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Info on chromosomes in general, esp. meiosis

Dr. van Buijtenen jvb at SILVA.TAMU.EDU
Mon Apr 5 12:32:46 EST 1999


On Mon, 5 Apr 1999, Stephanie wrote:

> Hello,
> I'm Stephanie (13)
> I am trying to plan, over my Easter holidays, a system for introducing
> diploid cell (Is that the right term? I mean the possibility of
> recessive genes, so double the DNA.)  into an artificial life game I
> play. I keep coming up against a major stumbling block: chromosomes.
> I especially want to clear up these points (I will try to get all the
> terms right):
> Do chromosomes carry dominant/recessive forms of the same gene (alleles)
> together on one chromosome, or are they carried on different chromosomes
> and the chromosomes 'pair up'?

Different alleles at the same locus are carried on different chromosomes.
They do pair up.

> If they don't pair up, why does colour blindness affect males
> predominantly?

The gene for color blindness is located on the X chromosome. Since males
have only one X chromosome it will always be expressed in the male if it
is present. 

> What exactly are chromatids, and why do we need them?
> What are double-stranded chromosomes, and why do we need them?
Good questions. I learned my cytology and DNA chemistry about 40 years
apart so I may be connecting them wrong. DNA is naturally double stranded.
The two strands are not identical, but complementary. The cell can
reconstruct double stranded DNA from either single strand. I expect a
single strand corresponds to a chromatid (a cytological term), but I am
not sure of that.

> Why, before a cell is about to undergo meiosis, does it have twice the
> genetic information in a normal cell, when, if it had half, it could
> just divide straight away?

I think your question is based on some wrong assumptions. For all
practical purposes all diploid cells have the same genetic information,
and they can divide any time via mitotic division. 

> What is the point of crossover? Does it give an evolutionary advantage,
> or is it just a side effect?
Crossing over recombines different mutations does giving rise to many new
combinations. This pool of variation is the raw material that natural
selection works on. Evolution would be much slower without recombination.

> I would be very grateful if anyone could give up a few minutes of their
> time to answer some of these questions for me.
> Thanks,
> Stephanie
> opes this helps. J. P. van Buijtenen

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