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Archiver > GENEALOGY-DNA > 2007-10 > 1191734272


From: "Ken Nordtvedt" <>
Subject: Re: [DNA] Why chromosomes ?
Date: Sat, 6 Oct 2007 23:17:52 -0600
References: <782313.68201.qm@web31513.mail.mud.yahoo.com>


Yes the number seems arbitrary or accidental, but my question was why
chromosomes at all? Why not just one big stretch of dna (X and Y left out
of the discussion)

Why the packaging of the dna into packets?


----- Original Message -----
From: "Jim T" <>
To: <>
Sent: Saturday, October 06, 2007 10:00 PM
Subject: Re: [DNA] Why chromosomes ?


> The number of chromosomes that a species has is the result of
> chance. Mistakes happen in meiosis. Sometimes an extra copy of
> chromosome is created. Sometimes a chromosome is lost.
> Sometimes a piece of a chromosome is lost. Sometimes two
> chromosomes fuse. Sometimes the entire genome is duplicated.
> The latter is called polyploidy and it is common in plants.
> Polyploidy is rare in animals, but it does occur, in salmon, for
> example. Most of the time, such mistakes result in no
> offspring, or in offspring that cannot reproduce. Sometimes
> they result in offspring that do reproduce and that pass the
> mistakes along to future generations.
>
> Extra copies of chromosomes are thought to enable evolution.
> When there are two copies of a gene, one can continue to perform
> its original function, leaving the other free to evolve to
> perform a different function.
>
> Although humans have 23 pairs of chromosomes, our closest
> relatives, chimpanzees, gorillas, and orangutans, have 24 pairs
> of chromosomes. Our species has lost a pair of chromosomes
> relatively recently in evolutionary history. Human chromosome 2
> appears to be a fusion of two ancestral chromosomes, with the
> loss of parts of both of the ancestral chromosomes.
> Fortunately, there is a lot of redundancy in our chromosomes.
>
> Jim Turner
>
> From: "Ken Nordtvedt" <>
> Putting aside the X and Y chromosomes, I wonder why nature chose
> to package our
> remaining genetic material into 22 chromosomes? Why not just
> have it has one
> long strand of our genes and so-called junk dna? It could
> still fragment into
> many pieces, cross over, and recombine the fragments from the
> two parents each
> generation. Segments of the dna could hop to other places from
> time to time;
> genes or segments of multiple genes could copy themselves in
> other places in the
> genome from time to time, and then diverge in function, etc.
> How autonomous
> are the chromosomes? Do they swim around in our cells
> separately or in a linked
> arrangement of all 22 ? Certainly the number of packages ---
> chromosomes ---
> changes from time to time in the world of life; different
> species have
> different numbers of these things. So this packaging can't be
> too sacred. Is one long
> strand of all our dna just too cumbersome a molecule?
>
> I realize this is a question mainly for the molecular
> geneticists who read this
> list; but perhaps others have come across the function of
> having separate
> chromosomes?
>
> Ken
>
>
>
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