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Mitochondrion- Imprinted to DNA?

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Mitochondrion- Imprinted to DNA?

Postby futurezoologist » Sun Mar 15, 2009 7:54 am

A question that has been lingering in the back of my head for a while now:

If a mitochondrion cell was supposibly engulfed by a eukaryotic cell and 'enslaved' into energy production then how was this mitochondrion cell imprinted into the DNA? or have i got my facts wrong somewhere?
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Postby MrMistery » Sun Mar 15, 2009 9:20 pm

well mitochondria still possess their own genes and are able to carry out protein synthesis in a completely independent manner from the rest of the cell. However, some genes have been transferred from the mitochondrion to the nucleus over evolutionary time.
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Postby futurezoologist » Mon Mar 16, 2009 8:54 am

Yeah, the point that i dont get though is how the mitochondria was able to 1. get into the cell without being decomposed by lysosomes(chance?) and 2. have its circular DNA joined to the DNA of the eukaryote(or is it?)?

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Postby biohazard » Mon Mar 16, 2009 1:33 pm

1. Getting into a cell was (and still is) pretty straightforward: many unicellular organisms (and some cells of multicellular organisms) even today phagocytose bacteria and other such particles. As a bacteria you only need to have either a bit of luck and get engulfed by a cell that for a reason or another cannot degrade you (e.g mutations that lead to dysfunctional lysosomal components), or you need to develop your own means to prevent digestion (e.g. enzymes or cell wall components that prevent endosome fusion with lysosomes.) There are bacteria that readily survive inside other cells, some do this even inside human macrophages that are evolved specifically to phagocytose and degrade bacteria and other stuff. Mycobacterium tuberculosis is one such bacterium.

So here we have a means to enter the host cell and stay alive. Let's see the DNA part.

2. In the beginning, the bacteria that were to become mitochondria probably just multiplied within the host cell and were split in two after each division of the host cell. This is basically how the mitochondria do even today. But back then, they had all the genes they needed in their own genome. During the course of time, they eventually lost all the genes that weren't necessary (such as enzymes or some structural proteins that also the host cell could produce). Now and then a bacterium living inside the cell would undergo lysis and die, and its DNA burst into the cytoplasm of the host. Usually the host's enzymes would break down the DNA, but by chance a piece of it may have got transferred into the host genome by means of recombination. Sometimes this might have killed the host or prevent it from multiplying, but sometimes a new gene product needed by the bacterium would have been coded by the host genome - and that is how eventually some of the mitochondrial genes might have ended up to the host cell genome.

Alternatively, chance mutations in the host genome may have started the cell to encode something that was actually useful for the endosymbiont (= mitochondrion), and thus it was evolutionary "feasible" to keep coding that product. At the same time, the mitochondrion itself may have ceased to code some similar component, because the host cell now did it. And so we now see "mitochondrial" genes in the host DNA, even though some of them may have evolved independently, not directly transferred. I'm not sure how likely this event would be, but hey, evolution has all the time in the world ;)
Last edited by biohazard on Tue Mar 17, 2009 1:43 pm, edited 1 time in total.
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Postby futurezoologist » Tue Mar 17, 2009 12:23 pm

Ahh thanks biohazard, makes much sense now :) . Just one thing though: do the mitochondria still reproduce themselves then or are they now completely coded for by the host cell and if they are then why do the mitochondrion still have DNA?

Thanks
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Postby biohazard » Tue Mar 17, 2009 1:52 pm

As far as I know, (in humans) the mitochondrial genome encodes only certain membrane proteins (which are mostly parts of the respiratory chain) and several types of tRNA/rRNA, so they rely on their host cell's nucleus for most of the protein synthesis and use these building blocks when they divide.

Because the main function of mitochondria is energy production, I think it makes sense that they have conserved only the genes that are required for their specialist function.

The exact content of mitochondrial genome varies between the species, though.
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Postby MrMistery » Tue Mar 17, 2009 11:35 pm

you need to also make a clear distinction between protein synthesis and "reproducing themselves". there are some mitochondrial mutants (the name eludes me now, and I don't have any textbooks around to easy look it up) that have mitochondria which completely lack DNA. Naturally those cells have a lot of problems, but the mitochondria still reproduce. this is because growth of a mitochondrion is completely dependent on proteins and lipids from the rest of the cell, and fission by itself doesn't require anything. it has been suggested that peroxisomes divide this way (assuming peroxisomes are indeed ex-endosymbionts, which I don't think we have enough evidence so far for...)
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Postby futurezoologist » Wed Mar 18, 2009 11:03 am

So then, (maybe i have my train of though wrong)in the mitochondrial mutants you were talking about Mistery; what determines the shape which these amino acids are packaged into(cristae, double membrane etc.) to create a mitochondrial functioning cell and for that matter what instructs the mitochondrion to undergo fission (does the 'host' cell code for these functions)?
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Postby biohazard » Wed Mar 18, 2009 2:07 pm

The host cell pretty much dictates these functions like it does with all membrane-bound organelles within its cytoplasm, the main difference being that mitochondria divide and many other organelles are assembled "from scratch". If these functions were not controlled by the host, the mitochondria might excessively multiply and kill the cell altogether.

Cristae and membranes are synthesised similarly as other cellular components: by guidance of the protein folding and auxiliary proteins.
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Postby futurezoologist » Thu Mar 19, 2009 10:22 am

I see, thanks very much for your help guys.
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