Molecular Basis of Inheritance

In 1953, Watson and Crick proposed the structure of DNA based on Rosalind Franklin's X-ray data. Seven facts:

1. DNA is a double helix — two strands wound around a common axis.

2. The two strands are antiparallel — one runs 5' → 3', the other 3' → 5'.

3. The strands are held together by complementary base pairing:

• Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds
• Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds

4. A purine (A or G) always pairs with a pyrimidine (T or C). Two purines or two pyrimidines wouldn't fit the helix width.

5. The helix has a diameter of 2 nm and a pitch of 3.4 nm (one full turn). There are 10 base pairs per turn, so each base pair is separated by 0.34 nm.

6. Chargaff's rules are direct consequences: A = T and G = C (in moles), so A + G = T + C.

7. The strands are complementary — given one strand's sequence, the other is determined. This is the foundation of DNA replication.

Worked example. A DNA sample contains 30% adenine. By Chargaff: T = 30%, so A + T = 60%. G + C = 40%, so G = C = 20% each.