Lesson 2 of 13 · 6 min
The double helix and central dogma
NCERT § "The DNA"
X-ray data plus a chemical clue gave Watson and Crick everything they needed for a physical model — one that had to explain why the amount of A always equals the amount of T, and G always equals C, in any DNA sample from any species.
The lesson in notes
In short
X-ray diffraction data from Maurice Wilkins and Rosalind Franklin, together with base-pairing, led James Watson and Francis Crick to propose the double-helix model in 1953.
Erwin Chargaff had shown that in double-stranded DNA the amounts of A and T are equal, and so are G and C, which fits with complementary pairing.
The two chains run antiparallel: one is 5′→3′ and the other 3′→5′. The sugar-phosphate backbones lie outside and the bases stack inside.
A pairs with T through two hydrogen bonds; G pairs with C through three. A purine always faces a pyrimidine, keeping the distance between the two strands uniform.
The chains coil in a right-handed manner. Each turn of the helix is 3.4 nm long (the pitch) and holds roughly 10 bp, so neighbouring base pairs are about 0.34 nm apart.
Stacking of one base-pair plane over the next adds stability to the helix on top of the hydrogen bonds.
Francis Crick proposed the central dogma: information flows DNA → RNA → protein. In some viruses information flows in reverse, from RNA to DNA.