Why are there 23 pairs of chromosomes?

[Graham Dellaire]

>   Netscape User <netuser at eromanga.pmb.ox.ac.uk> writes:
>  Hi,
>     Why are there only 23 pairs of chromosomes in human while simpler creatures may 
>  have many more chromosomes?  What are the mechanisms that decide the number of 
>  chromosomes in a creature?
>     All plausible suggestions will be much appreciated.
>  
>  best regards - teck sin
>  
>  
>>>>

First of all there is no real  correlation between the complexity of an organism and the number of
chromosomes.  We have 23 pairs and an animal much like a deer called a "Mutjak" only has 
four pairs (all be it very large ones).  Now can you say a the Mutjak is more or less complex then
ourselves.... probably not <grin>.   Plants are notorious for having huge numbers of chromosomes
in comparison to mammals.

My guess is the number of chromosomes we have is partly by chance.  I think that we may never
really be able to predict the original number of chromosomes that a mammalian "progenitor" species
had.  

Some possible limiting factors for genome size may include:

1. Replication of entire genome during one cell cycle
	a)-it is conceivable that beyond a certain limit the cell would have such a large genome
	it could not replicate it entirely during one cells cycle and the cell would die.
	
2. Minimum size of linear chromosome able to replicate
	a) there is probably a minimum size for a chromosome.... such that it would contain
	the necessary DNA structures (ORI/ORC's etc) for replication.  Mammalian cells
	are much more complicated than bacteria where the region required for efficient 
	replication is very small (0.5-1 kb)... whereas in mammalian cells this region may
	invlove Mb's of DNA.

3. Robertsonian Translocations/chromosome fusions

	a) there is evidence that many of our metacentric chromosomes may actually be derived
	from two or more acrocentric chromosomes (evidence from In situ hybridisation to 
acrocentric chromosomes of the mouse ... I think Chms 17 and 11 of the mouse are good examples)
	Therefore it is possible the number of chromosomes we have could increase or decrease
	through such a mechanism as fusion of centromeres and the accompanying
	rearrangements.


4.  Also the amount of non coding DNA tolerated (perhaps required<grin>) by an organism
may increase the overall genome size.  In yeast there are very few introns and bacteria lack them
all together (I believe, though recent studies may have found a few they are definately rare).  
Therefore to have the required number of genes for the organism you need less DNA.  Following
this we would require more DNA partly because of introns and other noncoding regions of the
genome and partly because of an increase "complexity" inherent in being mutlitcellular.


Well that is a few things that come to me right away if I think of any more I will try to post

Hopefully you get a few posts and between all of them there should be a few nuggets of
 (potential) truth!

G
	

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