Q.On complete hydrolysis DNA gives equimolar quantities of adenine and thymine.

Step 1. The Watson-Crick DNA double-helix model (section 14.4.3) states that the two strands of DNA are held together by hydrogen bonding between COMPLEMENTARY base pairs: adenine always pairs specifically with thymine (2 hydrogen bonds), and guanine always pairs specifically with cytosine (3 hydrogen bonds).

Step 2. Because this pairing is strict and one-to-one -- every single adenine base on one strand is hydrogen-bonded to exactly one thymine base directly opposite it on the complementary strand, all along the length of the double helix -- the total number of adenine bases in the whole DNA molecule must exactly equal the total number of thymine bases.

Step 3. Therefore, on complete hydrolysis of DNA (which breaks the molecule down into its free constituent bases, section 14.4.1), the amounts of adenine and thymine recovered are necessarily equimolar (and, by the same A-T/G-C pairing logic, guanine and cytosine are also recovered in equimolar amounts to each other).

Apply the Watson-Crick complementary base-pairing rule (A-T, G-C) -- since every base on one strand is paired one-to-one with its complementary base on the other strand, the two paired bases' total amounts in the whole molecule must be equal.

• Forgetting that this equimolar relationship is a consequence of the DOUBLE-stranded, base-paired structure specifically, and does not apply to RNA (which is single-stranded and has no fixed A:U 1:1 relationship in the same structural sense).
• Mixing up which bases pair with which (stating G pairs with A, or C with T, instead of the correct A-T and G-C pairing).