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Junk DNA

Keith Robison robison at mito.harvard.edu
Mon Jul 24 10:26:04 EST 1995

Shane McKee (shane at reservoir.win-uk.net) wrote:
: Stephen Karl writes:

: >  As far as "junk DNA" is concerned, I think it is always good to remember...
: >  There are two types of non-coding DNA of unknown origin or
: >       purpose: 
: >
: >       1) Garbage DNA and
: >       2) Junk DNA
: >
: >  As in life ... GARBAGE is something you throw out
: >                       and 
: >                JUNK is something you keep.

I believe Sydney Brenner is usually credited with originating this

: Seriously, though, looking from the viewpoint of the genome as a
: little ecosystem in its own right, all this becomes a lot clearer.
: Someone posted recently on the fact that two closely related
: species of deer differ greatly in the number of chromosomes - one
: had only four pairs, while the other had forty, or something like
: that. Anyone know if the one with fewer chromosomes has any less
: junk DNA than the other, or are the chromosomes just ten times as
: big?

They're 10X in size.  There's a clever trick published about a 
year ago which takes advantage of this.  A lot of genetics
can be done with somatic cell hybrids, cells which are a fusion
of human and non-human cells and which contain only a few
human chromosomes.  Classical somatic cell hybrids are made
with rodent cells, but it is very hard to purify the rodent
DNA away from the human, because the chromosomes are the 
same size.  But, if you make a muntjac-human somatic cell
hybrid, the human chromosome will be very well resolved from
the huge muntjac chromosomes (on a pulsed-field electrophoretic gel).

: Anyone care to speculate as to how this situation came about in
: these deer? (Even as a thought experiment, it can be quite
: instructive).

Blind chance, most likely.  It is not obvious that a particular
chromosome number has any particular advantage.

Keith Robison
Harvard University
Department of Cellular and Developmental Biology
Department of Genetics / HHMI

robison at mito.harvard.edu 

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