Question

Bacteriophage with radioactivity both in DNA and protein infects a bacterium. Bacterium becomes radioactive. Radioactivity occurs in

• A. DNA
• B. Both A and B
• C. All parts
• D. Protein

Answer 1

The correct option is A DNA

Part 1: Radiolabelling the bacteriophage
The DNA and protein of the bacteriophage is radiolabeled by culturing it in a medium containing radioactive phosphorus ${(}^{32}P)$ and radioactive sulphur ${(}^{35}S)$. The ${(}^{32}P)$ gets incorporated in the DNA as DNA has a sugar phosphate backbone. DNA does not possess sulphur and hence does not contain radioactive sulphur.
The 35S gets incorporated in the sulphur containing amino acids of the protein coat.

Part 2: Infection by bacteriophage
Like every other virus, bacteriophages are composed of a protein coat and a genetic material (DNA or RNA) molecule enclosed within the coat.
Bacteriophages attach themselves to specific host bacteria and insert their genetic material into the bacteria.
The genetic material takes control over the host machinery and directs the host cell to utilise its resources for creating new viral particles.
The protein coat does not enter the infected bacteria and is left behind.
Thus, during infection only radioactive phosphorus in the DNA enters the bacterial cells whereas the radioactive sulphur in the protein coat is left outside the cells.

Part 3: Protein
Infection of the bacteria by radiolabeled bacteriophage does not result in radioactivity in the bacterial proteins because the radioactive sulphur ${(}^{35}S)$ in the protein coat (of the bacteriophage) does not enter the bacterial cells.
The bacterial proteins do not possess radioactivity as the bacteria infected with radiolabeled phages are grown in normal-non-radioactive medium.

Part 4: DNA
Radioactive phosphorus enters the bacterial cells along with the radiolabeled viral DNA. Thus, radioactivity is found in the viral DNA molecules that are formed by replication within the bacterial cells.

Part 5: All parts
Apart from the viral DNA, the other parts of the infected bacterial cells do not show radioactivity as they are built using nutrients obtained from the non-radioactive culture medium.

Final answer
(b) DNA