DNA

Every living thing is built from instructions written in DNA. In your cells the DNA is kept in the nucleus, coiled up into chromosomes. A gene is a section of DNA that carries the instructions for one characteristic, such as the pigment in your eyes.

Word	What it is	How many
Nucleus	The control centre of a cell, where the chromosomes are kept	Usually one per cell (red blood cells have none)
Chromosome	One very long molecule of DNA, coiled up tight	46 in a human body cell, in 23 pairs
Gene	A section of DNA carrying the instructions for one characteristic	About 20,000 in a human
DNA	The molecule itself: a double helix, like a twisted ladder	About two metres of it in every cell with a nucleus

The DNA molecule is a double helix: two strands twisted round each other like a twisted ladder. The rungs are pairs of bases. There are four, called A, T, C and G, and A always pairs with T, and C always pairs with G. So if you know one strand, you know the other, and that is how DNA can be copied when a cell divides.

Who worked it out. In 1952 Rosalind Franklin’s team at King’s College London took X-ray photographs of DNA fibres. The most famous, Photo 51, has an X-shaped pattern that shows the molecule is a helix, and its spots give the molecule’s measurements. Maurice Wilkins worked on X-rays of DNA in the same department. In Cambridge, James Watson and Francis Crick used those measurements to build the double-helix model, published in 1953. Wilkins had shown Photo 51 to Watson without Franklin’s knowledge. Watson, Crick and Wilkins shared the Nobel Prize in 1962; Franklin had died in 1958, and the prize is not normally given after death, nor shared by more than three people.

Stretch X-rays cannot photograph a molecule like a camera. When X-rays pass through a crystal or fibre they scatter, and the pattern of spots they make can be decoded to work out the molecule’s shape. The pairing rule explains copying: unzip the ladder, and each half builds its missing half. The number of chromosomes says nothing about how complicated a living thing is: a dog has 78, a potato 48, a fruit fly 8.