A base pair's width comes from its rings
- The idea
- Predict
- See it
- Why
- Check
The idea
“A base pair fits the helix if it has the right number of hydrogen bonds, like A-T's 2 or G-C's 3.”
You learned 3 hydrogen bonds for G-C and 2 for A-T. Those numbers stick, so they feel like what makes a pair fit.
The short version
The mistake: A base pair fits the helix if it has the right number of hydrogen bonds, like A-T's 2 or G-C's 3.
What's actually true: Every base pair in DNA is one double-ring purine plus one single-ring pyrimidine, so every pair is the same width.
- A and G are purines, with two rings. C and T are pyrimidines, with one. In DNA, every pair is one of each.
- So every pair spans three rings, and the two sugar-phosphate backbones stay the same distance apart.
- Hydrogen bonds decide which bases match and how tightly they hold. G-C, with 3, holds tighter than A-T, with 2.
On the exam: Rings set a pair's width, one double plus one single. H-bonds set which bases match and how tightly.