Why are DNA molecules with a high percentage of guanine (G) and cytosine (C) more stable than DNA molecules high in adenine (A) and thymine (T) ?

The pairing of G-C in the DNA structure is done by three hydrogen bonds compared to the two in A-T pairing.

Explanation:

The DNA molecule is composed of two long polynucleotide chains made up of four types of nucleotide-containing subunits (Adenine, Guanine, Cytosine and Thymine). These bases pair up in the DNA to form the double-helical structure of the DNA. The pairing of these bases follow the complementary base pairing rule, where hydrogen bonds holds Guanine (G) to Cytosine (C) and Adenine (A) to Thymine (T).

Three hydrogen bonds pair up Guanine and Cytosine while two hydrogen bonds pair up Adenine and Thymine. The number of hydrogen bonds that pair these bases is responsible for how strong each pairing is, making G-C pairing stronger than A-T. Hence, the strong bonding in the G-C pairing will confer stability upon a DNA molecule rich in it.