>sbassi at CRIBA.EDU.AR (Sebastian Bassi) writes:
> Hello,
>> I'm studing medicine, and I'd like to understand what is
> IMPRINTING. I read books of genetics (like 'Genetics in
> medicine', by Thompson & Thompson), but I couldn't understand it.
> I'd like that someone could help, with a 'plain english
> explanation'.
> I know how is defined, but I think would be nice some example,
> with pedigree.
>>> Regards,
> Sebastian Bassi
> email: sbassi at criba.edu.ar>>>>>>>Okay here goes.... Imprinting as I understand it (by all means this subject
is greatly open to debate as to the true nature of imprinting)
involves the differential silencing (or expression, depends on point of view of "normal state")
of a gene copy (allele) in an individual organism (zygote) depending on the sex of parent that the
gene copy was recieved from (or as they say in the vernacular "gamete sex dependent").
So lets say you inherit a copy of gene Y from your DAD and a copy of the same gene from your
MOM. Then lets distinguish each alleles from each other as allele Ya , Yb, Yc etc
*=expressed and {}=silenced
DAD ------- MOM
Yb*{Yd} | Ya*{Yc}
|
|
You
Yb*{Ya}
Now this is one of four sets of alleles possible.
So if you noticed even though the Ya allele was expressed in you MOM it is not expressed in YOU!
This means that when you inherit the gene copy (allele) from your DAD it is expressed
and when you inherit it from your MOM it is silenced. This is called imprinting... whether you expressed
the copy of the gene or not was dependent on the sex of the donor of that allele (ie MOM or DAD).
Most likely imprinting (much like X chromosome inactivation in females) is a mode of limiting
gene dosage in the cell. Sometimes there can be such a thing as too much of a good thing <grin>!
Now the mechanism of Imprinting is under great debate. It is safe for now to say it is often
associated with methylation of nearby CpGp islands (groups of CGCG repeats that often coincide with
the start of coding regions or are within coding regions of the genome). Whether methylation is the
cause or effect of imprinting is debatable. Also methylation is not always correlated with silencing of the
gene either (look for the Ig2f and Ig2f-receptor genes in which one is expressed and the other silenced
upon methylation of the same locus, though which is silenced in the presence of methylation escapes me
right now <grin>).
Another mode of imprinting may involve the association of heterochromatin-assoc. proteins early in
development with a given region to be silenced (ie altering the chromosomal structure at that sight
such that perhaps transcription factors no longer have access to the DNA).
I hope this helps a little.....
Imprinting is a tough one : )
G. ______________________________________________________________________
Graham Dellaire Snail Mail:
Red Cross, Research
McGill University Montreal Blood Services
Faculty of Medicine 3131 Sherbrooke St. East
Div. of Experimental Medicine Montreal, QC, Canada
E-mail: popa0206 at po-box.mcgill.ca H1W 1B2
B2XE at musicb.mcgill.ca
WWW Page: http://www.medcor.mcgill.ca/EXPMED/expmed.html
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