🚀 go-pugleaf

rowe@pender.ee.upenn.edu (Mickey Rowe) writes:


>Does anybody know if all of these copies are *really* identical?
>Given that (in my understanding anyways) new genes arise largely from
>copies of previously existing genes, how is it that the sequences of
>the copies of the rRNA genes in _X. laevis_ don't mutate
>independently?  (I'm still close to the beginning of Futuyma's book,
>so please feel free to respond by telling me to keep reading :-)

>Mickey Rowe     (rowe@pender.ee.upenn.edu)


It's called concerted evolution.  Mechanistically, it occurs
through gene conversion.

Here's how it goes.  When chromatids pair during meiosis, recombination
normally occurs by an _exchange_ of part of the paired region.  So,
if you have:


	a-b-c-d  paired with
        e-f-g-h

you might get something out like:

       a-f-g-d   and
       e-b-c-h

But sometimes, instead of an exchange the information on one strand
displaces the information on the other (by mechanism which are still
under argument, I believe [corrections welcome]).  Thus, you
could end up with:

      a-b-c-d   and
      e-b-c-h


Because rRNA's are generally present as long tandem arrays, they
have many chances to recombine/convert, because pairing can occur
between ANY two members of the array.  Since this multiplies the
number of opportunities for conversion, conversion is observed
at a high frequency and, viola!*, all the rRNA's of a species
appear to evolve "in concert".


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

robison@biosun.harvard.edu 


* - pun not originally intended.  Too bad!