Answer the following questions based on Meselson and Stahl s experiment: (a) Write the name of the chemical substance used as a source of nitrogen in the experiment by them. (b) Why did the scientists synthesise the light and the heavy DNA molecules In the organism used in the experiment ? (c) How did the scientists make it possible to distinguish the heavy DNA molecule from the light DNA molecule ? Explain. ((1) Write the conclusion the scientists arrived at after completing the experiment.

Explanation

(a) Ammanium chloride (`NH_4Cl`). (b) To check if DNA replication was semi-conservative. (c) The heavy and light DNA molecules were distinguished by centrifugation in a cesium chloride density gradient. (d) The scientists concluded that DN A replicates semi-conservatively.