Mitochondria and plastids evolved from free-living prokaryotes into current cytoplasmic organelles through endosymbiotic evolution. Mitochondria are thought to be necessary for eukaryotic life to exist. They are known as the cell's powerhouses because they provide the majority of the energy or ATP required by the cell. The mitochondrial genome (mtDNA) is replicated separately from the host genome. Human mtDNA codes for 13 proteins, most of which are involved in oxidative phosphorylation (OXPHOS). The nuclear genome encodes the remaining mitochondrial proteins, which are then transported into the mitochondria. The genomes of these organelles have become far smaller than those of their free-living predecessors. This is mostly due to the widespread transfer of genes from prokaryote progenitors to the nuclear genome, followed by their elimination from organelle genomes. In evolutionary timescales, the continuous entry of organelle DNA into the nucleus has provided novel nuclear genes. Furthermore, Mitochondria depend on nuclear genes for essential protein production as they cannot generate all necessary proteins independently.