Assume a certain molecule of dna is composed of exactly 22 percent adenine. how much cytosine would you expect to find in this molecule

The amount of purine is always equal to the amount of pyramidine in a certain DNA molecule or can be written as:

[A] + [G] = [C] + [T]

where A is adenine, G is guanine, C is cytosine and T is thymine

But according to Chargaff’s rule, the amount of Guanine is equal to Cytosine, so that:

[G] = [C]

Hence,

[A] + [C] = [C] + [T]

Cancel [C]:

[T] = [A]

[T] = 22%

So that means:

[G] = [C] = 28%

So there is 28% Cytosine