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)

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 $60/2=30\%$.