Junk DNA

David B. Hedrick davidbhedrick at icx.net
Fri Apr 14 20:37:52 EST 2000


Steve:

	Eukaryote reproduction is not limited by the amount of DNA per cell. 
For bacteria, however, DNA synthesis is a significant fraction of the
cost of cell division.  So bacteria suffer stronger selection against
strains with more DNA sequences.  Bacteria grown in a laboratory often
lose capabilities not required by the culture conditions.  The viral
lifestyle even more strongly selects against amount of DNA - they
actually code 2 different proteins on opposite strands of duplex DNA. 
No one would have beleived that possible if they hadn't seen it.  
	That "junk DNA" hanging around a eukaryote genome might pick up new
functions, but there are salamanders with 10 times more DNA per cell
that you do.  
 
> I am not so confident that the "junk DNA" has no function and is just
> being carried by accident.  I prefer to think of it as the DNA for which
> we haven't yet figured out the function.
> 
> Nature is extremely conservative and doesn't seem to carry forward much
> unnecessary baggage.  Remember the long lists of functionless vestigial
> organs that were generated in the latter part of the 19th century.  We
> now recognize that most of them have important functions.
> 
> Jay Mone wrote:
> >
> > Here's a question for everyone to chew on...
> >
> > It is generally agreed that of the human genome, only about 20% is
> > used for
> > coding of information or other known functions.  What are the
> > prevailing
> > thoughts on why the other 80% of our genome seems to do nothing but
> > take up
> > space, and how we came to have so much junk DNA?
> >
> > In my general biology class, I often pose these questions to my
> > students for
> > thought.  As to how we came to have so much junk DNA, I have no idea.
> >  Might
> > this junk DNA actually provide a selective advantage?  Having so  much
> > wasteland in between the coding regions of the genome certainly
> > reduces the
> > probability that a mutation will occur in a coding sequence or it's
> > regulatory region.  It would be like trying to hit a tent in the huge
> > desert
> > by randomly lobbing missiles over the entire desert.  Its unlikely
> > that
> > you'll hit the tent.  Since mutations occur virtually every time a
> > cell
> > divides, this might be a mechanism to lessen the effects of such
> > random
> > mutational events.  Does this sound reasonable?  Has this or another
> > idea
> > been put forth recently?  And how did we come to get so much junk in
> > the
> > first place?
> >
> > By the way, in the other genomes sequenced so far (C.elegans,
> > Drosophila,
> > etc, do they also have large amounts of junk DNA?
> >
> > Jay Mone'

-- 
		~DBH

Technical writing, literature search, and data analysis at the interface
of chemistry and biology. 

	davidbhedrick at icx.net

	David B. Hedrick
	P.O. Box 16082
	Knoxville, TN 37996




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