UPSC Prelims 2021 Q70: Adenoviruses vs Retroviruses — Explained
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?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Answer: (b) 2 only
Statement-wise explanation
- 1: incorrect — The statement has the two genome types the wrong way round, and both halves are false. Adenoviruses have no envelope. Their genome is a single linear double-stranded DNA molecule of about 34–36 kilobase pairs. Retroviruses carry two copies of a single-stranded RNA genome (about 9.2 kb in HIV-1). Double-stranded DNA does appear in a retrovirus's life cycle, but only after reverse transcriptase copies the RNA into DNA. That DNA is a copy made inside the host cell, not the genome packed in the virus particle.
- 2: correct — Adenoviruses cause respiratory illness that ranges from a mild common cold to severe disease in people with weak immunity. Rhinoviruses cause most colds, so the word 'sometime' is accurate. AIDS is the late stage of infection by HIV. HIV is a lentivirus in the retrovirus family: it uses reverse transcriptase to turn its RNA into DNA, and integrase to insert that DNA into the host genome.
How to eliminate
You can solve this from NCERT alone. Class 12 Biology (Human Health and Disease) says HIV is a retrovirus with an RNA genome, and that it uses reverse transcriptase to make viral DNA. The prefix 'retro-' points the same way. The virus runs transcription backwards, from RNA to DNA, so its genome must be RNA. Statement 1 joins two claims with 'whereas', and such a statement is wrong if either half is wrong. Its retrovirus half is plainly false, so Statement 1 fails even if you know nothing about adenovirus DNA. That rules out every option that includes Statement 1. Statement 2 rests on two widely known facts. AIDS is caused by HIV, a retrovirus. Adenoviruses are cold viruses, and in 2021 Covishield was widely described as built on a 'weakened common cold virus from chimpanzees'. The softening word 'sometime' also makes the claim safe. Fact to remember: adenovirus = double-stranded DNA; retrovirus = single-stranded RNA plus reverse transcriptase.
Know more
Viruses are grouped by the nucleic acid they carry and how they copy it (the Baltimore classification). Adenoviruses and retroviruses sit at opposite ends.
| Feature | Adenovirus | Retrovirus (e.g. HIV) |
|---|---|---|
| Genome | Linear double-stranded DNA, ~34–36 kbp | Two copies of single-stranded (+) RNA, ~9.2 kb in HIV-1 |
| Baltimore group | I (dsDNA) | VI (ssRNA-RT) |
| Envelope | None; icosahedral capsid | Lipid envelope |
| Key enzymes | Uses host-cell machinery in the nucleus | Reverse transcriptase (RNA → DNA), integrase (inserts DNA into host genome) |
| Integration into host DNA | Normally does not integrate | Integrates as a 'provirus', which makes it hard to cure |
| Human diseases | Colds, sore throat, conjunctivitis, pneumonia, gastroenteritis | AIDS (HIV-1, HIV-2), some leukaemias (HTLV-1) |
| Use as a vector | COVID-19 vaccines: Covishield (chimpanzee adenovirus ChAdOx1), Sputnik V (human Ad26 for dose 1, Ad5 for dose 2) | Lentiviral vectors in gene therapy and CAR-T cell therapy |
Why reverse transcriptase matters: it breaks the usual flow of genetic information (DNA → RNA → protein). It is also an important drug target. Many HIV drugs, such as zidovudine and tenofovir, block reverse transcriptase.
The exception that causes confusion: Hepatitis B virus has a DNA genome but still copies itself through an RNA step using reverse transcriptase (Baltimore group VII). So 'uses reverse transcriptase' does not always mean 'RNA genome'. For retroviruses proper, the genome is always RNA.
Vector logic: adenoviruses can carry a large piece of foreign DNA and usually do not integrate it into host DNA, so they suit one-time vaccines. Retroviral and lentiviral vectors do integrate, so they suit lasting gene correction.
In the news
In 2021 India approved adenovirus-vector COVID-19 vaccines for restricted emergency use: Covishield in January, built on a chimpanzee adenovirus, and Sputnik V in April, built on human Ad26 and Ad5. June 2021 was also 40 years since the first AIDS cases were reported in 1981. Both events put adenoviruses and retroviruses in the news before the Prelims.