DNA's double helix has a wide groove where proteins read the code
Twist two strands of DNA around each other and the gaps between them form grooves of unequal size, 22 and 12 ångströms across. The wider one leaves the edges of the bases exposed, so proteins that switch genes on and off usually grip there, reading the sequence without ever unzipping the helix.
James Watson and Francis Crick published the double helix model in Nature in 1953, building on X-ray work by Rosalind Franklin and her student Raymond Gosling, whose image known as Photo 51 proved crucial, alongside Maurice Wilkins and others, plus Erwin Chargaff's data on base pairing. Linus Pauling and Robert Corey had wrongly proposed three strands. The structure revealed how genetic information is stored and copied, and Crick, Wilkins and Watson shared the 1962 Nobel Prize in Physiology or Medicine. Watson's 1968 memoir carried the phrase into popular culture.
The form that predominates in cells, B-DNA, is a right-handed spiral 23.7 ångströms wide that turns once about every 10.4 to 10.5 base pairs. Other shapes exist too. A-DNA, once thought to appear only in dried laboratory samples, turns out to occur in living things, and methylated stretches can flip into Z-DNA, which twists the opposite way. So many variants have been described that only the letters F, Q, U, V and Y remain unused.
The bonds joining the two strands are weak enough to part with gentle heat, enzymes or mechanical force, a process called melting. Regions rich in A and T separate more easily than those rich in C and G, which is why many genes begin with a TATA sequence that helps the transcription machinery open the helix. Heat works for strands up to roughly 10,000 base pairs, as in PCR, but longer stretches tangle, so cells pair unwinding helicases with topoisomerases that cut one backbone and let it swivel.
Though constantly jostled by water molecules, DNA is moderately stiff, behaving like a rigid rod over about 50 nanometres, roughly 150 base pairs. Sequence shapes how it bends: unstable base stacks on one side make it curve away, and A and T pairs tend to sit on the inside of tight bends, as in nucleosomes.
Source: Nucleic acid double helix
