(a) Meselson and Stahl carried out an experiment to prove the nature of DNA replication. Recall the experiment and answer the following questions. i) Which two types of nitrogen were used by them in their experiment and why? ii) Why did they take samples of E. coli at definite time intervals for their observation? iii) State the role of caesium chloride density gradient in their experiment. iv) Write the conclusions they arrived at. or (b) i) A true breeding tall pea plant with round seeds is crossed with a recessive dwarf pea plant having wrinkled seeds. Work out the cross up to F2 generation giving the phenotypic ratios of F1 and F2 generation respectively. ii) State the Mendelian principle that can be derived only with the help of such a cross.