The total number of hydrogen bonds of a DNA-double Helix strand whose one strand has the following sequence of bases is ________.
5′-G-G-C-A-A-A-T-C-G-G-C-T-A-3′

Solution

Chemistry ma’am · 3 steps

Count the G and C bases and the A and T bases on one strand: every G–C pair gives 3 H-bonds, every A–T pair 2.

1. Base pairing rule. In DNA, A always pairs with T through 2 hydrogen bonds, and G always pairs with C through 3 hydrogen bonds. Each base on the given strand makes exactly one pair, so counting the bases on one strand is enough.

   Base on given strand	How many	Pairs with	H-bonds per pair	H-bonds
   G	4	C	3	4 × 3 = 12
   C	3	G	3	3 × 3 = 9
   A	4	T	2	4 × 2 = 8
   T	2	A	2	2 × 2 = 4

2. Add them up. There are 13 bases: 7 G–C pairs and 6 A–T pairs. So the total is 7 × 3 + 6 × 2 = 21 + 12.
3. So. The double helix holds 33 hydrogen bonds.

The common slip: 66. Counting the bases of both strands separately counts every pair twice and gives 66, not 33.

The idea to keep. Hydrogen bonds sit between the two strands, one set per base pair. Count pairs from one strand only: G–C gives 3, A–T gives 2.

Answer from the NTA final answer key. Solution by PrepWiser.