Question

A double stranded DNA has 30% Thymine. The percentage of cytosine is

• A. 30%
• B. 70%
• C. 20%
• D. 15%

Hint:

In DNA, the base pairing rule is A = T and G = C. Use this to find the percentage of other bases when one is known.

Answer 1

The Correct Option is C

In double
stranded DNA, the amount of adenine (A) equals thymine (T), and the amount of cytosine (C) equals guanine (G). Given: Thymine = 30% ⇒ Adenine = 30% So, A + T = 60% Remaining 40% is C + G Hence, Cytosine = 20% and Guanine = 20%

Answer 2

Elimination approach:
Since DNA is double-stranded, Chargaff's rule requires \( A = T \) and \( G = C \), so the four bases must always satisfy \( A + T + G + C = 100\% \) with \( G = C \).
Given \( T = 30\% \), we also get \( A = 30\% \), so \( A + T = 60\% \) and the remaining \( 40\% \) must be shared equally between G and C.
Checking the options: \( 30\% \) and \( 70\% \) would push the total base percentage beyond \( 100\% \) once G is added, and \( 15\% \) would leave only \( 30\% \) for G+C (short by \( 10\% \)) instead of the required \( 40\% \).
Only 20% satisfies \( 2C = 40\% \), confirming Cytosine \( = 20\% \).