Re: Junk DNA: A hypothesis

From: Tim Tyler (tim_at_tt1lock.org)
Date: 01/25/05 

Date: Tue, 25 Jan 2005 00:23:02 -0500 (EST)

Larry Moran <lamoran@bioinfo.med.utoronto.ca> wrote or quoted:
> Tim Tyler <tim@tt1lock.org> wrote:
> > Larry Moran <lamoran@bioinfo.med.utoronto.ca> wrote or quoted:

> >> Why do you feel the need to find an adaptionist
> >> explanation for junk DNA?
> >
> > I don't. However, I do think it is worthwhile examining
> > the various selective forces that act on junk DNA, in the
> > hope of learning more about them.
>
> What are the "selective forces" that act on junk DNA?

They look something like this:

Likely negative selective forces
--------------------------------

1) Junk DNA has a metabolic cost. In increases nutrient
   demands, and takes time and energy to copy.

2) Junk DNA harbours self-replicating DNA capable of
   horizontal transmission between loci - such DNA
   acts like a germ-line virus, and causes mutations.

...and probably many others.

Likely positive selective forces
--------------------------------

1) Junk DNA acts as spacing, and affects linkage.

   Decreasing linkage can have selective benefits - e.g.
   when it separates genes that benefit from recombination,
   such as disease-restance genes.

2) "Junk" DNA improves evolvability by creating
   "neutral networks" - and allowing scratch space
   to allow evolution to work.

3) Junk DNA reduces the chance of crossovers occurring
   in the middle of genes that don't like being split -
   and can increase the chance or crossing over in the
   middle of any genes that need to be expressed in
   a range of forms.

4) Junk DNA can act as a sponge for intracellular mutagens
   that bind to DNA - decreasing the effective mutation rate
   in coding regions.

5) Junk DNA changes the nuclear volume - and having a large
   nucleus can be beneficial[1].

...and probably many others.

[1] "Eukaryotic non-coding DNA is functional: evidence from the
     differential scaling of cryptomonad genomes"

     - http://calorierestriction.org/pmid/?n=10902541 

-- 
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