Path: utzoo!attcan!uunet!munnari!otc!metro!basser!usage!bio73!root
From: root@bio73.unsw.oz (Karl Redell )
Newsgroups: sci.bio
Subject: Re: Mortality
Message-ID: <1131@bio73.unsw.oz>
Date: 15 Jun 88 02:27:41 GMT
Organization: Uni of NSW, Sydney, Australia
Lines: 32

In New Scientist 29 October 1987 Pg. 39 in article titled: Faulty genes
lead to old age - Janet Mohun writes:

"...Biologists now know that methyl groups (-CH3) attached to a cell's DNA
can in some way control whether or not a particular gene is expressed.  Any
change in methylation results in a corresponding change in control of gene
expression..."

"...Sebastion Fairweather suggested that the loss of methyl groups is a
normal part of ageing.  This is because genes will remain "silent" or
unexpressed so long as specific cytosine bases within that gene remain
methylated.  A gradual loss of methyl groups early in development is
therefore necessary to allow the normal pattern of gene expression to follow.
    Fairweather, of the Radcliffe Infirmary in Oxford, said there is now
strong evidence that the rate at which methyl groups are lost from an
animal's cellular DNA is directly related to that animal's lifespan.  DNA
from human cells loses methyl groups slowly as the cells age..."

"...Why should the loss of DNA methylation shorten a cell's life?  The
theory is that, as cells age, some methyl groups are lost, perhaps because
the enzyme methylase is less active or because there is damage to the DNA.
This allows the expression of genes that previously were silent.  The result
is that cells may produce proteins that are harmful to them, perhaps
activating oncogenes or inhibiting systhesis of DNA..."

"...In a review of the inheritance of epigenetic defects Holliday says
that defects in the cell would tend to appear after the loss of several
methyl groups, rather than just one, because genes tend to have a cluster
of methylated cytosine residues and not just one(Science,vol238,p163).
    "It would be possible, therefore, to have many silent epigenetic
defects which produce a deleterious phenotype only after a given period
of time or a given number of cell divisions," he said."...