Mitochondrial DNA and recombination

Question

Firstly I could do with a brief description of mitochondrial DNA. How does the structure of DNA in mitochondria compare to animal DNA (for the sake of simplicity let's say human - some animals might have unusual DNA structure) and what living organism is the mitochondrial genome most akin to? (Circular like bacteria maybe?) and are the mitochondria within a single human homogenous?

Secondly, and most importantly to my aim, does the mitochondrial genome recombine in anyway? Is the process of recombination affected by its structure? Are there any patterns/rules in mitochondrial recombination? Is there DNA transfer between mitochondria which could have a similar effect?

I am trying to think about how male deleterious alleles can spread differently in diverged populations given potential recombination to female beneficial (or recombination away from female deleterious mutations).

Answer 1

Mitochondrial DNA is circular, like bacterial DNA, and around 16.6 Kb long. It codes for 37 genes in total but the majority of these are simply the machinery for gene expression (curiously, mitochondria have a slightly different codon->amino acid translation 'table' to nuclear DNA) and so just 13 code for proteins. Single mitochondria usually have multiple copies of the DNA.

Mitochondria are thought to be most closely related to the bacteria Rickettsia, an obligate intracellular parasite that causes typhus. Mitochondria are so degenerate, however, that they've lost nearly all the genes that normal bacteria have.

While there is some small level of heterogeneity in mitochondria in a single person because mitochondria are exclusively transmitted down the female line most people typically only start with a single clonal population of mitochondria. Because of this, there is no other mitochondrial population present that the mitochondrial DNA could undergo recombination with.

Mitochondrial DNA does undergo recombination with nuclear DNA, however, in that genes can be transfered from the mitochondria to the nuclear DNA and because of this the majority of genes necessary for mitochondrial function are now coded not on the mitochondrial DNA but on the nuclear DNA and their protein products transported into the mitochondria after translation.

Answer 2

Addition to Jack Adley's answer:

Evolutionarily, mitochondria are thought to be descended from α-proteobacterium (Rickettsia are also descendants of this bacterium).

There are evidences that nuclear genes have also been imported to organellar minigenome. This can also happen because of recombination. I am not sure about the case of mitochondria, though. Since the current hypothesis is that organeller DNA that got leaked because of organeller damage, got incorporated in the nuclear genome, there not much of a likelihood that active recombination takes place between the two. However inter-mitochondrial recombinations have been reported, which perhaps happen during mitochondrial fusions.

Answer 3

Mitochondrial chromosomes do not undergo recombination. They replicate their DNA in an asexual manner, producing identical copies (except for the occasional mutation). Mitochondria do not exchange or transfer DNA between themselves as they have no mechanism for that.

Very rarely a piece of DNA might get out of a mitochondrion and into another or into the cell's nucleus. The latter type of transfer is believed have happened multiple times in the more than a billion years eukaryotic cells have existed. Most of the genes needed for mitochondrial functioning are located in the nucleus, not in the mitochondria, and many of those genes probably had such a mitochondrial source. Having mitochondrial genes in the nuclear genome is advantageous, exactly because it allows them to undergo recombination and thereby allows improvements to spread through a population of organisms.

Answer 4

Recombination of mitochondrial DNA does seem to be possible, but it doesn't seem anywhere near as common as nuclear recombination. This is important because mtDNA is subject to much higher mutation rates, and so would be subject to the same kind of degeneration you see in the Y chromosome - i.e. Muller's Ratchet, if it wasn't so small. Nevertheless mtDNA does generate considerably over a multicellular organisms life span and this is one possible mechanism for aging.