In Hershey and Chase experiment, DNA was radiolabelled with:

The correct answer is Option (2) → P³²

Hershey and Chase wanted to find out whether DNA or protein enters the bacterium when a virus infects it. They grew viruses on two media, one with a radioactive form of phosphorus and one with a radioactive form of sulphur, so that either DNA or protein would carry a label.

(1) S³⁵ — Incorrect. Sulphur is present in some amino acids, so it is found in protein but not in DNA. Radioactive sulphur labelled the protein of the virus.

(2) P³² — Correct. DNA contains phosphorus in its phosphate groups, but protein does not. Viruses grown with radioactive phosphorus therefore had radioactive DNA but no radioactive protein.

(3) K³² — Incorrect. This is not an isotope used in the experiment. Potassium is not a part of DNA.

(4) N³⁵ — Incorrect. Nitrogen is found in both DNA and protein, so it could not tell the two apart. The isotopes used were of phosphorus and sulphur.

NCERT Extract:
The Genetic Material Is DNA – …