posted
I think I've asked this before, but I didn't get a satisfactory answer. Or maybe no one had an answer that my biologically-uneducated self could understand ...
So, in high school biology, I learned that humans have 46 chromosomes, and chimpanzees have 48. So I'm assuming that, more than likely, our common ancestor had either 46 or 48 chromosomes.
How did one of the branches of that ancestor's descendants gain or lose a pair of chromosomes without wreaking havoc on their ability to function? I mean, a mutation here and there can be pretty minor, but a whole chromosome? Isn't there important stuff on those? How do you just lose one or gain one and still go on functioning ... and then even beat out the competition to be naturally selected?
Do we have an official answer to this? I've heard creationists bring it up before, and honestly, I have no idea what to say about it ...
Posts: 1907 | Registered: Feb 2000
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posted
To understand this you have to understand the way chromosomes, genes and DNA work. And its bloody complicated stuff that we're still working out. I'll try to explain it to the extent that I understand it, dunno how well I can do but here goes:
Basically, we have a ton of DNA, several billion basepairs (2.3 billion is the number I found online) spread out on those chromosomes. Not all of those basepairs encode genes that actually do anything. A lot of it is random 'junk' DNA that comes from many places. Gene's that got destroyed by random mutations, viral DNA that got inserted (possibly destroying other genes in the process) and then negated by mutation, transposons that travel through the DNA randomly copying and inserting themselves, so on and so forth. The result is that many genes are destroyed and lose their function. The actual DNA that was once those gene's remains on their chromosomes, but doesn't do anything anymore. Just sits there and continues to get copied. This stuff tends to build up the more DNA a species has. Humans have quite a bit of it. As do chimps.
DNA can also move. The mechanisms for this aren't entirely understood (by me at least). But I know from some volunteer research I did in a genetics lab at the university that we know gene's and DNA in general can shift around. Gene's found in the mitochondrian in some plants were found in the nuclear chromosomes in other plants further down the evolutionary chain and genes found on one chromosome in an earlier plant were found duplicated on several or moved to another in plants later on.
So the final answer to your original question, with all that background is: gene's that were truely necesary likely migrated to other chromosomes and the rest was just junk DNA that got destroyed. That is extremely simplifying it of course. If you want a better answer try looking it up online or digging up a genetics text book somewhere and browsing that.
Posts: 3295 | Registered: Jun 2004
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posted
I once saw a picture of a comparison of human and chimp chromosomes. Turns out that apparently one of the human chromosomes matches up very well with two chimp chromosomes (so half of human chromosome 2 looks like one chimp chromosome and the other half looks like a different chimp chromosome). Here's an answer to this question in layman's terms. Given that the other primates have 24 pairs, it's speculated that a common ancestor would have had 24 pairs and that two pairs merged somewhere between the ancestor and us, leaving us with 23 pairs but the same amount of genetic information. Even more interesting is the fact that there are sequences characteristic of telomerase (which normally occurs on the ends of chromosomes) in the center of human chromosome 2. See the paper Origin of Human chromosome 2. Also, a nice picture.
posted
Chromosomes are basically just a storage method for DNA (they provide other capabilities as well, but those are largely independent of the details of the storage).
As such, changes in chromosomes actually don't matter all that much. There are, in fact, oodles of species which can under certain circumstances have offspring with double the normal number of chromosomes, which breed true (its even more usual for polyploids to be sterile, though).
Also, humans have chromosomal abnormalities all the time, you've probably met dozens or even hundreds of people who don't have the normal number of chromosomes (this is usually due to sex chromosomes; there are XXYs and XYYs and X's, et cetera).
As for chromosome fusing, we've never seen it happen, but then again, it would be incredibly rare. However, we do know that absent telomeres, chromosomes will naturally fuse together. It is more than conceivable that some enzyme came along (likely due to a virus or somesuch) which, say, lopped off the telomeres of a couple of chromosomes (perhaps matching a particular pattern that appeared on the ends of those two particular chromosomes), and that those chromosomes then fused. So long as the fusing was symmetrical (which would make sense given this enzyme hypothesis), the cell shouldn't have a problem.
Posts: 15770 | Registered: Dec 2001
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posted
Since the pairing of chromosomes is so linked with the process of mating, how does a single, mutated instance of chromosomes shuffling get passed on to the next generation? I mean, let's say that our ancestor DOES have a couple of fused chromosomes. With whom did he mate? And how did their chromosomes divide and combine with one another in a zygote if they didn't line up?
Posts: 1907 | Registered: Feb 2000
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posted
Chromosomes don't actually "line up" in the way you'd imagine. That said, both polyploidy and aneuploidy are major causes of human reproductive failure and are generally fatal in humans (although they're actually quite common in some other species.) Many genetic disorders -- like Downs syndrome and Turner syndrome -- are chromosomal in nature and can be expressed sexually in children, although it's far more common for these disorders to result in sterility; I think Downs syndrome in particular is known for causing almost 100% sterility in males, with one or two notable and fairly famous exceptions.
But that's the key, really: even one or two notable exceptions mean that it's theoretically possible for the chromosomes to essentially "let it slide" in some way, thus creating a new hybrid. That this happens only rarely -- and, as a consequence, through a mechanism we haven't sufficiently observed to understand -- means that, thankfully, chromosomal changes aren't widespread in humans the way they are in plants. But it also means that they can happen.
Posts: 37449 | Registered: May 1999
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posted
First, if it were caused by a relatively specific pattern matching enzyme (which is a bit of a misnomer, as all enzymes are pattern matchers), then the cell division wouldn't be a problem, the pairs would both merge.
So instead of chromosome pair 1 and chromosome pair 2, we'd have chromosome pair 3 in which each member was a merge of members of chromosome pair 1 and 2.
As for mating, there's a number of possibilities. One is that the enzyme which caused the merge in the first individual's sex cells was still around, meaning when the sex cells combined the corresponding chromosome pairs in the other individual's sex cell got merged as well. Another is that two individuals with the same abnormality did happen to have sex (we are talking about millions and millions of years here; few people realize how awesome that span of time is). There are more, but those should get you thinking.
Posts: 15770 | Registered: Dec 2001
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