In every human cell there are hundreds of mitochondria, which are required for oxidative phosphorylation as well as many other metabolic processes. Each mitochondrion contains approximately 5 mitochondrial DNA molecules. These circular DNAs of 16.5 kb in size contain only 39 genes . Mutations in mitochondrial DNA are responsible for many diseases. Alterations in these molecules may also play a role in ageing and in tumour formation.