From Prokaryotes to Eukaryotes--Development of the Nucleus

One of the factors used to differentiate between prokaryotic and eukaryotic cells is the existence of a nucleus surrounded by a membrane, containing individual chromosomes, in eukaryotes.If I recall correctly, primitive eukaryotic cells obtained as symbionts other prokaryotes, which became organelles like mitochondria, chloroplasts, etc. As far as I know, the incorporation of a symbiont is not how the nucleus developed.My question is whether there exist any theories that address the evolutionary development of the nucleus.

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"Chance is a minor ingredient in the Darwinian recipe, but the most important ingredient is cumulative selection which is quintessentially nonrandom."
--Richard Dawkins, The Blind Watchmaker

Thanks for the sources Wounded King (a bit late, I knowsorryI went through another I lost my password phase but had intended to respond immediately). I have another [slightly tangential] question. Again it’s about the evolution of eukaryotic cells. An endosymbiotic origin for mitochondria and chloroplasts certainly makes sense, given that they contain their own DNA that replicates when the organelle divides, and that the genes that the organelles encode are actually transcribed inside of the organelle, and are subsequently translated on organelle ribosomes.My question is: how does this incorporation work? I mean, on the surface it seems soLamarckian. How does the DNA within the nuclear membrane come to know how to encode for the development of chloroplasts, for example?My original guess was that perhaps the chloroplast’s DNA made it into the nucleus of the cell and some sort of synthesis occurred. However, I’m almost certain that DNA can’t pass through the nuclear membrane, so now I’m thinking perhaps it was the RNA analogue of the chloroplast’s DNA that made it into the nucleus and then some synthesis occurred (perhaps via some sort of reverse transcription). I’m quite sure RNA can cross the nuclear membrane. I’m probably way off the mark, but none of the textbooks I have seem to address this issue.My question, put simply, is what is the mechanism by which nuclear DNA comes to code for organelles incorporated by endosymbiosis?This message has been edited by Snikwad, 02-04-2005 15:57 AM

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"Chance is a minor ingredient in the Darwinian recipe, but the most important ingredient is cumulative selection which is quintessentially nonrandom."
--Richard Dawkins, The Blind Watchmaker

Which I suppose is what I’m asking. How does it do this? mRNA has the potential to cross the nuclear membrane because before leaving the nucleus, it has added to it around 200 or so adenine nucleotides to the 3’ end of it. If I recall correctly this is called a poly-A tail, and it’s what controls how mRNA moves across the nuclear membrane. Note that as far as I know, this is just movement from the nucleus into the cytoplasm. If DNA is capable of moving from the mitochondria and/or chloroplasts into the nucleus, how does it do this?

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"Chance is a minor ingredient in the Darwinian recipe, but the most important ingredient is cumulative selection which is quintessentially nonrandom."
--Richard Dawkins, The Blind Watchmaker

Yes and no.It’s my understanding that as time went on, a whole lot of the genes that were present in the original bacteria that would ultimately become the chloroplasts and mitochondria came to be incorporated into the nuclear genome of the cell, and so fewer and fewer parts of the mitochondria and chloroplasts were encoded by the organelle genomes themselves. The question I’m asking (which I may not have expressed clearly enough) is how does this incorporation work? How does organelle DNA become incorporated into the nuclear DNA? Can organelle DNA cross through the nuclear membrane? If so, how does organelle DNA cross through the nuclear membrane?I think I see what you’re saying. Since the whole cell divides it is not necessary for the nuclear genome to end up coding for the organelles, and the organelles could just divide independently in the cytoplasm, and when the cell splits, each cell ends up with half of the organelles, which can then independently produce (by which I mean, the organelles’ genomes could produce) more of the organelles. This would preserve the existence of organelles within the cells throughout all subsequent generations. The issue here is that the nuclear DNA came to incorporate the genes present in the mitochondria and chloroplasts. How does this happen?Note that I am not challenging a symbiotic origin for mitochondria and chloroplaststhere’s no other plausible scenario that I know of. I’m just interested in the mechanism by which organelle genes are incorporated into nuclear DNA.Did that help clarify what I’m asking?

