Received: by alpheratz.cpm.aca.mmu.ac.uk id WAA01152 (8.6.9/5.3[ref pg@gmsl.co.uk] for cpm.aca.mmu.ac.uk from fmb-majordomo@mmu.ac.uk); Fri, 16 Feb 2001 22:15:56 GMT From: <Zylogy@aol.com> Message-ID: <f7.766ea00.27beffef@aol.com> Date: Fri, 16 Feb 2001 17:13:03 EST Subject: Re: Genome Project To: memetics@mmu.ac.uk CC: Zylogy@aol.com Content-Type: multipart/alternative; boundary="part1_f7.766ea00.27beffef_boundary" Content-Disposition: Inline X-Mailer: 6.0 sub 10506 Sender: fmb-majordomo@mmu.ac.uk Precedence: bulk Reply-To: memetics@mmu.ac.uk
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Most eucaryotic genes are actually composites, made up of mix-n-match domains
(such as beta pleated or alpha helix) in the corresponding protein sequences.
Intervening are the now famous introns, which get edited out and my actually
be tags which participate in some sort of economic system. Some geneticists
have speculated that the much more streamlined genes of bacteria (lacking in
any intervening sequences, as well as "junk" intercoding sequences) are the
end result of a long term process of deletion of such, and that the ancestral
condition may be more like that of eucaryotes.
The total number of coding eucaryotic domain sequences is very much smaller
than the total number of genes (one still must allow for mutation over long
stretches of time, which ends up tweaking each particular copy of a domain
(used for particular larger proteins), so that in the end you have a "family"
of near to not so near identical domain members. Interestingly, the SHAPE of
the final product in protein is much more conserved than the sequence which
codes it, showing that there has been severe selectional pressure at that
level (forced "false" convergence by weeding out divergent members, except in
those very rare cases where the change isn't deleterious).
Bacterial genes also encode domains, but because of the lack of intervening
sequences it is nearly impossible to recombine on this level and get a viable
product. Bacteria have many fewer genes than eucaryotes (only several
thousand). Interestingly, eucaryotes are known to be multicellular in origin-
various organelles are known or thought to be bacterial or viral originally-
this includes mitochondria and chloroplasts (which have their own remnant
bacterial circular chromosomes- coming in very handy for family analyses),
centrioles, and a couple of other bodies I can't remember the name or
function of (any really good grad level cellular biology text will discuss
this).
We know that many of the original functions of these subcellular bodies
genewise were either physically transferred to the nucleus or had equivalents
in the main genome whose products get transported to those bodies. It may be
that some of the genetic structure of the nucleus derives from the
combination of the various contributions by the different organelles- leading
to a larger number than found in any single bacterium or virus. Heck, there
are unicellular eucaryotes with more than one nucleus!
The multi-chromosomal content of the nucleus may itself be the remnant result
of such fusion of single bacterial chromosomes (though the loss of
circularity is problematic- may be that the current bacterial situation is an
innovation).
There are actually a couple of projects already in progress trying to
determine what is the minimal number of genes needed to run a cell. The
researchers try to tear away all the extra gizmos and leave only
maintainance, housekeeping, reproductively salient forms. You'd be surprised
how small that number actually may be.
Much of the complexity of the human genome is likely given over to the
maintainance of our multicellularity, on the one hand, and adaptation ability
(such as different regimes of temperature, pressure, immunity, etc.) on the
other. Both of these have a tendency to "hardwire" in gene structure itself
over time if the species is stable (which is why there are no new body plans
being generated in nature now, even though there was an explosion of such
during the late precambrian and cambrian periods). Plants produce a very
large number of poisons to deter grazers- each variation requires a number of
enzymes, so more genes.
So in general, the more a body has to do, the larger the numbers of gene
products needs to be, specialized to do ever more detailed work in larger
numbers of "compartments". The number of gene products can be increased
either by simply increasing the numbers of genes, or having the ability to
edit. Editing itself can be at the level of the RNA transcript prior to
translation to protein, or of the protein itself.
Neat, huh?
And lets not forget that the numbers of knobs and switches on gene products
has increased over phylogeny, allowing ever greater numbers of regulatory
interactions. Makes you wonder what's next. I guess that would be us, with
our language and culture, sciences and theories. Now we can alter the system
from outside! The drawing hand reaches back upon itself, changing...
Jess Tauber
zylogy@aol.com
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