Question

Escherichia coli was infected with bacteriophage having radioactive ${(}^{32}P)$ DNA in a culture. It was blended, centrifuged and distribution of ${}^{32}P$ determined. What does the experiment show?

• A. Nothing is proved
• B. DNA is not involved in heredity
• C. DNA is the genetic material
• D. Protein is not the genetic material

Answer 1

The correct option is C DNA is the genetic material

Part 1: Introduction to the experiment
Bacteriophages attach themselves to specific host bacteria and insert their genetic material into the bacteria.
The genetic material carries all the genetic information related to the structure and function of the virus and can direct the synthesis and assembly of new phage particles.
The genetic material of the virus takes control over the host machinery and directs the host cell to utilise its resources for creating new viral particles.
Like every other virus, bacteriophages are composed of a protein coat and a nucleic acid (DNA or RNA) molecule enclosed within the coat.
The given experiment aims to prove whether the nucleic acid or the protein coat serves as the genetic material for the bacteriophage.

Part 2: Procedure
In the given experiment, a bacteriophage having DNA radiolabeled with radioactive $P^{32}$ is used to infect E.coli cells.
After a while the mixture was blended and centrifuged.
Centrifugation results in the bacterial cells to settle at the bottom of the containers whereas the supernatant, consisting of culture medium and particles outside the bacterial cells, float above.
The two fractions were then tested for the presence of radioactivity.

Part 3: Result
The bacterial cells settled at the bottom of the centrifuge vessels showed radioactivity but the supernatant did not.

Part 4: Protein is not the genetic material
The experiment does not prove that protein is not the genetic material. This is because the protein coat was not radiolabeled and hence we cannot track whether it was inserted within the bacterial cells or not.

Part 5: DNA is the genetic material
The results showed that the phage DNA carrying radioactive phosphorus was inserted within the bacterial cells and helped in the formation of new viral particles which also carried radiolabeled DNA.
Thus all the bacterial cells that were infected by newly formed viral particles also showed radioactivity as the radiolabeled DNA was inserted into them during infection.
This shows that DNA is responsible for the transfer of genetic information from one generation of viruses to the next generation.
Thus it proves that DNA is the genetic material.

Final answer
(d) DNA is the genetic material