The human has about 200 000 different genes. The genes are arranged on separate, very long pieces of DNA, each one of which is wound up to form a chromosome. Humans have 46 chromosomes in total – 23 pairs. One of each pair is passed down from your mother, one of each pair is passed down from your father. This means you have two copies of every gene – these different copies are called alleles.
A gene is a section of deoxyribonucleic acid that codes for a functional protein. Every living organism has a large collection of genes, each one coding for the production of a different protein.
When a gene is expressed to produce a protein, the chromosome has to be unwound so that the protein making template can be made. The DNA untangles and then the DNA double helix itself unwinds so that an RNA (ribonucleic acid) copy can be made. This messenger RNA acts as a template and passes out of the nucleus to the ribosomes, the protein making machines in the cell.