Answer the following questions based on Meselson and Stahl's experiment of E. coli :
(a) Write the name of the chemical substance used as the only source of nitrogen in the experiment.
(b) Why did they allow the synthesis of the light and the heavy DNA molecules in the organism ?
(c ) How did they distinguish the heavy DNA molecules from the light DNA molecules? Explain.
(d) Write the conclusion the scientists arrived at, at the end of the experiment.