A DNA segment has the sequence 5'-G A A T T C G C-3'. When it pairs with its complementary strand, how many hydrogen bonds are formed in total between the two strands?
Answer
20 hydrogen bonds
The correct answer is 20 hydrogen bonds — Add the contributions: 12 + 8 = 20 hydrogen bonds.
Step-by-step solution
- Write the complementary strand using base‑pairing rules: G↔C, A↔T, T↔A, C↔G.
- Obtain the complement: 3'-C T T A A G C G-5'.
- Count the number of G–C pairs (3 hydrogen bonds each) and A–T pairs (2 hydrogen bonds each).
- Identify G–C pairs at positions 1, 6, 7, and 8 → 4 pairs × 3 = 12 bonds.
- Identify A–T pairs at positions 2, 3, 4, and 5 → 4 pairs × 2 = 8 bonds.
- Add the contributions: 12 + 8 = 20 hydrogen bonds.
Common mistake: Using 2 bonds for G–C or 3 bonds for A–T
Concept tested: Hydrogen bonding in DNA base pairing
Published · How this solution was prepared and checked
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