Question

Choose the right combination of scientists and the organisms used for their experiments
i. Griffith - Streptococcus pneumoniae, mice
ii. Griffith - Bacteriophage, mice
iii. Hershey - Chase - E.coli, $T_2$ bacteriophage
iv. Hershey - Chase - Streptococcus pneumoniae, $T_2$ bacteriophage

• A. i and iii
• B. ii and iii
• C. i and iv
• D. ii and iv

Answer 1

The correct option is A i and iii

Griffith discovered the “transforming principle” in 1928.
The bacterial transformation was shown by Frederick Griffith in his experiments on Streptococcus pneumoniae. He took two strains of Streptococcus pneumoniae and infected them with mice.
They were R(non-virulent)and S(virulent) strains. S-strain was able to cause pneumonia infection in the mouse while R strain was not. When heat-killed S-strain was mixed with live R-strain, it was able to cause infection in the mouse. He witnessed a miraculous transformation in the bacteria and stated that something was being passed on to the R-strain from the heat-killed S-strain bacteria.

Alfred Hershey and M. Chase (1952), gave unequivocal proof that DNA is the genetic material.
Their experiment proving DNA as the genetic material was based on the principle of transduction. It is the process by which DNA is transferred from one bacterium to another by a virus. They worked with bacteriophages and E.coli bacteria. They found that it was DNA that was getting transferred from bacteriophages into the bacteria by using the radioisotopic technique.

Radioactive elements ${}^{32}P$ and ${}^{35}S$ were used in Hershey and Chase’s experiments. Phosphorus forms an important constituent of DNA and while sulphur is present in certain amino acids like methionine which are involved in the formation of proteins. Moreover, using the radioactive isotopes of these elements made it easier to trace them during the experiment. The experiment that proved DNA as the genetic material was based on the principle of transduction. Radioactive phosphorus $\left({}^{32}P\right)$ and radioactive sulphur $\left({}^{35}S\right)$ were used separately to culture bacteriophages. The DNA of bacteriophage was radiolabeled with ${}^{32}P$ and protein with ${}^{35}S$. These radiolabeled bacteriophages were then made to infect E. coli bacterial cells. It was seen that the bacterial cells contained radioactive viral DNA $\left({}^{32}P\right)$, but not radioactive viral protein$\left({}^{35}S\right)$. This means that the bacteriophages transferred their DNA to the E. coli bacteria.
Hence this experiment proved that the DNA, and not protein, is the genetic material passed from the virus to bacteria.