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Biology

Class 12 · Science & Environment

Ch 8 · Microbes in Human Welfare

Real UPSC Prelims questions from 2016–2026, linked to this book. Try each one first, then open the answer. 2026 answers follow UPSC's provisional answer key.

Prelims 2016 · Q59

Which of the following statements is/are correct?

Viruses can infect

  1. 1.bacteria
  2. 2.fungi
  3. 3.plants

Select the correct answer using the code given below.

  1. (a)1 and 2 only
  2. (b)3 only
  3. (c)1 and 3 only
  4. (d)1, 2 and 3
Show answer

UPSC's answer: (d)

NCERT answers it: Class 12 Biology, Ch 5 (Molecular Basis of Inheritance); Class 12 Biology, Ch 8 (Microbes in Human Welfare).

The one fact NCERT doesn't give: Some viruses, called mycoviruses, infect fungi.

Class 12 Biology: bacteriophages are viruses that infect bacteria (ch 5), and ch 8 names plant viruses. With the fact, all three are true. Answer (d).

Hindi version: UPSC's official question paper (2016), question 59 ↗

Prelims 2017 · Q59

Biological Oxygen Demand (BOD) is a standard criterion for

  1. (a)Measuring oxygen levels in blood
  2. (b)Computing oxygen levels in forest ecosystems
  3. (c)Pollution assay in aquatic ecosystems
  4. (d)Assessing oxygen levels in high altitude regions
Show answer

UPSC's answer: (c)

NCERT answers it: Class 12 Biology, Ch 8 (Microbes in Human Welfare).

Class 12 Biology, ch 8: BOD is the oxygen that bacteria would use to break down all the organic matter in a litre of water. So it measures organic matter: the higher the BOD of waste water, the more it pollutes. Answer (c).

Hindi version: UPSC's official question paper (2017), question 59 ↗

Prelims 2021 · Q69

Consider the following:

  1. 1.Bacteria
  2. 2.Fungi
  3. 3.Virus

Which of the above can be cultured in artificial/synthetic medium?

  1. (a)1 and 2 only
  2. (b)2 and 3 only
  3. (c)1 and 3 only
  4. (d)1, 2 and 3
Show answer

UPSC's answer: (a)

NCERT answers it: Class 8 Curiosity, Ch 2 (The Invisible Living World: Beyond Our Naked Eye); Class 12 Biology, Ch 8 (Microbes in Human Welfare).

Class 12 Biology, ch 8 shows colonies of bacteria and of fungi growing in petri dishes. Class 8 Curiosity, ch 2: viruses multiply only after they enter a living cell, so they can't grow on an artificial medium. Answer (a).

Hindi version: UPSC's official question paper (2021), question 69 ↗

Prelims 2021 · Q70

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?

  1. (a)1 only
  2. (b)2 only
  3. (c)Both 1 and 2
  4. (d)Neither 1 nor 2
Show answer

UPSC's answer: (b)

NCERT answers it: Class 12 Biology, Ch 8 (Microbes in Human Welfare); Class 12 Biology, Ch 7 (Human Health and Disease).

Class 12 Biology: retroviruses have an RNA genome (ch 7), so 1 is false. Adenovirus causes respiratory infections (ch 8), and HIV, which causes AIDS, is a retrovirus (ch 7), so 2 is true. Answer (b).

Beyond NCERT: adenoviruses in fact have double-stranded DNA genomes, while retroviruses carry RNA. So statement 1 gets both viruses wrong. Source: Gu et al., 'Molecular characterization and evolutionary analysis of human adenovirus type 55', Virus Genes (2025), on PubMed Central (US National Library of Medicine), PMC12263808

Hindi version: UPSC's official question paper (2021), question 70 ↗

Prelims 2022 · Q38

Consider the following statements in respect of probiotics:

  1. 1.Probiotics are made of both bacteria and yeast.
  2. 2.The organisms in probiotics are found in foods we ingest but they do not naturally occur in our gut.
  3. 3.Probiotics help in the digestion of milk sugars.

Which of the statements given above is/are correct?

  1. (a)1 only
  2. (b)2 only
  3. (c)1 and 3
  4. (d)2 and 3
Show answer

UPSC's answer: (c)

NCERT answers it: Class 12 Biology, Ch 8 (Microbes in Human Welfare).

The one fact NCERT doesn't give: Probiotic bacteria help break down lactose, the sugar in milk.

Class 12 Biology, ch 8: lactic acid bacteria live in our stomach and check disease-causing microbes, so 2 is false. Both remaining options have 1; with the fact, 3 is true. Answer (c).

Beyond NCERT: probiotics include bacteria such as Lactobacillus and Bifidobacterium and yeasts such as Saccharomyces boulardii, so statement 1 is true as well. Source: NIH NCCIH, 'Probiotics: What You Need To Know' (nccih.nih.gov)

Hindi version: UPSC's official question paper (2022), question 38 ↗

Prelims 2023 · Q18

Consider the following statements:

  1. 1.Some microorganisms can grow in environments with temperature above the boiling point of water.
  2. 2.Some microorganisms can grow in environments with temperature below the freezing point of water.
  3. 3.Some microorganisms can grow in highly acidic environment with a pH below 3.

How many of the above statements are correct?

  1. (a)Only one
  2. (b)Only two
  3. (c)All three
  4. (d)None
Show answer

UPSC's answer: (c)

NCERT answers it: Class 12 Biology, Ch 8 (Microbes in Human Welfare).

The one fact NCERT doesn't give: Some microbes grow even above 100 °C: in the laboratory, a methanogen (Methanopyrus kandleri strain 116) multiplied at up to 122 °C under high pressure.

Class 12 Biology, ch 8: microbes live deep inside geysers (thermal vents) where it may be as hot as 100 °C, under snow several metres thick (2), and in highly acidic places (3). NCERT stops at 100 °C, the boiling point; with the fact, statement 1 is true too. Answer (c).

Hindi version: UPSC's official question paper (2023), question 18 ↗

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