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Info on chromosomes in general, esp. meiosis

Aoife McLysaght amclysag at tcd.ie
Tue Apr 6 12:18:46 EST 1999

> 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'?

Everybody has two of every chromosome type (types are numbered 1-22 and then
the sex chromosomes X and Y). Each chromosome type consists of what can be
thought of as a particular list of genes. Each chromosome only has one form
of each gene in this list. Two chromosomes of the same type (same number)
will have the same list (e.g. eye colour, hair colour etc.) but will have
different variations/forms(alleles) of this list ... so one might have blue
eyes, brown hair, and the other might have green eyes, blonde hair. Usually,
someone's eyes are the same colour as each other so only one of these eye
colour genes comes through (dominates).

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

Whenever you see something that predominately affects men you can guess that
it is because it is on the X chromosome. This is because men only have one
of these and women have two. If a woman has an X chromosome with the gene
for colour-blindness on it she will also have another X chromosome with
another gene which may be for 'not colour blind' (or normal) which is
dominant over the other gene. Because men only have one X chromosome, if
they have a gene for colour-blindness it will always come through.

> What exactly are chromatids, and why do we need them?

When a cell is duplicating all of its genetic material in meiosis (like you
talk about later) it does this by making copies of each chromosome ...
before the cell divides the copies stay attached to each other at one point
... each of the copies is called a chromatid. If you ever see a picture
(photograph) of a chromosome, it is usually at this stage of cell division
and looks like a large 'X', however, it is probably better to consider it
like a 'H' with each vertical bar of the 'H' as a chromatid, and the
horizontal bar as the attachment between the two.

> What are double-stranded chromosomes, and why do we need them?

Chromosomes are made of DNA. All of our DNA is double stranded.

> 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?

It actually doubles everything first then gives half of the original amount
to each of four new cells.

The reason it needs to have half the genetic information is because after
meiosis cells will become either sperm (male) or egg (female) ... when the
sperm and the egg join they then have the correct amount of chromosomes
again (two of each). If you didn't halve the genetic information in these
circumstances the child would have twice the correct amount and the
grandchild would have four times the correct amount of chromosomes and this
can cause many and complex problems.

The way the genetic information in halved is not entirely random. After
meiosis the new cells will each have one (instead of two) of each chromosome

In mitosis (ordinary cell division that takes place as we grow ... and as
our hair and nails grow etc.) is different to this and it involves the cell
making copies of all of the chromosomes and then splitting in half with one
full set of chromosomes in each new cell.

> What is the point of crossover? Does it give an evolutionary advantage,
> or is it just a side effect?

An important part of evolution is allowing changes to happen ... crossing
over is another way of making changes ... so in the above example, a
crossover may have the effect of 'shuffling' the chromosomes so that blue
eyes and blonde hair are together on one chromosome and green eyes and brown
hair are together on the other.

> I would be very grateful if anyone could give up a few minutes of their
> time to answer some of these questions for me.

I hope this helps. I'm sorry if it gets too complicated but they were
complicated questions. I tried my best to explain things in a simple way but
still give you the information you were looking for.  Meiosis and mitosis
are the most difficult to explain ... you probably will need to find a book
or a web page with diagrams to illustrate what is going on. I was so
impressed by the fact that someone your age is interested in these things
(maybe there is such a thing as an educational computer game!) that I
thought I should answer and let you decide for yourself if it is too

Good luck,


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