA rather than DNA, and RNA templates are chemically less stable, causing more copying errors. The consequence is faster viral evolution.
• B. Reverse transcriptase lacks proofreading (3'→5' exonuclease) activity, so errors are not corrected during synthesis. The consequence is that nearly every new HIV genome copy carries at least one mutation, generating rapid diversity including drug-resistant and immune-evasive variants.
• C. Reverse transcriptase works at low pH in the acidic endosomal environment, which increases error rates. The consequence is that some mutations are beneficial and help the virus survive.
• D. Reverse transcriptase must copy both strands simultaneously, increasing errors. The consequence is that viral diversity is reduced because defective copies are more common.

Answer: (B)

The key mechanistic reason is the absence of 3'→5' proofreading exonuclease activity. Human DNA polymerase detects and excises misincorporated nucleotides as it goes; reverse transcriptase cannot. For a ~10 kb HIV genome, ~1 error per 10,000 nt means nearly every viral copy has at least one mutation. Most mutations are deleterious, but the sheer number of variants generated means drug-resistant and immune-evasive mutants arise rapidly — this is why HIV evolves resistance to single antiretroviral drugs within weeks. The template being RNA (vs DNA) contributes less to the error rate than the absence of proofreading.

Question 2

During reverse transcription, what does the RNase H activity of reverse transcriptase do, and why is it essential for the process?

• A. It repairs misincorporated nucleotides in the newly synthesized cDNA strand, improving fidelity.
• B. It degrades the viral RNA template strand as cDNA is being synthesized, allowing the second DNA strand to be copied from the cDNA.
• C. It processes the tRNA primer used to initiate synthesis, preparing it for reuse in subsequent rounds of replication.
• D. It cleaves the integrated provirus from the host chromosome when the virus needs to reactivate.

Answer: (B)

RNase H (H = hybrid) degrades the RNA strand of an RNA:DNA hybrid. As reverse transcriptase synthesizes the cDNA copy of the viral RNA genome, RNase H simultaneously degrades the RNA template behind it — essentially erasing the original RNA genome as DNA is written. This degradation is essential because the second DNA strand must be synthesized using the cDNA as a template: the RNA template must be removed before the complementary DNA strand can be made. Without RNase H, the virus would be stuck with a single-stranded cDNA that cannot be integrated into the host genome as a double-stranded provirus.

Question 3

Once integrated into the host cell's chromosomal DNA, the HIV provirus can persist for the lifetime of the cell and be inherited by all daughter cells after division.

• T. True
• F. False

Answer: True

Integration is what makes retroviruses so clinically challenging to cure. The integrase enzyme inserts the proviral DNA (double-stranded DNA produced by reverse transcription) into the host chromosome, where it becomes a permanent part of the cell's genome. When the infected cell divides, both daughter cells inherit the provirus. In latently infected long-lived cells (such as resting memory CD4+ T cells), the provirus can remain transcriptionally silent for decades, evading immune detection and antiretroviral drugs that only target actively replicating virus. This reservoir of latently infected cells is the primary barrier to HIV cure.

Question 4

Reverse transcriptase violates the central dogma by allowing information to flow from protein sequence back into nucleic acid sequence.

• T. True
• F. False

Answer: False

Reverse transcriptase violates the classical version of the central dogma in a specific and limited sense: it allows information flow from RNA to DNA, which Crick's original 1958 statement did not anticipate. The 'violation' is RNA → DNA, not protein → nucleic acid. The flow protein → nucleic acid would be a much more radical violation — and is not what reverse transcriptase does. In his 1970 revision, Crick explicitly acknowledged that RNA → DNA was possible (discovered by Temin and Baltimore) while continuing to exclude protein → nucleic acid. Reverse transcription expands the information flow diagram, not inverts it.

Question 5

Why does the error-prone nature of reverse transcriptase make combination antiretroviral therapy (rather than a single drug) necessary for HIV treatment?

Model answer: With ~1 error per 10,000 nucleotides and ~10,000 new virions produced per day in an untreated patient, the viral population generates enormous genetic diversity. For any single antiretroviral drug, a resistant mutant — one or two substitutions away from the wild type — is almost certainly already present in the viral population before treatment begins. A single drug eliminates drug-sensitive viruses but leaves resistant variants to proliferate, producing rapid treatment failure. Combination therapy (typically three or more drugs targeting different viral processes) requires the virus to simultaneously acquire multiple independent resistance mutations in the same genome, which is far less likely to occur by chance. The probability of resistance to all three drugs simultaneously is the product of the individual probabilities — essentially negligible.

This is the direct clinical application of understanding RT's error rate. The rapid generation of viral diversity means evolutionary space is densely sampled each replication cycle. Any single target can be 'escaped' by a single mutation; requiring simultaneous escape from three targets creates an evolutionary bottleneck that the error-prone RT cannot overcome fast enough. This principle — high mutation rate → pre-existing diversity → single-drug resistance → need for combination therapy — applies broadly to other rapidly evolving pathogens.