Question: 140. If Adenine makes 30 \% of the DNA molecule, what will be the percentage of Thymine, Guanine and Cytosine in it?
(1) \mathrm{T}: 20 ; \mathrm{G}: 30 ; \mathrm{C}: 20
(2) \mathrm{T}: 20 ; \mathrm{G}: 20 ; \mathrm{C}: 30
(3) \mathrm{T}: 30 ; \mathrm{G}: 20 ; \mathrm{C}: 20
(4) \mathrm{T}: 20 ; \mathrm{G}: 25 ; \mathrm{C}: 25
Answer: Option (3)
Explanation:
According to Chargaff’s rule, in a DNA molecule the amount of adenine is equal to thymine and the amount of guanine is equal to cytosine.
If adenine constitutes 30 \% of the DNA, then thymine will also be 30 \%.
The total percentage of bases is 100 \%, so the remaining percentage after adenine and thymine is:
100 - (30 + 30) = 40 \%This remaining 40 \% is equally shared by guanine and cytosine.
Thus, guanine is 20 \% and cytosine is 20 \%.
Therefore, the correct answer is option (3).