Re: recombination question
- From: "Ron O" <rokimoto@xxxxxxx>
- Date: Thu, 12 Oct 2006 00:22:01 -0400 (EDT)
Jeremy Targett 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.
Homologous recombination that occurs during meiosis does not "mash up"
anything. Just parts are switched. In any two members of a species
the genes are nearly identical and recombination just swaps out the
sequence of one chromosome for the nearly identical sequence of
another.
Say you have a sequence: ACGTTAACGAGAGTCGG on one chromosome and on the
homologue the sequence was acattaacgagagccgg and a recombination event
occurred in the midle of the sequence you would get one recombinant
product like ACGTTAACGAgagccgg and the other acattaacgaGAGTCGG
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?
Crossovers occur within genes. Look up MHC genes and recombinants in
humans and you should pick up a couple of papers. The old literature
on yeast is full of detecting recombinants within genes between two
mutants that restore function of the gene because the mutations are in
different parts of the gene.
Genes do not have longevity because of the recombination that doesn't
occur in them. Recombinants can either be selected for or against just
like new mutations.
If it does happen, what are the usual consequences? Does the individual
usually die?
No.
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?
In yeast recombination within genes is quite common because a
centiMogan (1% recombination) is around 3 kb? less? I can't recall.
The average protein coding gene is around 1 kb of coding sequence along
with introns. In humans 1 centiMorgan is about a megabase, but some
genes like the Duschennes muscular dystrophy gene are more than a
megabase in length.
How many crossover points do there tend to be per chromosome, roughly,
in human recombination?
Nearly all chromosomes are over 50 centiMorgans in length (I can't
think of an example of one shorter than 50 cM. This is recombination
distance and not base-pair length of the chromosome). During Meiosis
at least one recombination event occurs on a chromosome during pairing.
It can be resolved so that a recombination occurs or the resolution
can just have the a little gene conversion with no recombination of the
parts of the two chromosomes. I don't know if it is still going
around, but the claim was that recombination might be required for
accurate pairing of the homologs. Big chromosomes can be several
hundred centiMorgans in length.
There are hotspots of recombination where the frequency of
recombination is much higher. Recombination rates can be different on
different chromosomes within a cell. In chickens there are macro
chromosomes and smaller chromosomes called micro chromosomes. 1
centiMorgan averages out to around 300,000 bp on the macro chromosomes,
but only 50,000 bp on the microchromosomes.
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.
No one is born knowing everything.
Ron Okimoto
.
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- From: Jeremy Targett
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