If a DNA Sample Were Composed of 10% Thymine, What

Question

If a DNA sample were composed of 10% thymine, what would be the percentage of guanine?
A) 10
B) 20
C) 40
D) 80
E) impossible to tell from the information given

Accepted Answer

In DNA, thymine always pairs with adenine, and guanine always pairs with cytosine. This means that if a DNA sample has 10% thymine, it must also have 10% adenine (since the percentages of thymine and adenine are always equal). The remaining 80% is composed of guanine and cytosine. Since adenine and thymine together make up 20%, and there is 10% thymine, there must be 10% adenine. This means that the remaining 80% must be divided evenly between guanine and cytosine, so the percentage of guanine is also 40%. Therefore, the answer is C.