What is DNA?

DNA is the most famous molecule in the world, but what goes on inside that double helix? At the heart of it are four chemicals called nucleotides. Each contains a pentagon-shaped sugar molecule, a structure called a phosphate group and a protein fragment called a base.

The sugar and the phosphate group are the same for every nucleotide. They join together side by side to form the backbone of each DNA strand. Like LEGO bricks, they only connect one way round; the phosphate of one nucleotide slots into the sugar of the next. This means that DNA strands have a definite ‘up’ and ‘down’. The top is called the five prime (5’) end, and the bottom the three prime (3’) end.

The two strands of a DNA helix face in opposite directions, one running five prime to three prime, and the other three prime to five prime. They twist together in a right-handed helix. If you look closely you’ll notice that the alternating twists are uneven: one big (the major groove) and one small (the minor groove).

The bases connect the two DNA strands together like the rungs of a ladder. They form pairs, known as base pairs, linked by strong interactions called hydrogen bonds. There are four bases – adenine (A), cytosine (C), guanine (G) and thymine (T) – and they are extremely picky about the bonds they will form. Adenine will only pair with thymine, and guanine will only pair with cytosine. This means that if one strand carries the sequence ATGC, the other has to have the sequence TACG.

This strictness allows a cell to easily make copies of its genetic code. It simply unzips the two strands and uses them both as templates to assemble two new strands, copying the matching nucleotides into the empty slots.

The most important role for DNA is to carry information from one generation to the next. This information is written in code, using the