Mitochondrial synthesis of nucleic acid and protein synthesis

Mitochondria are equipped with a special genetic apparatus. Mitochondrial genes encode special ribosomal RNA (15S and 21S), the ribosomal protein var-a, all mitochondrial tRNAs (more than 20), three of the nine subunits of cytochrome oxidase, apocytochrome B, and a few subunits of ATPase. Most of the mitochondrial proteins are encoded by nuclear DNA and synthesized in the cytoplasm.

Replication of mitochondrial DNA
Mitochondrial division is not coupled to cell nuclear division. Mitochondrial DNA (mtDNA) is found in the matrix. Human mtRNA is circular dsDNA composed of 16,569 bp. The light (L) and heavy (H) chains are distinguished according to their density when centrifuged in a CsCI gradient. Replication begins at a defined position (lori-H). It starts with displacement of the H chain by the newly synthesized H chain (so-called D-loop). The L chain is a template chain. After the DNA-polymerase &gamma; reaches 2/3 of the total length of the DNA, it releases ori-L, from which the synthesis of the new chain L starts in the opposite direction. By the DNA-gyrase, both of the new double helixes are gyrating into the superscrews.

Mitochondrial transcription
Transcription is catalyzed by a special mitochondrial RNA polymerase. In human mitochondria, it is initiated from only two sites, and the resulting transcripts are then split and edited into functional RNA. These are two rRNAs, 22 types of tRNAs and 13 different mRNAs, containing very few untranslated sequences.

Mitochondrial translation
Some mitochondrial tRNAs read up to 4 kinds of codons. The mitochondrial genetic code has its own specificities:
 * AGA and AGG are terminators, outside the mitochondria they encode arginine;
 * UGA, on the other hand, is not a terminator in the mitochondria, but a codon for tryptophan;
 * AUA encodes methionine, not isoleucine.

None of the mitochondrial translation products leave the organelle. Most of the mitochondrial components are encoded by nuclear DNA and are synthesized by eukaryotic cytosolic ribosomes as protein precursors with signal sequences. They are then transported into the mitochondria by a process requiring ATP. It is possible that some mitochondrial genes moved into the nucleus during the evolution of life and vice versa. The expression of mitochondrial and nuclear genes for mitochondrial components is coordinated.

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