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"Chance is a minor ingredient in the Darwinian recipe, but the most important ingredient is cumulative selection which is quintessentially nonrandom."
--Richard Dawkins, The Blind Watchmaker

From Mitochondrial genomes: anything goes by Burger et al., The question is not whether the incorporation happens. I’m asking about the chemistry that is involved in incorporating mitochondrial DNA, for example, into the nuclear DNA. How does mtDNA make its way into the nuclear genome? More specifically, what allows it to cross the nuclear membrane? As I said above: I’m wondering if something similar happens to DNA so it can cross the nuclear membrane. That’s my basic question. What allows mtDNA to cross the nuclear membrane?My second question is, once mtDNA is inside of the nucleus, how is it incorporated into the nuclear DNA? What is the chemical process that occurs? This is assuming that this is how it happensI’m still wondering whether it is the RNA analogue of the mtDNA sequence that crosses the nuclear membrane, and then is subsequently reverse-transcribed back into DNA, and then incorporated into the nuclear genome. Yes, yes, it seems ridiculously roundabout, but I still would not be surprised if this is what went on. Depends on what you mean by the fence.Yes, it is evident that evolution is currently the only theory capable of explaining the observed biodiversity on earth. Unless you’ve got a better one that explains all the evidence in a scientific manner, I’m sticking to evolutionary theory.However, don’t let the fact that I quote Dawkins fool youI’m a Christian myself. So I suppose we’re on the same side of the fence in that respect. Hyper-parsimonious indeed. Just remember that the theory that explains everything, explains nothing. God did it, won’t get you very far in understanding the universe we inhabit.

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"Chance is a minor ingredient in the Darwinian recipe, but the most important ingredient is cumulative selection which is quintessentially nonrandom."
--Richard Dawkins, The Blind Watchmaker

Thanks pink sasquatch! This is precisely the kind of answer I was looking for. It’s so blatantly obvious I don’t understand how I could have overlooked it. So I take it’s an insertion mutation that gets overlooked in the DNA proofreading process. Where is it likely that this insertion occurs? When DNA replicates you end up with two double-stranded molecules of DNA, each one with one strand of old DNA. Would the mtDNA have a tendency to be inserted into the old DNA strand? If so, how (by which I mean by what chemical process) is the old DNA strand broken up so as to permit the insertion of the mtDNA sequence? If, on the other hand, the insertion occurs on the new complement segment, how does it manage to combine with the base pairs on the old DNA strand? Let’s say we start off with the following DNA molecule: AATTCGATTCCG
TTAAGCTAAGGCAnd when the double-stranded DNA splits, you get: AATTCGATTCCGAnd TTAAGCTAAGGCNow, let’s say we want to insert the following sequence from mtDNA: GACTTAThe first strand copies perfectly, so we end up with a double-stranded DNA molecule that looks like AATTCGATTCCG
TTAAGCTAAGGCBut the mtDNA sequence is inserted into the second complementary strand that is being constructed: TTAAGCTAAGGC
AATTCGGACTTAATTCCGHow would the DNA molecule come together to form a double helix if the base pairs don’t match up?Edited to add:Ok, I’ve been doing some reading, and perhaps the way in which the double helix forms is through something similar to excision repair. Carrying on with the example I used: TTAAGCTAAGGC
AATTCGGACTTAATTCCGMaybe the error is recognized and, using 3’ and 5’ nucleases, either side of the error in the original [red] DNA strand is cleaved, and the gap is filled using the complementary [yellow and orange] DNA strand as a template.So you end up with TTAAGC
AATTCGGACTTAAnd then, using the complementary [yellow and orange] DNA strand as a template: TTAAGCCTGAAT
AATTCGGACTTAAnd you end up with a DNA molecule capable of forming a double-helix. And with the crazy mtDNA mutation to boot.Is this anywhere in the ballpark?This message has been edited by Snikwad, 02-07-2005 01:28 AM

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"Chance is a minor ingredient in the Darwinian recipe, but the most important ingredient is cumulative selection which is quintessentially nonrandom."
--Richard Dawkins, The Blind Watchmaker