Question

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

Answer 1

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