Question

If the amount of adenine in a DNA is 30%, then according to Chargaff's rules, the amount of Uracil and Guanine present in the DNA respectively are:

• A. 0% and 30%
• B. 30% and 0%
• C. 0% and 20%
• D. 20% and 30%

Hint:

Always check the molecule type in the question! Uracil is the "imposter" in DNA questions; its value will always be $0$ for a DNA molecule.

Answer 1

The Correct Option is C

Step 1: Chargaff's Rule states that in DNA, $A=T$ and $G=C$. Also, the total percentage of purines equals pyrimidines ($50%:50%$).
Step 2: If $A = 30%$, then $T$ must also be $30%$. Together, $A + T = 60%$.
Step 3: The remaining $40%$ must be $G + C$. Since $G=C$, then $G = 20%$ and $C = 20%$.
Step 4: Uracil is not present in DNA; it is only found in RNA. Therefore, Uracil is $0%$.