DNA Structure

DNA (deoxyribonucleic acid) carries the instructions for building and running every living thing. Its shape is a double helix - two strands twisted around each other like a spiral ladder. The sides of the ladder are a backbone of alternating sugar and phosphate; the rungs are pairs of bases.

DNA is a long polymer built from repeating units called nucleotides. Each nucleotide has three parts joined together: a deoxyribose sugar, a phosphate group, and one nitrogen base. The sugar and phosphate form the backbone; the base sticks inwards to form half of a rung.

There are four bases: Adenine, Thymine, Cytosine and Guanine. They pair in a fixed way - A always pairs with T, and C always pairs with G - held by weak hydrogen bonds. This is complementary base pairing, so one strand's order tells you the other. Chargaff's rule follows: amount of A = T and C = G.

A gene is a section of DNA that codes for one protein. The order of bases is a code: each group of three bases is a codon that names one amino acid, and amino acids join to build proteins. In our cells the long DNA is wound around histone proteins and packed into chromosomes - humans have 23 pairs.