NCERT Class 12 Biology: Chapter – 6 Molecular Basis of Inheritance Part 9 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Long Answer Type Questions

Question 1:

Give an account of Hershey and Chase experiment. What did it conclusively prove? If both DNA and proteins contained phosphorus and sulphur do you think the result would have been the same?


Hershey and Chase Experiment

1. In the year 1952, Alfred Hershey and Martha Chase did an experiment to detect the presence of genetic material. For this experiment they took a virus called ‘Bacteriophage’ .

2. The virus was grown in two separate mediums. One medium consisted radioactive phosphorus (P32) . The other medium consisted of radioactive Sulphur (S35) .

3. Medium containing radioactive Phosphorous had radioactive DNA because DNA is a phosphorus-based structure. Medium containing radioactive sulphur had radioactive protein.

4. Now, these viruses were injected into a bacterium called E. coli.

5. For this incorporation of Virus into E. coli, viral coats were removed by agitating them in the blender.

6. E. coli infected with virus had radioactive DNA and no radioactive proteins were present.

7. This showed that ‘DNA’ was the genetic material that got transferred from virus to E. coli and not protein.


This experiment concluded that DNA is the genetic material that transfers from one cell to another and not protein.

In such case,

a. Radioactive 35 S and 32 p labelled protein capsule would show no radioactivity in the cell, but radioactivity would be detected in the supernatant.

b. Radioactive 35 S and 32 p labelled DNA: Radioactivity is detected in the cells but not in the supernatant.

Such case would lead to non- conclusive experiment results.

Question 2:

During the course of evolution why DNA was chosen over RNA as genetic material? Give reasons by first discussing the desired criteria in a molecule that can act as genetic material and in the light of biochemical differences between DNA and RNA.


The desired criteria in a molecule that can act as genetic material are as follows:

1. It should be chemically and structurally stable.

2. It should be able to replicate, that is replication should take place.

3. Mutation or gradual and slow changes should take place.

4. Should be able to express itself in Mendelian.

5. It must be able to transfer from parent to progeny for further evolution.

Biochemical Differences between DNA and RNA

  • Both DNA and RNA show complementarity of base pairs and hence are capable of replication
  • As shown by Griffith experiment, DNA is more stable than RNA because it could survive even heat-killing during the experiment.
  • Group is present in RNA. This makes RNA labile and degradable; which is not the case with DNA.
  • Both RNA and DNA can carry on mutations. But DNA being more stable is better suited for long term storage of mutations.

Hence, DNA was preferred as the genetic material during the course of evolution.