Mitochondria can also synthesize proteins

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After M. and S. Nass discovered mitochondrial DNA (mtDNA) in 1963, a complete set of components for DNA replication, transcription, and protein translation was found in mitochondria, including RNA, DNA polymerase, RNA polymerase, tRNA, ribosomes, and amino acid activating enzymes, indicating that mitochondria possess an independent genetic system as cells differentiate. Although mitochondria can also synthesize proteins, their synthetic capacity is limited. Among the more than 1,000 proteins in mitochondria, only about a dozen are synthesized by them themselves. Mitochondrial ribosomal proteins, aminoacyl-tRNA synthetases, and many structural proteins are encoded by nuclear genes; after being synthesized in the cytoplasm, they are transported to the mitochondria in a targeted manner, which is why mitochondria are referred to as semi-autonomous organelles. By culturing cells with labeled amino acids and using chloramphenicol and actinomycin to inhibit mitochondrial and cytoplasmic protein synthesis respectively, it was found that the polypeptides encoded by the mitochondrial DNA of human cell structures include 3 subunits of cytochrome c oxidase, 2 subunits of F0, 7 subunits of NADH dehydrogenase, and 13 other polypeptides such as cytochrome b. In addition, mitochondrial DNA can also synthesize 12S and 16S rRNA as well as 22 types of tRNA.