Question

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

C?
a) 20%
b) 30%
c) 40%
d) 50%

Answer 1

Using Chargaff's rules, we determine that the amount of cytosine in a DNA sample with 20% thymine is 30%, as adenine and thymine are equal and so are guanine and cytosine.

Step-by-step explanation:

Percentage of Cytosine in DNA

According to Chargaff's rules, in DNA, the amount of adenine (A) always equals the amount of thymine (T), and the amount of guanine (G) always equals the amount of cytosine (C). If 20% of a DNA sample is thymine, then 20% must also be adenine due to base pairing. That accounts for 40% of the DNA, leaving 60% for guanine and cytosine. Since G pairs with C, they must be present in equal amounts. Therefore, the remaining 60% of the DNA is equally divided between guanine and cytosine, making the percentage of cytosine (C) in the sample 30%.