Question

The final proof for DNA as the genetic material came from the experiments of

• A. Hargobind Khorana
• B. Avery, Mcleod and McCarty
• C. Hershey and Chase
• D. Griffith

Answer 1

The correct option is C Hershey and Chase

Part 1: Hershey and Chase
In 1952 Alfred Hershey and Martha Chase conducted an experiment in which they infected Escherichia coli with $T_2$ bacteriophage to prove that the genetic material is DNA and not proteins.

● The $T_2$ bacteriophage is made up of DNA and protein coat. It infects the bacteria and multiplies inside it.
● DNA contains phosphorus and protein contains sulphur.
● One batch of the phage was grown in a medium with radioactive phosphorus ${(}^{32}P)$, and the other was grown in a medium with radioactive sulphur ${(}^{35}S)$.
● The phage DNA was incorporated with radioactive phosphorus and the phage protein was incorporated with radioactive sulphur.
● Then these labelled phages were allowed to infect bacteria grown in a normal medium.
● After the infection, the infected bacteria was broken down by blender and centrifuged.
● The bacterial cells infected with phages having radiolabeled DNA showed the presence of radioactive phosphorus inside the cell.
● In case of the bacterial cells infected with radiolabeled protein coat, radioactive sulphur was present outside the cell.
● This shows DNA is the genetic material not protein.

Part 2: Avery, Mcleod and McCarty
Once the presence of a transforming principle was proved by Griffith’s transformation experiments, Avery, Macleod and McCarty performed transformation experiments using biomolecules such as DNA, RNA and proteins.Their aim was to identify the chemical nature of the transforming principle. They found that transformation occurred only in the presence of DNA showing that the transforming principle is DNA. However, their experiment was not accepted as conclusive proof.

Part 3: Hargobind Khorana
Har Gobind Khorana developed a chemical method that was used in synthesising RNA molecules with defined combinations of bases.

Part 4: Griffith
Frederick Griffith first conducted transformation experiments using R-strain and S-strain of Streptococcus pneumoniae. The R-strain transformed into S-strain, by uptaking the genetic material from heat killed S-strain. It was termed a transforming principle. Thus, in Griffith’s transformation experiment the presence of genetic material was proved. But the biochemical nature of genetic material was not defined.

Final answer
(a) Hershey and Chase