(a) Hershey and Chase carried their experiment in three steps : infection, blending, centrifugation. Explain each step.

(b) Write the conclusion and interpretation of the result they obtained.

OR

Taking an example of white-winged moths and dark-winged moths of England in pre and post industrialised era, explain evolution by natural selection.

(a) The Hershey and Chase experiment involved two separate sets of bacteriophages. They carried out the following processes:

Infection: In one of the bacteriophages, the proteins were labelled with sulphur (S³⁵), and in the second bacteriophage, the DNA was labelled with Phosphorous (P³²). These are radioactive probes. These radioactive labelled (e.g., P³² and S³⁵) bacteria are allowed to infect separately of E.coli.

Blending: After the infection of E.coli by the bacteriophage, the cells of E.coli are gently agitated in a blender. The blending process separates the phage particles from the bacteria.

Centrifugation: Then, the virus particles were separated from the bacterial cells by centrifugation. The bacterial cells are heavier as compared to virus particles, and thus they form a pellet at the centrifuge bottom, and since virus particles are lighter. So, they settle on the sides of the centrifuge.

(b) Interpretation: They found that the sulphur labelled supernatant, which contains the radioactive sulphur probe had not entered the bacterial cells. Instead, the bacterial pellet which has radioactive phosphorous has entered the bacterial cell.

Conclusion: This proved that DNA is the genetic material which is transferred.

OR

Before the beginning of industrialization, the number of white-winged moths was higher than dark-winged moths due to the tree barks covered in white lichens which prevented thewhite-winged mothsfrom predators whereas the dark-winged moths were easily visible to the predators and thus got preyed upon.

After the industrialization the number of dark-winged moths was higher than white-winged moths due to the huge amount of soot and smoke deposited on the tree barks which turned them dark, and this helped the dark-winged moths to hide from predators whereas the white-winged moths were easily visible to the predators and thus got preyed upon.

Thus, in these cases, natural selection occurred and naturally selected only those moths, which were a better fit for the natural surroundings.