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Junk DNA

Michael E. Zwick mezwick at ucdavis.edu
Fri Aug 4 14:59:20 EST 1995


In article <3voejj$400 at ixnews5.ix.netcom.com>, francone at ix.netcom.com
(Frank Francone) wrote:
> Our working hypothesis is that the "Junk" code plays a structural role
> in allowing the genome to direct the attention of the crossover
> operator to locations where high fitness blocks of code are not
> disrupted.  So far, ou
> r results are consistent with this hypothesis. (The work is available
> as a TR out of Univ Dortmund and will be availible in "Advances in
> Genetic Programming II" due out sometime this year.)
> 
> I am quite interested in a biologist's perspective on this subject. Is
> there any evidence that the junk code in DNA plays any kind of a
> structural role in crossover, mutation? 

In fact, the biological pattern is the opposite of what you suggest.  In
Drosophila for example, the "junk" DNA (or perhaps heterochromatin)
suppresses meiotic crossing over - so that most cross over events occur in
the more heavily coding euchromatic regions in the middle of chromosome
arms.  Many fewer crossing over events occur near the centromere.

 Is there any evidence, however
> indirect, tha
> t the junk code plays any role whatsover?

There is some evidence that this DNA may be important for chromosome
segregation.  However, this is still primarily correlational evidence
(i.e. there is a lot of repetitive DNA near the centromere, and we know
the centromere is important for chromosome segregation in meiosis and
mitosis).  In fact, the eucaryotic centromere may be composed of
repetitive sequences.  See Gary Karpin's work on Drosophila centromeres. 
There also has been some work showing that some alpha-satellite sequences
are sufficient to act as centromere (I am afraid that I don't have the
reference at hand)

Alternatively, a simple increase in the number of repetitive sequences can
lead to heterochromatin forming - see S. Henikoff's elegant work on this
issue.

Based on my reading of the literature, there are far more ideas about
possible functions that may be played by these sequences than there is
strong evidence supporting these ideas.  These sequences still are pretty
mysterious.



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