Question

If a double-stranded segment of DNA has been found to have 23 purines and 15 thymines, how many guanines are there?

Answer 1

Answer: 4 Guanines

Explanation: Following Chargaff's ratio that describes the ration of purines to pyrimidines as 1:1, if there are 23 purines, it follows that there should be 23 pyrimidines; 15 Thymines should therefore be hydrogen bonded to 15 Adenines by complementary base pairing.

We are then left with 8 nucleotides composed of guanines and cytosines in complementary base pairs. This means that half that number (4) should be guanines and the rest cytosines.