The genome of an organism is all of its genetic material. It is the complete set of DNA in the organism, including every gene and the DNA between the genes. In humans and other animals and plants, this DNA is stored in chromosomes in the nucleus of cells.
A human body cell has 23 pairs of chromosomes, which makes 46 chromosomes in total. Together these chromosomes carry the human genome, which contains around 20 000 genes. Each species has its own genome, and the genome of one individual differs slightly from that of another, except for identical twins.
Nearly all the body cells of one organism contain a copy of the same genome. Different cells use different genes, so they carry out different functions. A nerve cell and a muscle cell have the same genes but make different proteins.