UPSC Prelims 2025 Q49: Viruses in Oceans, Phages & Host Cells

Consider the following statements:

I. No virus can survive in ocean waters.

II. No virus can infect bacteria.

III. No virus can change the cellular transcriptional activity in host cells.

How many of the statements given above are correct?

  1. Only one
  2. Only two
  3. All the three
  4. None

Answer: (d) None

Statement-wise explanation

  • I: incorrect — The ocean holds more viruses than anything else that lives there. One millilitre of coastal surface seawater can hold up to about 10 million virus particles, and the total in the oceans is estimated at about 10^30. Marine viruses kill an estimated 20% of microbial biomass every day, so they clearly survive and stay active in seawater.
  • II: incorrect — Bacteriophages ('phages') are viruses that infect bacteria. They were discovered by Frederick Twort (1915) and Félix d'Herelle (1917). The NCERT Class 12 Hershey–Chase experiment used a bacteriophage infecting E. coli to show that DNA is the genetic material. Phage therapy uses such viruses to kill drug-resistant bacteria.
  • III: incorrect — Many viruses take over the host's gene-expression machinery. Soon after phage T4 infects E. coli, it chemically modifies the host RNA polymerase (ADP-ribosylation by the Alt protein) and makes proteins such as AsiA. Together these shut down host promoters and redirect transcription to phage genes. Animal viruses do the same. HIV's Tat protein drives transcription, and the E6/E7 proteins of HPV, a cancer-causing virus, change host cell gene expression.

How to eliminate

All three statements start with 'No virus can'. That is an absolute claim, and one counter-example is enough to make it false. Statement II falls on NCERT knowledge alone: bacteriophages are by definition viruses that infect bacteria (Hershey–Chase experiment, Class 12 Biology). That removes 'All the three'. Statement III also falls on basic reasoning. A virus has no transcription machinery of its own, so it must use and redirect the host's. Viruses that cause cancer, such as HPV, work by changing host gene expression. Statement I is the one fewest people know, but the oceans are full of bacteria and plankton, which are hosts for phages, so viruses must be there too. Once you see that every statement is a sweeping 'no virus' claim that one known example breaks, 'None' follows. Fact to remember: viruses are the most abundant biological entities in the sea.

Know more

Viruses: obligate parasites of every kind of life

A virus is a piece of nucleic acid (DNA or RNA) inside a protein coat. It has no metabolism and no ribosomes of its own, so it can multiply only inside a living host cell. Every group of organisms has its own viruses.

Host group Example virus Note
Bacteria Bacteriophage T4, lambda, T2 T2 was used in the Hershey–Chase experiment
Cyanobacteria / algae Cyanophages, coccolithovirus Control plankton blooms
Plants Tobacco mosaic virus (RNA) First virus identified (Ivanowsky 1892; Beijerinck named it 'contagium vivum fluidum')
Animals / humans HIV, HPV, influenza, Nipah, Ebola Many are zoonotic

Two life cycles of phages

  • Lytic: the virus takes over the cell, makes copies and bursts the cell (lysis).
  • Lysogenic: the viral DNA joins the host chromosome as a 'prophage' and is passed on quietly until it is triggered.

Hijacking the host Viruses redirect host transcription to their own genes. T4 modifies E. coli RNA polymerase so that it prefers phage promoters. Retroviruses such as HIV insert their DNA into the host genome. Oncoviruses (HPV, hepatitis B) switch on host growth genes.

Viruses in the sea Marine viruses, mostly phages, number up to about 10 million per millilitre in coastal surface water. They kill about a fifth of microbial biomass daily. This releases nutrients back into the water (the 'viral shunt'), which shapes carbon cycling and plankton blooms.

Why it matters now Phage therapy is being studied as a way around antimicrobial resistance (AMR), and scientists are now using AI to design phage genomes.

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