Why are there 23 pairs of chromosomes?

Graham Dellaire popa0206 at PO-Box.McGill.CA
Mon Jul 10 08:05:42 EST 1995

>   dcurtis at hgmp.mrc.ac.uk (David Curtis) writes:
>  Since I'm being encouraged to increase the intellectual content of
>  this group, I'll make the following contribution.
>  As pointed out elsewhere, the number of chromosomes has little
>  relation to the number of genes. All animals share common ancestry, so
>  the fact that nowadays they have different numbers of chromosomes
>  means that at some point in evolution the number changed from parent
>  to child through trisomies, fusions and other rearrangements.

I have wrestled with this line of thinking for a while now.  It is more likely 
as you describe here that the change in number of chromosomes was 
in smaller increments than say whole genome duplications as has been
hypothesized to explain comparative mapping between species and within
species.  It would seem highly unlikely that "gross" rearrangements if not 
lethal in there own right would then be heritable.  How would you find a
another memeber of your "new species" with a similarly amplified and rearranged
genome?  You would be sterile for all intents and purposes.  One mechanism
that can drive changes and provide amplified regions of DNA .... and thus provide
regions for comparative mappers to find <grin> is gene conversion between
repeat like elements.

This process has very recently been shown to occur at a very
high rate!  At such a rate that most probably the majority of our cells are actually
genetic mosaics. ( I will try to find out the a reference... it has to do with studies of Line-1 sequences 
and there movement in embryonic cells)  As well gene conversion has been hypothesized to been 
involved in the homology search at meiosis before synapsis and during.

 In fact,
>  these events have carried on occurring until very recently in
>  evolutionary terms, and presumably may continue to do so. Except that
>  for humans almost all trisomies are lethal and certainly compromise
>  the capacity to reproduce dramatically. Are all tetrasomies lethal?

I wonder how any complex organism can tolerate tri or tetrasomies...
I guess it depends on what genes are duplicated and how bad the ensuing
rearrangements will be after duplication.   Q. Are people with Downs Syndrome
partially or totally sterile?

> I  would expect so. If so, it would seem likely that humans will be
>  "stuck" with 23 chromosomes indefinitely. Would most people agree? 

Hmm on the surface perhaps yes.  But in the end who knows... "Nature" has
its surprises... the post with about the Mutjac's one with 4 and the other with 40 chromosomes
is interesting as they are supposedly quite similar.  Maybe there are some humans
in the  Ozarks (spell?) of Arkansaw with more or less than the normal number of chromosomes.


>  On a tangentially related point, we know that the genome size depends
>  largely on how much junk DNA is around, but can anybody explain _why_
>  the puffer fish has so little junk DNA? 

Hmm.  How about this.  Perhaps the fish arose from a progenitor species that
could not tolerate retroviral or retroposon like invasion of its genome.... consequently
you have very little repeats and intron-like "junk".  I am out on a limb here completely
as I have little experience with fish genomes in general....are they all largely junk free?
or is the puffer a real exception to the rule.


>  Dave Curtis (dcurtis at hgmp.mrc.ac.uk)
>  Institute of Psychiatry, London 
>  http://www.iop.bpmf.ac.uk/home/depts/psychmed/general/dcurtis/dcurtis.htm

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