Re: recombination question


"DrBenway" <DB@xxxxxxxxxxxxx> wrote in message news:egckq3$nnq$1@xxxxxxxxxxxxxxxxxxxxxx
On Sun, 8 Oct 2006 01:54:53 -0400 (EDT), Jeremy Targett
<other@xxxxxxxxxxxxxxxxxxxxx> wrote:

A question that I'm sure someone here can answer for me:

In my basic understanding of genetic recombination, two chromosomes
recombine with crossings at essentially random locations in the DNA
sequence. Most of the crossing points, if selected randomly, occur in
long stretches of DNA that do not code for anything in particular, and
may well be junk for all we know. But the tiny bands along the
chromosome that contain genes are not vanishingly small and in fact
sometimes a crossing point must occur within a gene. When that happens,
if the two chromosomes have different alleles at that point in the DNA
sequence, the resulting gene will be a mash-up of the original two,
which I would guess is very unlikely to work.

My questions are:

does what I described actually happen or does something prevent
crossover within certain segments - and that's why genes have such
longevity?
If it does happen, what are the usual consequences? Does the individual
usually die?
How likely is it that a random crossover point in the DNA sequence falls
within a gene, rather than in the stuff between genes along the
chromosome?
How many crossover points do there tend to be per chromosome, roughly,
in human recombination?

as you can see I have only a very elementary understanding of this, I'm
a musician whose wife is taking elementary biology and I was just
looking at her textbook trying to understand this! Thanks for any
answers.


Hi
I'm very much a newbie in this area as well but let me take a quick
shot at your question, hopefully it will encourage some of the local
experts in the group to join in.



From my understanding whether the "individual" will die or not
does not come into play here. Since "Crossover" happens during
mitosis of the Gametes even a totally deformed Gene and hence
Chromosome would only affect the viability of 1 sperm or egg.


I think there are a lot more failed impregnations than is often
assumed. It is the first successfully sperm not just the first sperm
to arrive. Even then if lucky enough to form a fetus, this mashed gene
(mutated allele) would have a better chance of being recessive to the
other parents good gene.

Hope someone will chime in and correct me if I've steered you wrong
here. (At a level we can both understand <grin>)

You are doing fine except you said "mitosis of the gametes". No doubt
you meant to say "meiosis".

As a novice I find that in this beautifully incredible science
(genetics) I have a tendency to conceptually grasp the physical
processes (Mitosis Meiosis, Crossover) first.

And then later if you are lucky, put them at an actual place or time
or scale in the bigger process of life. Then it's like a second flash
of insite.

My response was really geared around the way I interpreted
your phrase "Does the individual usually die?"

Crossover is a cellular phenomenon
An "Individual" does not yet exist
(even in the most fundamental views that I am aware of?)

There are some single-celled organisms and a few plants (IIRC)
in which meiosis results in something that would have to be
called an individual. What you say is correct for animals.

Along the same lines as your question, crossover is somewhat
understandable as a random process for the 22 Somatic Chromosomes.
But what mechanism controls the crossover in the vastly different X
and Y Chromosomes?

My understanding is that the X crosses over in women (who have two of
them) during the meiosis leading to the egg. X does not cross-over
in men. Y never crosses over.

(Especially if as Dawkins says it's not even on a "gene boundary " or
even on a specific start or stop code!)

Hope we can both get some additional answers and knowledge here

DB





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Relevant Pages

  • Re: recombination question
    ... recombine with crossings at essentially random locations in the DNA ... sometimes a crossing point must occur within a gene. ... if the two chromosomes have different alleles at that point in the DNA ... How likely is it that a random crossover point in the DNA sequence falls ...
    (sci.bio.evolution)
  • Re: recombination question
    ... In my basic understanding of genetic recombination, ... recombine with crossings at essentially random locations in the DNA ... sometimes a crossing point must occur within a gene. ... if the two chromosomes have different alleles at that point in the DNA ...
    (sci.bio.evolution)
  • Re: Blog: The Scars of Evolution.
    ... 23 pairs of chromosomes, while the other great apes such as gorillas ... gene, but ours is "broken", disabled by mutations. ... humans have many characters in common with apes. ... Common descent explains the differences between insectivorous bats, small insectivorous birds and large insectivorous insects far better than any reasonable assumption of common design. ...
    (talk.origins)
  • Re: recombination question
    ... understandable as a random process for the 22 Somatic Chromosomes. ... My understanding is that the X crosses over in women (who have two of ... only the "common" gene area's in the X&Y crossover ...
    (sci.bio.evolution)
  • Re: Origin of sex - not why but who and when?
    ... is that the homologous chromosomes would be free to diverge until they ... gene families with two members. ... This is more complicated if the bdelloids ...
    (talk.origins)