Mitochondrial DNA replication

The replication of mt DNA is carried out by DNA p-ax that operates independently of nuclear DNA p-ase, as mt DNA synthesis can take place anytime during the cell cycle i.e. they are not restricted to S-phase when nuclear DNA replicates.

In 1971 Jerome Vinogard and Piet Borst independently discovered that mt DNA mols exhibit a small 100p.displaced from the main circle and mode of replication is semi conservative. Denaturation of DNA mols causes a small fragment of S.S.DNA to be released, suggesting that loop is caused by displacement of one of the two strand of double helix, therefore it is termed as ‘D’ loop (Displacement loop)

Mitochondrial DNA is replicated by the DNA polymerase gamma complex which is composed of a 140 kDa catalytic DNA polymerase encoded by the POLG gene and two 55 kDa accessory subunits encoded by the POLG2 gene. The replisome machinery is formed by DNA polymerase, TWINKLE and mitochondrial SSB proteins. TWINKLE is a helicase, which unwinds short stretches of dsDNA in the 5′ to 3′ direction. All these polypeptides are encoded in the nuclear genome.
mt DNA codes for ribosomal RNA’s, t-RNA’s and small mitochondrial polypeptides

Several evidence suppose the conclusion that mt DNA serves as a template for transcription of mt RNA’s. 2nd-mitocondria contains their own RNA-p-ase that can be distinguished from nuclear. RNA’s by differing susceptibility to inhibitors. 3rd ethidium bromide inhibits mt DNA synthesis and not nuclear DNA synthesis supports the conclusion that independent RNA synthetic pathways exist in nuclear and mitochondria.
mitochondria synthesis proteins using a unique sets of ribosomes, tRNA’s and protein factors.

The mRNA’s transcribed by mt DNA are translated into polypeptides within mitochondria using. Protein synthesizing machinery that differs from the machinery present in the cytorol.
Ribosomes Mitochondrial ribosomes differs from cytoplasmic ribosomes in size & chemical contraction.