Miscounting hydrogen bonds in DNA base pairs
- The idea
- Predict
- See it
- Why
- Check
The idea
“I count the hydrogen bonds on each strand, and I'm never sure which pair gets three.”
Each base seems to come with its own bonds, so counting both strands feels thorough. And on paper, A-T and G-C are easy to swap.
The short version
The mistake: I count the hydrogen bonds on each strand, and I'm never sure which pair gets three.
What's actually true: A-T pairs have 2 hydrogen bonds and G-C pairs have 3, and each bond is counted once per pair.
- The two strands are held together by hydrogen bonds between paired bases. Each bond links a base on one strand to its partner on the other.
- A-T pairs form 2 hydrogen bonds and G-C pairs form 3. That's one reason DNA rich in G-C takes more heat to separate.
- To count, multiply A-T pairs by 2 and G-C pairs by 3, then add. Don't double it for the second strand.
On the exam: Count pairs, not bases. Each A-T pair has 2 hydrogen bonds and each G-C pair has 3.