VARIATION IN DNA CONTENT AMONGST DIFFERENT SPECIES

Andrew Cockburn afc at gnv.ifas.ufl.edu
Fri Jan 28 11:47:13 EST 1994


In article <1994Jan26.225939.20159 at ac.dal.ca>, aroger at ac.dal.ca writes:
>> Mustafa
> 
> The book you should read is a compilation by Tom Cavalier-Smith
> called "The Evolution of Genome Size", published sometime in the
> mid-eighties.
> 
> The majority of multicellular eukaryotic genomic DNA is not
> made up of coding sequence which has obvious phenotypic effect
> on the organism (at least on structures above the level of the
> cell).  It is largely made up of repetitive DNA.  That is either
> repeated short sequences (5 or less bases) or repeated longer
> sequences, like Alu elements.  Coding sequence for many animals
> and plants may make up as little as 5% or less of the total 
> genome size.  

It is correct that most genomic DNA in higher eucaryotes is noncoding.
However, the argument that most of this is repetitive is blatantly
homocentric.  In Drosophila and many other insects, a third or less of
the DNA is repetitive.  Most of the "junk" DNA is single copy.  This
is also true in other groups, Arabidopsis and nematodes come to mind.
Even in humans a big chunk is single copy.  (Too bad that is true-
else the genome project would be lots easier.)

> The various theories explaining why genomes like these have
> evolved are outlined in Cavalier-Smith's book.
> 
> Andrew Roger
> Dept. of Biochem
> Dalhousie University
> aroger at ac.dal.ca  

Making a large genome from a small genome is trivial, which is why there
are so many theories to explain it :-).

What I would like to see is an explanation of how a small genome
can evolve from a large genome full of repeats.  I believe that this
happens in insects fairly frequently.  Allen Spradling suggested to me
that this might be tied up with the mechanism of polytene
chromosome formation, which involves deletion of repetitive DNA.

I know about gene loss in obligate parasites, I want to know how to get
rid of the junk and keep all the functional stuff.

Andrew Cockburn



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