Every base pair is one purine and one pyrimidine
- The idea
- Predict
- See it
- Why
- Check
The idea
“Partner bases are the same kind, so A and T are both purines, and two like bases can pair.”
"Complementary" sounds like "alike," so partner bases seem like a matched set. A base pair is really a big piece fitted to a small one.
The short version
The mistake: Partner bases are the same kind, so A and T are both purines, and two like bases can pair.
What's actually true: Every pair joins a two-ring purine, A or G, to a one-ring pyrimidine, C, T or U. That keeps the helix the same width.
- Adenine and guanine are purines, with two fused rings. Cytosine, thymine and uracil are pyrimidines, with one. Uracil takes thymine's place in RNA.
- Large plus small gives every rung the same width, so the helix stays about 2 nm across.
- Hydrogen bonds pick the exact partner. A and T share two, and G and C share three. So A pairs with T, or U in RNA, and G pairs with C.
- Since every pair has one of each, double-stranded DNA is half purines and half pyrimidines. Chargaff measured this. A equals T and G equals C.
On the exam: Pure As Gold. Purines are A and G, and every pair has one purine and one pyrimidine.