Some mutations sound too good to be true

Faith, I'm not sure, but I was under the impression that the DNA replication process is such that EVERY TIME a cell divides, the daughter cells will have slightly different (mutated) DNA due to the copying "errors."But maybe I got the wrong impression from some of the material I read so long ago. I'm having trouble understanding what you're asking here. Perhaps you have an intuitive definition for the word "mutation" which equates to "an ugly mistake in body form"...like a cow being born with an extra leg growing out of its back...or a three-eyed fish...or something.At the genetic level (which one can't know anything about unless one first knows there is DNA), a mutation just means that the DNA is not the same...is changed somehow (the order of the nucleotides is different).If the mutation (change in one or more of the nucleotides on the DNA strand) occurs because of the replication process, then it is called a "copying error." This occurs, AFAIK, in all cells...it does not necessarily mean there will be a corresponding ugly, harmful body change or structure.--Jason

The mistakes can happen during replication of the chromosome, but it can happen at other times too, for example when repairing damage to the DNA caused by chemicals, UV irradiation, ionising radiation, chemicals etc.It is impossible to estimate how often mutation happens and ho often it is deleterious, versus neutral or positive because the really deleterious ones don't survive to be sampled and so you don't even know they existed.I can't really help you with the history of mutation and the molecular basis for it - I have enough trouble just managing to cope with the present day facts about it!I'll try to get on to "copying" errors tomorrow night. Can you find some time to have a look on the internet for some piccies of the structure of DNA? I don't mean intricate diagrams, just simple schematics showing the double stranded nature of the beast? Or maybe another of you kind people out there could supply Faith with one. If you can lodge that in your head, it could make it simpler for you to follow the next installment.If at any time I go too fast, rein me in and ask questions.

OK, I HAVE to leave EvC for a while or I will turn into a pumpkin or something. And yes this is all very useful and I hope to think about it more.But it still leaves the questions how often these mistakes occur in the replication process normally, and how many of these mistakes are good, vs. neutral vs. bad.{Edit: Never mind, I think you just answered this above.}Oh, and another question left over from Crashfrog's chart. The same chemical is created by different combinations. How can this be?This message has been edited by Faith, 09-20-2005 05:27 PM

Trixie,in one of the posts I assumed that something meaningful could be said about how the error-rate in replication itself might also be subject to natural selection. In the sense that replication-mechanisms that are now in existence, have been selected on the basis of the fact that their error-rate produces an optimal balance between introducing variations and producing meaningless garbage, on average.Has that sort of thing been discussed in literature? (is this still enough on-topic, btw? lol)

Change for a dollar.
Four quarters.
Ten dimes.
Twenty nickles.
One Hundred pennies.or any combination of the above totally 100 cents.

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Aslan is not a Tame Lion

No, I just think its being understood as a copying error is relatively new. In the "olden days" I had the idea it was something that randomly changed or destroyed a gene, but a gene wasn't known to be this string of base pairs on a long DNA strand, though it was a location in a chromosome. What caused it included radiation as a possibility. But maybe I just didn't read deeply enough on the subject back then.This message has been edited by Faith, 09-20-2005 07:53 PM

I thought chemical formulas were a little more exact than that. H20 is water and no other formula creates water. On the amino acids chart TGG is Tryptophan and no other combination makes Tryptophan. Just funny that you get, say, glutamine, out of both CAA and CAG, and Leucine out of six different combinations. Seems like there should be an explanation.

Lemon jello vs orange jello.This message has been edited by jar, 09-20-2005 07:05 PM

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Aslan is not a Tame Lion

quote:

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I thought chemical formulas were a little more exact than that. H20 is water and no other formula creates water. On the amino acids chart TGG is Tryptophan and no other combination makes Tryptophan. Just funny that you get, say, glutamine, out of both CAA and CAG, and Leucine out of six different combinations. Seems like there should be an explanation.

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I think what you're asking for is an explanation into how coding works. The reason you can code for glutamine in different ways is that a lot of what happens on large molecules like those is determined by shape. It's like turning a key in a lock, the latches have to be in the right places. If they're similar, you can code for a molecule in different ways.

Great question, very insightful. Allow me to try to answer it.Amino acids are "prepped" for use in the cell by being attached to little strands of RNA called tRNA; the tRNA has a outlooped end with the complimentary codon for the amino acid that attaches to it, something like this:The bottom end, the "anticodon", is where the tRNA is bound to the mRNA; the tRNA tags line up along the mRNA and in doing so the amino acids are lined up next to each other, where they form the chemical bonds that turn them into proteins. The top end, the acceptor end, is lightly bonded to the amino acid.It's like a little tag or label for the amino acid that the cell uses to push it around. How are aminos attached to the appropriate tRNA tags? I don't know how that works.

It starts to get pretty hairy at this point, but it's really interesting. What's rather counterintuitive is that AA end of the tRNA molecule itself is effectively neutral with regard to amino acids; it doesn't care which one gets attached. Charging of the amino acid end of the tRNA is performed by an enzyme, the tRNA synthetase, which makes the call after 'sniffing' the DHU loop of the tRNA. If you simply had to point to a single place in the whole process as the place where the actual 'translation' from nucleotide triplet to amino acid takes place, this would seem to be the most likely candidate.

Well, when were talking about bacteria, it's perfectly reasonable for no copying errors.However, my post was in regards to their immunity to the antibotic.So by "exactly the same", I mean exactly the same amount of resistance. Two cells could be radically different in size, color, etc. But none of that is relavent to the scenaro I'm describing

My understanding is that bacteria do not possess the error-correction mechanisms of eucaryotic cells.

I think yours is the clearest explanation but it is still puzzling how one kind of substance can take the place of a completely different kind without changing the overall result.Is jar right that there is a difference -- on the order of orange versus lemon jello? Or is there none at all if the shape of the molecule is the important thing?

I get a lot out of diagrams generally speaking, but I have to admit that I don't have any idea what this one is trying to tell me, or in fact how your post answers my question about how different codons can produce the same chemical.