Mitochondrial DNA

Factual Questions
1

Someone told me that we don’t know if each and every
mitochondria is genetically identical in each cell.

Is that correct?

I thought there might be differences between mitochondria in different kinds of cells, i.e., each mitochondria in the liver cells will be identical to all other mitochondria in all the other liver cells. (But now I can’t find any references to the point.) Is that correct?

Thanks,
Jois

2

The DNA in all the mitochondria in one person’s body is identical (except for occasional mutations), but the mitochondria themselves can be very different. Mitochondrial DNA has a pretty limited number of genes on it, and if I recall correctly, none of them code for entire, functioning proteins. They just code for polypeptide subunits that associate with other subunits coded for in the nuclear DNA to make functioning proteins. As far as I know, the polypeptides coded for in the mitochondrial DNA are all too big to be efficiently transported in from the cytoplasm, so they’re made in place. If it wasn’t for this, mitochondrial DNA would have disappeared long ago.

The proteins and polypeptides that enter from the cytoplasm are different in different cell types, so mitochondria can vary quite a bit from one cell type to another.

There are some mitochondrial disorders where a defect in the mitochonrial DNA causes problems in only one cell type because the subunits made in the mitochondria don’t mesh with the ones coded for by nuclear DNA in certain tissues, but work fine in others.

3

Several of the mitochondrial genes code for entire, functional subunits of the F0-F1 ATP Synthetase complex . There’s no “just” about these gene products since they comprise the very core of eukaryotic energy metabolism.
Mitochondria come strictly from the maternal side, so there’s no obvious mechanism for heterogeneity. Also, with DNA probe technology as good as it is you’d expect that references to such heterogeneity would be easy to find. They’re not.

4

I’m not sure if this is exactly what you’re looking for, but “mtDNA heteroplasmy” means having two different forms of mtDNA in the same body. Tsar Nicholas II had heteroplasmic mtDNA, as did (apparently) his mother and brother. The two versions differed in only one base pair.

see, for example, http://www.faseb.org/genetics/ashg00/f1929.htm

5

Nice tries, still clear as mud!

Would the mitochondria in my liver cells all look exactly alike?

Would the mitochondria in my liver cells look exactly like the mitochondria in my muscle cells?

And in case it makes any difference:

Would the mtDNA in my liver cells all look exactly alike?

Would the mtDNA in my liver cells look exactly like the mtDNA in my muscle cells?

Please try again!

Jois

6

Would the mitochondria in my liver cells all look exactly alike?

Liver is kind of a bad choice for this question because the liver is so amorphous/changeable and carries out so many different biochemical reactions. However, the shape content and structure of of your liver mitochondria depend on what the liver is involved in metabolizing. If it’s detoxyfing strange chemicals the mitochondria will look different than if it’s doing glycogen metabolism.
Would the mitochondria in my liver cells look exactly like the mitochondria in my muscle cells?

NO, they’ll look much different because the reactions that the respective mitochondria are carrying out are much different. The protein content of the mitochondrial membrane varies between cell types.

The DNA of mitochondria in different cells is the same, but that’s no guarantee that the same proteins, polysaccharides and lipids will be put together in the same proportions to make mitochondria in every cell.

7

Better, thank you!

My college biology text has the mtDNA organelles in animals look exactly like the mtDNA organelles in plants. (Curtis & Barnes about 1987)

Function can or will differ according to the needs of the nuclear DNA in the cells where the mtDNA organelles are located, right?

Jois

8

Something like that. Shape depends on function. You seem to be attempting to interpret nuclear DNA as some sort of central processing unit in a single cpu paradigm for cell function. It’s more of a template for some of a cells possible components that gets different pieces expressed depending on the cells type, history and the external environment. There’s no “thinking” about the cells needs involved. It’s all an extremely complicated stimulus->response system.

9

OK!

Please check page 7 or the contribution of Dr. Sims -

http://mmserver.cjp.com/gems/medpsych/2.1.Kim.pdf

(And I do not have MELAS! I thought what Bibliophage did was a good idea and found this site under “mtDNA heteroplasty” in Google.)

I can (after reading that) understand the problems with the disease and mtDNA problems in general are not easy to diagnose or see because of heteroplasty. That the clinical expression will depend on reaching a mutant burden level - enough wild-type mtDNA remain functioning to cover the lack or changed functioning in the mutants.

The problem that remains is: do we inherit heteroplasty in mtDNA or do we inherit homoplasty and it mutates easily?

I can see that we could inherit the mutated mtDNA and I can see that we could inherit the wild mtDNA. Can some people inherit both at the same time?

Thanks,

Jois

10

That’s a very nice disease mechanism !

From page 7, it sounds as if people are inheriting a mix of mitochondria which can shift over time, but reference 2 (Archives of Neurology) would be the place to find out for sure.

11

Thanks, I doubt I’ll catch up with Archives of Neurology, but I’ll keep looking.

Jois