DNA, chromosomes and mutations

DNA is a long molecule shaped like a twisted ladder, called a double helix. It carries the genetic code. DNA is coiled into structures called chromosomes, which are found in the nucleus. Human body cells have 23 pairs of chromosomes, 46 in total. Sex cells (egg and sperm) have only 23, one from each pair, so when they join at fertilisation the baby has 46 again.

Genes code for proteins, and proteins build and run the body. A mutation is a change in the DNA. Most mutations have no effect, some are harmful, and a few are helpful. Mutations are a source of new variation. Variation can also come from the environment: a plant may be tall because of its genes, or because it got plenty of light and water.