Consider the following statements: 1. Adenoviruses have single-stranded DNA genomes whereas retroviruses have double-stranded DNA genomes. 2. Common cold is sometime caused by an adenovirus whereas AIDS is caused by a retrovirus. Which of the statements given above is/are correct?
Contents12
- A1 only
- B2 only
- CBoth 1 and 2
- DNeither 1 nor 2
Show answer
Answer: (B) 2 only
Statement 1 is wrong — it reverses the facts:
Adenoviruses have double-stranded DNA genomes (not single-stranded).
Retroviruses have RNA genomes (not double-stranded DNA).
Retroviruses carry RNA which gets reverse-transcribed into DNA inside the host cell — hence the name 'retro' (backward).
Statement 2 is correct:
Adenoviruses can cause:
- colds
- sore throats
- bronchitis
- pneumonia
- pink eye.
AIDS is caused by HIV, which is a retrovirus that stores genetic information as RNA and reverse-transcribes it into DNA to replicate.
Only
Statement 2 is correct.
Answer: (b).
COVID-19 vaccine development in 2020-21 brought viral vector vaccines like those using adenoviruses into global spotlight, making virus genome structures a current topic.
UPSC is testing whether students can distinguish between DNA viruses (adenoviruses) and RNA viruses (retroviruses like HIV) - a fundamental classification in virology.
The question deliberately reverses the genome types in statement 1 to trap students who memorize without understanding the underlying biology.
Adenoviruses: Structure & Diseases
Science And Technology Adenoviruses double-stranded DNA common cold
Adenoviruses: DNA Structure & Disease Spectrum
Adenoviruses have double-stranded DNA genomes (not single-stranded)
Cause common cold, sore throat, bronchitis, pneumonia, and pink eye
Named after adenoids where they were first isolated
Non-enveloped viruses with icosahedral capsid structure
Adenoviruses are DNA viruses that infect humans and animals, causing respiratory and eye infections. The key exam fact: they contain double-stranded DNA, not single-stranded DNA as UPSC often tests in reversed statements.
Adenovirus Characteristics
Feature | Details |
|---|---|
Genome Type | Double-stranded DNA (linear) |
Envelope | Non-enveloped |
Shape | Icosahedral capsid |
Size | 70-90 nanometers |
Host Range | Humans, animals, birds |
Diseases Caused
Respiratory: Common cold, sore throat, bronchitis, pneumonia
Ocular: Pink eye (conjunctivitis), epidemic keratoconjunctivitis
Gastrointestinal: Gastroenteritis in children
Severe infections: Can cause serious illness in immunocompromised patients
Trap: UPSC reverses genome types — Adenoviruses have double-stranded DNA, not single-stranded
Confusion: Don't mix with rhinoviruses (RNA) which also cause common cold
Retroviruses: HIV & AIDS
Science And Technology retroviruses AIDS double-stranded DNA
Retroviruses: RNA Genome & Reverse Transcription
Retroviruses have RNA genomes (not DNA)
HIV is a retrovirus that causes AIDS
Use reverse transcriptase to convert RNA → DNA
Named 'retro' because they reverse normal DNA→RNA flow
Retroviruses are unique RNA viruses that reverse the normal genetic flow. Instead of DNA→RNA→protein, they go RNA→DNA→RNA→protein using reverse transcriptase enzyme.
Retrovirus Replication Process
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`****RNA genome** enters host cell**
Retrovirus carries single-stranded RNA`"]
s2["`****Reverse transcriptase** converts RNA→DNA**
Unique enzyme synthesizes DNA from RNA template`"]
s3["`****DNA integrates** into host chromosome**
Viral DNA becomes part of host cell genome`"]
s4["`****Viral proteins** produced from integrated DNA**
Host cell machinery makes new virus particles`"]
s1 --> s2
s2 --> s3
s3 --> s4Major Human Retroviruses
Retrovirus | Disease | Target Cells | Key Feature |
|---|---|---|---|
HIV-1 | AIDS | CD4+ T cells | Most common globally |
HIV-2 | AIDS (milder) | CD4+ T cells | Mainly West Africa |
HTLV-I | T-cell leukemia | T lymphocytes | Endemic in Japan, Caribbean |
HTLV-II | Hairy cell leukemia | T lymphocytes | Rare, drug users |
Trap: Retroviruses have RNA genomes, not double-stranded DNA as stated in the question
Memory Aid: RETRO = RNA Enzyme Transcribes Reverse Order
Confusion: HIV integrates into host DNA but its own genome is RNA
DNA vs RNA Viruses
Science And Technology single-stranded DNA double-stranded DNA
Viral Genome Classification: DNA vs RNA Types
Viruses classified by nucleic acid type: DNA or RNA
DNA can be single-stranded or double-stranded
Reverse transcriptase unique to retroviruses
RNA viruses mostly single-stranded (except reoviruses)
Major Virus Types by Genome
Genome Type | Structure | Examples | Key Disease |
|---|---|---|---|
Double-stranded DNA | Linear/Circular | Adenovirus, Herpesvirus | Common cold, chickenpox |
Single-stranded DNA | Linear/Circular | Parvovirus | Fifth disease |
Single-stranded RNA | Linear (+/- sense) | Retrovirus, Influenza | AIDS, flu |
Double-stranded RNA | Segmented | Rotavirus | Gastroenteritis |
Replication Strategies
DNA viruses: Use host cell DNA polymerase for replication
RNA viruses: Carry own RNA-dependent RNA polymerase
Retroviruses: Unique reverse transcriptase converts RNA→DNA
Integration: Only retroviruses and some DNA viruses integrate into host genome
Classic Trap: UPSC swaps genome types between virus families
Memory: Adenovirus = Always Double-stranded DNA
Don't confuse: Virus genome type vs what it produces in host cell
Reverse Transcription Process
Science And Technology
Reverse Transcription: Breaking the Central Dogma
Reverse transcriptase synthesizes DNA from RNA template
Reverses normal DNA→RNA flow to RNA→DNA
Essential for retrovirus replication and PCR technology
Won Nobel Prize for its discoverers in 1975
Reverse transcription violates the central dogma of molecular biology. Normal flow is DNA→RNA→Protein, but retroviruses reverse this using reverse transcriptase enzyme to go RNA→DNA.
Reverse Transcription Steps
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`****RNA template** binding**
Reverse transcriptase binds to viral RNA`"]
s2["`****DNA synthesis** begins**
Enzyme synthesizes complementary DNA strand`"]
s3["`****RNA degradation****
Original RNA template is degraded`"]
s4["`****Second DNA strand** synthesis**
Double-stranded DNA molecule completed`"]
s5["`****Integration** into host genome**
Viral DNA inserts into host chromosome`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5Applications of Reverse Transcription
Application | Purpose | Field |
|---|---|---|
RT-PCR | Amplify RNA sequences | Medical diagnostics |
cDNA libraries | Clone genes from mRNA | Genetic engineering |
HIV testing | Detect viral RNA | Disease diagnosis |
Gene therapy | Deliver genes using viral vectors | Medical treatment |
Key Point: Reverse transcriptase is error-prone → high HIV mutation rate
Don't confuse: RT-PCR (detects RNA) vs regular PCR (detects DNA)