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 17%, A 33%, T 33%
B) G 8.5%, A 50%, T 24.5%
C) G 34%, A 24.5%, T 24.5%
D) G 17%, A 16.5%, T 32.5%.
Explanation
According to Chargaff ’s rule, the amount of adenine is always equal to that of thymine and the amount of guanine is always equal to that of cytosine, i.e., A = T and G = C. Also, the purines and pyrimidines are always in equal amounts, i.e., A + G = T + C. Now, given dsDNA has 17% cytosine and hence guanine will be also 17%. So, A + T must be 66%, therefore, percentage of adenine or thymine would be 66/2 = 33%.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.