Q.102 Retroviruses, like the influenza virus, escape the detection of pre-existing antibodies in the host by
generating surface antigen variants. They do so:
(A) By editing the surface antigen post-translationally
(B) Because RNA polymerase of these viruses display high mutation rate during RNA synthesis
(C) Because the surface antigens are attacked by the proteases present in the host cell
(D) Because DNA polymerase of the host mutates the viral genome in the infected cell

Retroviruses generate surface antigen variants through high mutation rates during reverse transcription by viral RNA-dependent DNA polymerase (reverse transcriptase), not RNA polymerase.
However, the question contains a factual error: influenza is an RNA virus (orthomyxovirus) using RNA polymerase, while retroviruses use reverse transcriptase. The correct mechanism for both is still high polymerase error rates. Answer: (B).

Antigen Variation Mechanism

Retroviruses (HIV, HTLV) and influenza escape antibodies via antigenic drift—point mutations in surface glycoproteins (gp120 for HIV, HA/NA for influenza). Reverse transcriptase (retroviruses) lacks 3’→5′ exonuclease proofreading (error rate ~10⁻⁴/base), generating quasispecies diversity. Influenza RNA polymerase similarly error-prone during replication.

Option Breakdown

• (A) Post-translational editing: Wrong—no evidence Abs/glycoproteins chemically modified post-translation; variation genomic.​

• (B) High mutation rate polymerase: Correct—RT (retroviruses)/RNA pol (influenza) error-prone; ~1 mutation/genome/replication cycle enables immune escape.​

• (C) Host proteases attack antigens: Wrong—proteases degrade, don’t create functional variants; cleaved proteins lose antigenicity.​

• (D) Host DNA polymerase mutates: Wrong—host DNA pol high-fidelity (proofreading); viral genome replicated by viral enzymes before integration.

Introduction to Retroviruses Surface Antigen Variants

Retroviruses surface antigen variants generation via error-prone reverse transcription enables immune evasion—key GATE Life Sciences virology. Influenza/retroviruses mutate HA/gp120 faster than host Ab response, despite question mixing RNA vs retroviral polymerases.​

Mutation Mechanics

• Retroviruses: RT copies RNA→DNA (10⁻⁴ errors/nt); HIV env hypervariable regions evolve ~10⁻³/substitution/site/year.

• Influenza: RNA pol lacks proofreading; seasonal drift requires vaccines.

• Outcome: Neutralizing Abs obsolete; new variants infect.​

Why Other Options Fail

GATE Clarification

Retroviruses surface antigen variants PYQs test antigenic drift universally. Note: influenza ≠ retrovirus (RT vs RNA pol), but mechanism identical: polymerase infidelity. Answer: (B). Master quasispecies concept.