If 20% of a DNA sample is made up of thymine, T, what percentage of the sample is made up of cytosine, C?

Question

Faith Weeks

Biology

4 September, 17:28

If 20% of a DNA sample is made up of thymine, T, what percentage of the sample is made up of cytosine, C?

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Urijah Johnson · Answer 1

DNA consists of four nitrogenous bases: A, T, C, and G. Because A and T form a pair and C and G form a pair, the percentage composition of A is equal to T and the percentage composition of C is equal to G. This is called Chargaff's Rule.

In this sample, 20% is T. Because the percent composition of A is equal to T, we can expect 20% to also be A. That means that A and T make up 40% of the sample. That leaves 60% of the sample as C and G. The percentage composition of C must be equal to G, so divide 60% by 2 to get 30% G and 30% C.

So the answer is 30%.