junk DNA

Ian A. York iayork at panix.com
Sat Apr 15 09:08:14 EST 2000


In article <200004150928703.SM00402@[216.220.164.17]>,
Jay Mone <jaymone at paonline.com> wrote:
>
>It doesn't work that way.  The error rate is a probability, with
>>something like 1 error in 100,000 (or so) for eucaryotic DNA.
>>Doubling the amount of DNA also doubles the amount of errors.
>
>But it should work this way.  As a example, consider your windshield in a
>light rain.  Draw a 1 cm diameter circle on the windshield, and nest it in a
>10 cm diameter circle.   Which circle gets more hits (mutations)?
>I agree that the more DNA you have, the more mutations you'll accumulate.
>However, if the mutations are in non-coding regions, who cares?

You didn't read what I said.  You double the total amount of DNA and you
double the total number of errors.  But if the coding amount is still 1X,
that region still gets 1X the number of errors.

To use your analogy, draw your 1 cm circle on your windshield; that's the
coding region.  Will there be fewer drops of rain hitting it if you make
the windshield ten times larger?

>If a selective advantage to remove junk DNA existed, then we would have
>gotten rid of junk DNA a long time ago.

Only if the selective advantage was high enough to counteract random
increases.  By your argument, there would never be any inherited
disorders, because the selective disadvantage of having hemophilia would
instantly get rid of it.  There's a balance between random disadvantage
and selective advantage.

Secondly, the advantage could be counteracted by the costs of any system
required to achieve that advantage.  You can imagine, perhaps, that a
mechanism that removes large chunks of DNA would need massive
error-checking to ensure that it doesn't take away essential stuff, so
that a system that efficiently removed the (mildly disadvantageous) extra
DNA would cost more than the benefits.  Or, as evolution works, would cost
more in the short term, regardless of long-term benefits.

Similarly, there could be a selective advantage, but no mechanism easily
achievable to reach that advantage.  Humans would no doubt have a
selective advantage if they had a fusion reactor in their stomachs, but it
ain't gonna happen.

That's no to say that any of those arguments applies in this case, but you
always have to consider the null hypothesis.  You can't simply declare
that because something is there it must have a selective advantage, or
that becase it hasn't been removed there's no advantage to doing so.

Ian 
-- 
    Ian York   (iayork at panix.com)  <http://www.panix.com/~iayork/>
    "-but as he was a York, I am rather inclined to suppose him a
     very respectable Man." -Jane Austen, The History of England




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