What are bacteriophages?

The correct answer is Option (2) → Viruses that infect bacteria

Viruses are named or grouped by the host they infect. Bacteriophages (“bacteria eaters”) are viruses that infect bacteria. A phage attaches to the bacterium and injects its genetic material; the cell then makes more virus particles. Hershey and Chase used such phages to prove that DNA is the genetic material.

Option (1) Viruses that infect plant cells, such as tobacco mosaic virus — Incorrect. Tobacco mosaic virus (TMV) infects plants, not bacteria. It is the book’s example of a virus in which RNA is the genetic material.

Option (2) Viruses that infect bacteria — Correct. Bacteriophages are viruses that infect bacteria. Because they multiply inside bacterial cells, they are also used as cloning vectors.

Option (3) Viruses that can transform normal animal cells into cancerous cells, such as retroviruses — Incorrect. Retroviruses in animals can transform normal cells into cancerous cells. They infect animal cells, so they are not bacteriophages.

Option (4) Viruses that infect the nose and respiratory passage, such as rhino viruses — Incorrect. Rhino viruses cause the common cold in humans. They infect the nose and respiratory passage, not bacteria.

NCERT Extract:
The Genetic Material is DNA – “The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952). They worked with viruses that infect bacteria called bacteriophages. The bacteriophage attaches to the bacteria and its genetic material then enters the bacterial cell. The bacterial cell treats the viral genetic material as if it was its own and subsequently manufactures more virus particles.”