2021 GS Paper I, all questions

UPSC Prelims 2021 · GS Paper I

Question 70: Science and Technology

Consider the following statements :

  1. 1.Adenoviruses have single-stranded DNA genomes whereas retroviruses have double-stranded DNA genomes.
  2. 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?

a1 only
b2 only
cBoth 1 and 2
dNeither 1 nor 2

AnswerUPSC’s answer is (b).

Adenoviruses have double-stranded DNA genomes, and retroviruses such as HIV have single-stranded RNA genomes that are copied into DNA by reverse transcriptase, so statement 1 is wrong. Adenoviruses can cause common cold-like respiratory illness, and AIDS is caused by HIV, which is a retrovirus, so statement 2 is correct. Remember: retroviruses carry RNA and use reverse transcriptase.

  • 1. Adenoviruses have double-stranded DNA; retroviruses have RNA genomes.
  • 2. Adenoviruses sometimes cause common colds, and HIV, the cause of AIDS, is a retrovirus.

How you could have got this

  1. If you were sure statement 1 is not correct, (a) and (c) go: each of them includes 1.

    What you needed to know: Adenoviruses have double-stranded DNA; retroviruses have RNA genomes.

  2. Then, if you were sure statement 2 is correct, (d) goes: it leaves 2 out.

    What you needed to know: Adenoviruses sometimes cause common colds, and HIV, the cause of AIDS, is a retrovirus.

Only (b) is left. This one needs every statement.

Which options go is worked out from UPSC’s key and the options, not by a model.

Why each option is right or wrong
  • (a) Wrong: statement 1 is false.
  • (b) Right: only statement 2 is correct.
  • (c) Wrong: statement 1 is false.
  • (d) Wrong: statement 2 is correct.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper I, Series A. The paper on upsc.gov.in.