DNA structure

A twisted ladder built from four bases.

The idea

DNA is a polymer of nucleotides, each made of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine, thymine, guanine, or cytosine. Phosphodiester bonds link sugar to phosphate along each backbone, and the two strands run antiparallel — one 5′→3′, the other 3′→5′ — twisting into a right-handed double helix about 2 nm wide with roughly 10 base pairs per 3.4 nm turn, as Watson and Crick showed in 1953 using Franklin’s X-ray images. Bases pair across the middle by hydrogen bonds, always a two-ring purine with a one-ring pyrimidine so that every rung is the same width: A with T by two hydrogen bonds, G with C by three. That complementarity explains Chargaff’s rules (%A = %T and %G = %C) and lets either strand serve as a template for copying the other.

Work through an example

A DNA sample is 30% adenine. By Chargaff’s rules thymine is also 30%, so A + T = 60% and the remaining 40% is split equally: 20% G and 20% C. To write the complement of 5′-ATGGCA-3′, swap A↔T and G↔C and reverse the direction: 3′-TACCGT-5′. Scale check: the diploid human genome holds about 6.4 billion base pairs; at 0.34 nm per pair that is roughly 2 m of DNA folded into a nucleus about 6 µm across.

What to watch for

The two strands are held together by hydrogen bonds, not covalent bonds; the covalent phosphodiester bonds run along each backbone. That is why heating DNA to about 95 °C in PCR separates the strands without breaking either one, and why G–C-rich DNA, with three hydrogen bonds per pair, needs a higher temperature to melt. A related slip: uracil belongs to RNA — DNA uses thymine.