All living cells except bacteria contain mitochondria – “the powerhouses of the cell” – to convert food into the useable energy form ATP. Mitochondria house their own DNA (mtDNA) which in humans has 13 protein-coding genes vital for ATP production. All other ~1000 mitochondrial proteins are encoded by genes in the nucleus and need to be imported into mitochondria.
Copying the mitochondrial genome is essential to maintain the thousands of mtDNA molecules each cell needs to function, grow and divide. Mitochondria copy their own genomes using a dedicated replication machinery of proteins. Many human mitochondrial diseases are caused by mutations in this machinery which leads to increased replication errors. These errors result in mtDNDA mutations or loss of mtDNA molecules. Errors in mtDNA replication are also believed to accumulate and contribute to the ageing process and age-related diseases (e.g. diabetes mellitus type II and Parkinson disease).