Q. 22.  If Meselson and Stahl's experiment is continued for four generations in bacteria, the ratio of 15N/ 15N:15N/14N:14N/14N containing DNA in the fourth generation would be

(a)  1 : 1 : 0

(b) 1 : 4 : 0

(c) 0 : 1 : 3

(d) 0 : 1 : 7


(d) Meselson and Stahl found that DNA of the first generation was hybrid or intermediate N15 and N14. It settled in caesium chloride at a level higher than the fully labelled DNA of parent bacteria N14  N14 in 1 : 3 ratio. The fourth generation after 80 minutes contained 12.5% N15  N14 and 87.5% N14 N14 DNA in 1 : 7 ratio.