In Nucleic Acids you met the nucleotide: a sugar, a phosphate and a base. DNA is two long chains of nucleotides wound around each other. Its shape explains both how it stores information and how it is copied.
Key idea
DNA is a double helix. Sugars and phosphates form each strand's backbone. The bases point inward and pair: A with T (two hydrogen bonds) and G with C (three). The two strands run in opposite directions: they are antiparallel, one 5′→3′ and the other 3′→5′.
Why it works
Why only A–T and G–C? A and G are large bases with two rings; T and C are small, with one ring. A large base always pairs with a small one, so the ladder has the same width all along. Of the large-small pairs, only A–T and G–C line up their hydrogen bonds. So each strand fixes the other: if you know one, you know its partner.
Worked example: Writing the partner strand
One strand reads 5′-ATGCCA-3′. Write the partner strand lined up under it.
- Pair base by base: A→T, T→A, G→C, C→G, C→G, A→T.
- The partner runs the other way, so label its ends 3′ and 5′.
Answer: 3′-TACGGT-5′
Worked example: Chargaff's rule
In a sample of DNA, 30% of the bases are A. What percent are G?
- Every A pairs with a T, so T is also 30%.
- A + T = 60%, which leaves 40% for G + C.
- G pairs with C, so they are equal: 40% ÷ 2.
Answer: 20% G (and 20% C).
The pairing rule came from two lines of evidence. Erwin Chargaff measured the bases in DNA from many species and found that A always matched T and G matched C, though the A amount differed from species to species. X-ray pictures of DNA taken by Rosalind Franklin showed a helix. In 1953 James Watson and Francis Crick put these facts together in the double-helix model.
Common mistake
Two common slips: pairing A with G, and thinking all four bases must be 25%. The rule is A = T and G = C. A and G can be quite different: human DNA has more A–T pairs than G–C pairs. Check your answer: %A + %T + %G + %C must make 100.