Question

If a double stranded $DNA$ has $20\%$ of cytosine, what will be the percentage of adenine in it?

• A. $20\%$
• B. $40\%$
• C. $30\%$
• D. $60\%$

Answer 1

Answer: (C)

$30\%$

Answer 2

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 $20\%$ cytosine and hence guanine will be also $20\%$. So, $A + T$ must be $60\%$. Therefore, percentage of adenine would be $60/2 = 30\%$.