A genome is the complete genetic code of an organism
The code exists as a substance called DNA, Deoxyribonucleic Acid, which takes a shape similar to a ladder twisted into a helix
The DNA ladder contains a massive three billion individual steps and carries all of the code required to make the biochemicals needed for life
A full copy of the DNA code is found in the nucleus of each of the body's 10 trillion cells and, if unravelled and straightened, would be about 1.8 metres long
If all of this information was printed it would produce a stack of paperback books 60 metres high or fill 200 telephone directories
All of the DNA code is assembled from just four chemicals represented by the letters A, C, G and T. They join in pairs to make a single DNA ladder step.
Most of the DNA code, about 97 per cent, is made up of repeated sequences and "junk" that has no apparent purpose
Individual working parts of the code are called genes and these in turn provide the instructions needed to make amino acids, the 20 building blocks which assemble to form proteins
The genes are grouped into clusters called chromosomes and there are 23 pairs of these, half inherited from the mother and half from the father
Humans have far fewer genes than expected, between 30,000 and 40,000. A fruitfly has 13,000 genes.
DNA differs by only 0.2 per cent between humans and 4 per cent between humans and chimpanzees.