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DNA and Chromosome evolution

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

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DNA and Chromosome evolution

Postby Troy » Sat Apr 14, 2007 8:17 am

Hello all!

I'm new to Biology, so maybe the questions sound a bit stupid - sorry for that :)

I've stumbled upon an interesting short article about the possible danger of Y-chromosome in men loosing its genes throughout the evolution, because it is unpaired (the X-chromosomes are always paired and thus can correct random errors). That could inflict danger on reproduction, however MIT researchers claim that the Y-chromosomes have found an other way to survive by encoding the major genes as palindromes in DNA...

Now, what I can't understand is the following:

what I assume (please correct, if wrong):
1. DNA is a long double-stranded (ds) molecule. It splits into two single-stranded (ss) counterparts only during replication. This is the place where the mutations can take place, however there always remains the second single-stranded counterpart, which will probably remain unchanged.
2. chromosomes are nothing else than a complete (3 billion base pairs), double-stranded DNA molecules.

what I do not understand:
1. How to X-chromosomes couple? Can two double-stranded DNA's couple?
2. What is the danger with Y-chromosome? It is dsDNA, and is "secured" by the fact that mutation will occur only in a single strand, while the other one will survive. Why is the palindromic algorithm needed?
3. How does DNA implement palindromic algorithm? Does it simple bend and copy itself like that "__" into "U" transform.

It'd be great if you could help me or point an easy-to-read article, where everything is rigorously defined!

Thanks in advance!
Troy
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Postby Darby » Sun Apr 15, 2007 10:02 pm

You have a very narrow concept of mutations. Although a lot of mutations might fit your scenario (and one replicant would be mutated even then), there are lots of other types of mutations.

The non-pairing of the Y affects variation of some male traits, but why that would affect the chromosome itself, or make errors any more likely to be repaired, is unclear (but a lot of the stuff they're saying about unpaired chromosomes don't seem to make complete sense).

Homologous chromosomes like female X's pair during meiosis and can exchange pieces, including unequal pieces that can produce extra gene copies. This is when the copies separate, too - if one is mutated as you suggest and passed on, the other one isn't. You don't have a "good" copy to cover your "bad" one.

The palindromic part, I have no clue - couldn't tell you why it helps.
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Postby Troy » Mon Apr 16, 2007 4:24 pm

Darby, thanks for your reply.

From now on, i assume that everything written in "what I assume" section in the first post is correct.

Darby wrote:there are lots of other types of mutations.

happening not during replication? what are they?
is it true that their probability is much lower? (Unless the certain chemicals are pumped into the cell specifically to induce mutations)

Darby wrote:The non-pairing of the Y affects variation of some male traits, but why that would affect the chromosome itself, or make errors any more likely to be repaired, is unclear.


change in traits is a direct consequence of change in genetic code, isn't it?

Darby wrote:This is when the copies separate, too - if one is mutated as you suggest and passed on, the other one isn't. You don't have a "good" copy to cover your "bad" one.

sorry, I don't understand what you mean here :)
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Postby Darby » Mon Apr 16, 2007 4:55 pm

Probably the most common mutations happen during breakage repair of the DNA. Even without well-known inducers (like ionizing radiation or a whole list of chemicals), DNA spontaneously breaks and reconnects - there's a whole system in place to splice the pieces back together. But errors, both in splicing and in changes to the "cut" ends, should produce a decent amount of changes to the original DNA. And it's worse with multiple breaks.

Then there are large-scale errors, movements of bits of chromosome around, losses or additions of sections, integration of viral genes (and sometimes bacterial genes), a whole menu of others.

The point I made about the single Y is that pairing has, so far as I know, no effect on damage repair. It affects variation in individuals (since you're working with just single paternal alleles with selected male traits), but even male traits are hugely affected by genes on the other non-sex chromosomes, so that's a fairly minor issue.

But it's not like your cell knows how to fix a problem with your paternal Chromosome 3 by checking it against the maternal Chromosome 3. Paired or unpaired shouldn't affect repair.

For the last point, say that during replication, an error occurs in one of the new strands. That changes one of the two resulting chromosomes, but they have been made to be distributed into a new cell (both for mitosis and meiosis). The chromosome with the mistake will be going to one daughter cell, the one without it to the other. If the mistake produces a major negative consequence, one daughter cell line should be screwed up, and may even all die, but the other daughter cell line won't be affected. In mitosis, you may lose some replacement cells but not all; in meiosis, you may lose some offspring but probably not all.

Did that make any more sense-?
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Postby Troy » Mon Apr 16, 2007 8:41 pm

Paired or unpaired shouldn't affect repair.


Now everything is on its place! hehe :)

Thank you very much!!!
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