Evolution and genetic significance of mitochondria
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Mitochondria are constituted of two different membrane systems which may well have quite different origins in replication and evolution. It is hypothesized that the outer membrane is entirely derived from cytosol constituents and activities. The inner membrane and contained matrix is derived from an aerobic promitochondrion which was originally a free living procaryote. It became an endosymbiont in an anaerobic host cell which supplied it with the end products of its glycolytic pathway. These the symbiont converted to CO[subscript 2], H[subscript 2]0 and energy through a system homologous to the present day Krebs TCA cycle and the electron transport system. The original endosymbiont lost its cell wall and became a mitochondrion by a gradual process of integration into the host cell. In this process there was transferred to the nucleus of the host cell the genetic material in the form of DNA from the promitochondrion which was involved in the transcription of messenger RNA coding for the soluble components of the inner membrane-matrix system. This class of soluble inner membrane-proteins contains such enzymes as those involved in the synthesis of isoleucine and valine in Neurospora mitochondria, the Krebs TCA cycle enzymes and cytochrome c. These enzymes are apparently all synthesized in the cytosol and transferred to their appropriate positions within the mitochondria. Their functional integration within the inner membrane-matrix is dependent on the structure of the inner membrane. It is probable that most if not all are allotopic proteins. The inner membrane contains insoluble components which must be synthesized within the mitochondria, since they cannot be transported from the cytosol into the mitochondria. The function of the mitochondrial DNA is to transcribe the RNA necessary for the constitution of a mitochondrial-protein biosynthetic system, and perhaps to furnish messenger RNA for the translation of certain proteins in the inner membrane as well as some that may function in the protein biosynthetic system. A minimal amount of DNA must remain within the mitochondrion both for this reason, and possibly also because some of the proteins must be synthesized by translation of RNA simultaneously with its transcription from DNA as occurs in bacteria.