Q. In sea urchin DNA, which is double stranded, 17 % of the bases were shown to be cytosine. The percentages of the other three bases expected to be present in this DNA are :
(a) G=8.5%, A=50%,T=24.5%
(b) G=34%,A=24.5%,T=24.5%
(c) G=17%,A=16.5%,T=32.5%
(d) G=17%,A=33%,T=33%
Solution :
Correct option : D According to Chargaff’s rule all cellular DNAs, regardless of the species, number of adenosine residues is equal to number of thymidine residues which means that A=T; and the number of guanosine residues is equal to the number of cytidine residues; G= C. In the given question, there is 17% cytosine which means guanine percentage is also 17%. Thus, percent of G and cytosine = 34; percent of A+T= 100-34= 66; Adenine percent= 66/2= 33; thymine percent= 33. Thus, the correct answer is D.