Question

If one DNA strand in a duplex is composed of 50% pyrimidine bases, and one in eight bases on the complementary strand is a G, what is the T content of the first strand?

Answer 1

According to Chargaff's rule, the DNA from any cell of an organism should have 1:1 ratio (base pair rule) of Purine bases (Cytosine and Thymine) and Pyrimidine bases (Adenine and Guanine). The amount of Guanine should be equaled to Cytosine and the amount of Adenine should be equal to Thymine. This rule is followed in both strands.

It is given, one in eight bases on the complementary strand is G. Thus on the first strand, one in eight bases is C.
Then, the number of G in the first strand is one in eight bases too (as G=C).

So, among 8 bases on the first strand, 2 of them are C and G, and the rest 6 bases are A and T combined.
As the amount of A and T is equal, we can say that A is 3 out of 8 bases and T is 3 out of 8 bases.

Therefore, the content of T on the first base is three in eight bases or 37.5% of the bases.