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As I was studying for my GRE exam, I was reading on Mitochondria. I then remembered that they undergo replication similar to bacteria. So I googled for info and found on Wikipedia that there is a " near-absence of genetic recombination in mitochondrial DNA" https://en.wikipedia.org/wiki/Mitochondrion#Replication_and_inheritance

So then, if there is a lack of genetic diversity, how can these mitochondria still be functional? If we assume they are affected by random mutations as DNA can be. Is it the case that they replicate infrequently enough to not suffer ill-effects of low diversity?

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Your claim that "there is a lack of genetic diversity" in mitochondria is not correct. There is no connection between recombination and mutation rates per se. In fact, mitochondria have much higher mutation rates than nucleus and different types of mitochondrial DNA can even co-exist in one organism, a phenomenon called heteroplasmy (which is found at a frequency of ~90% in human).

But: there is a connection between mutation fixation rate and effective population size. Because mitochondria are normally inherited through maternal line, effective population size for their genome is smaller than that of the nuclear genome, and thus any mutations in the mitochondrial DNA reach fixation faster, but again this is counteracted by high mutation rates.

There is no problem with functionality: as always if something gets broken (and it is easy to break something in the mitochondrial genome, since genes are normally tightly packed in it) respective genotype is eliminated or reduced in frequency by natural selection. In the case of mitochondria this selection acts both upon the level of individual organisms (intra-individual somatic pool of mitochondria: positive and negative selection) and the level of populations (mostly purifying selection in humans). It is nevertheless true, that given the virtual lack of recombination, selection becomes the only force of purging deleterious mutations.

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    $\begingroup$ Since mitochondria have a very small genome, with very little intergenic DNA, the mutations are likely to affect the essential respiratory genes (amongst others). Since many of these genes perform vital functions, many mutations may not be tolerated. I am not sure about it but would this also lead to loss of diversity? On the other hand, I had read somewhere that mutation rate in mitochondria is higher: perhaps due to high ROS concentration (my guess). Basically, how strong is the selection pressure against variation i.e. is there a sort of permanent bottleneck? $\endgroup$ – WYSIWYG Jul 24 '15 at 8:58
  • $\begingroup$ @WYSIWYG Yes, the fact that the mt genome is mostly coding reduces the potential allelic and nucleotide diversities in comparison to a randomly mutating sequence. And yes, mt mutation rates are much higher than in nucleus. Concerning the bottleneck: I've added a ref to a review. Nevertheless, there are always enough neutral mutations (3rd position and others) to contribute to the genetic diversity. $\endgroup$ – har-wradim Jul 24 '15 at 9:59
  • $\begingroup$ @WYSIWYG Yea I guess I wasnt thinking about how important mitochondria are, and that a mutation would have to be purged out. $\endgroup$ – Ro Siv Jul 24 '15 at 17:12

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