DNA: the double helix

The nucleus of every cell holds chromosomes, and each chromosome is one very long molecule of DNA tightly coiled up. DNA is shaped like a twisted ladder, a double helix. The two sides of the ladder are backbones made of sugar and phosphate. The rungs are pairs of bases, held together by weak hydrogen bonds. There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C), and they always pair the same way: A with T and G with C. The order of the bases is a code. A gene is a stretch of DNA whose code tells the cell how to make a particular protein, which in turn produces a trait such as eye colour.