Question

The following questions are based on the classic experiment conducted by Hershey and Chase.

(i) Why were bacteriophages chosen by the scientists?

(ii) Name the radioactive isotopes used and state the reason for using them.

(iii) Describe the steps of the experiment.

(iv) Why is this experiment considered a major breakthrough in the field of molecular biology?

Hint:

Think of 32P for DNA, 35S for protein, blending and centrifugation.

Answer 1

Step 1: Part (i): Why bacteriophages:
A bacteriophage is a virus that infects bacteria. It is made of only two parts, a protein coat and DNA.
So one of these two must be the genetic material, and each can be labelled separately.
Phages infect bacteria quickly and multiply inside them in a short time, so results come fast.

Step 2: Part (ii): Isotopes used:
Radioactive phosphorus, \( \mathrm{{}^{32}P} \), was used to label DNA. DNA has phosphate groups, but protein has no phosphorus.
Radioactive sulphur, \( \mathrm{{}^{35}S} \), was used to label protein. Protein has sulphur in cysteine and methionine, but DNA has no sulphur.

Step 3: Part (iii): Steps of the experiment:
1. One lot of phages was grown on a medium with \( \mathrm{{}^{32}P} \) and another lot on a medium with \( \mathrm{{}^{35}S} \).
2. These labelled phages were allowed to infect E. coli bacteria.
3. The mixture was blended in a blender to shake off the phage coats from the outside of the bacterial cells.
4. It was centrifuged. The light viral coats stayed in the supernatant and the heavy infected bacteria went to the pellet.
5. Radioactivity was checked. \( \mathrm{{}^{32}P} \) was found in the pellet (inside bacteria) and \( \mathrm{{}^{35}S} \) in the supernatant.

Step 4: Part (iv): Why a breakthrough:
Only DNA entered the bacteria, and new phages were made from it. So DNA, not protein, is the genetic material.
This ended the long debate and gave firm proof for the DNA as genetic material.

Final Answer:
Phages were chosen because they have only DNA and protein. \( \mathrm{{}^{32}P} \) labels DNA and \( \mathrm{{}^{35}S} \) labels protein. Only DNA entered the bacteria, which proved that DNA is the genetic material.