There is a theoretical possibility that a mitochondrion could eliminate
those mutations that cause a distortion of the double-stranded mtDNA,
because such mutations could be physically detected by a set of enzymes.
The enzymes could then destroy the entire double-stranded copy of the
circular mtDNA genome containing the distorted mtDNA, or alternatively
tag it for 'export' and cause it to be ejected from the mitochondrion.
(The ejecting of genetic material through a conjugation tube is a known
bacterial behaviour. Mitochondria are thought to have a bacterial
ancestor.)
The destroyed or ejected copy of the mtDNA genome could be replaced,
sooner or later, by the replication of an unmutated copy in the same
mitochondrion.
It is interesting to note that enzymes in the nucleus try to do the
same thing. But of course, having detected a chromosome distorted by a
mutation they cannot destroy the chromosome. If they did that they
would not have an unmutated copy to replicate and so make good the
loss. The homologous chromosome is not an identical copy. So the best
they can do is to 're-pair' the distorted part of the double-stranded
nuclear DNA. This is a chancey business because they cannot 'know'
which base or bases to leave in and which to cut out. But it is better
than doing nothing.
Oxygen free radicals produced by the respiratory chain in mitochondria
can damage mtDNA and thus increase the mutation rate. It is conceivable
that a high mtDNA mutation rate in somatic cells is an indication that
this hypothetical system for eliminating mutations that cause
distortions in double-stranded mtDNA has failed in those cells.
It is perhaps worth noting that if such a system is at work in the
mitochondria of female germline cells it might be applied with
different vigour in different species of animal. In that case it is
possible that the mtDNA mutation rate in humans is much lower than has
been assumed in studies of human evolution based on the 'mtDNA sequence
divergence rate'. As far as I know this rate has never been objectively
measured.
Some indication of whether mitochondria do use the hypothetical system
outlined above could perhaps be gained from cell cultures in vitro,
using X-irradiation as a mutagenic agent to accelerate the mutation
rate.
Andrew Gyles
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