Keith Robison robison at mito.harvard.edu
Mon Jun 24 08:17:29 EST 1996

MIKE  LAMB (mikejonlamb at earthlink.net) wrote:
: I was not contending that cancer prevention was the primary benefit of 
: introns. Not by any stretch.  I believe that introns facilitate the 
: splicing and introns probably have other functions and benefits.   
: "If introns really distract mutagens from hitting exons, wouldn't the
: likleyhood of a deletion in an intron/exon increase? Wouldn't this
: increase the likleyhood of a frame shift?"

: Mutagens cause point mutations far more often than frameshifts. Why 
: would a deletion occur? Mutagens bind and prevent correct reading of the 
: codon. They do not normally cause deletions to my knowledge. 

It depends on the mutagen.  Many cause point mutations.  But go back
to the classic Crick et al papers on the genetic code -- how did they
figure out it is read in threes?  By using a class of mutagens
(acridines) which tend to induce single base insertions and deletions.

In any case, it makes no sense to talk about frameshifts due to a mutation
_within_ the intron.  The intron is spliced out long before the ribosome
sees the RNA.  Splice site alteration mutations, on the other hand,
can be devastating by altering the splicing pattern to remove or insert
exons or change their length.

Keith Robison
Harvard University
Department of Molecular & Cellular Biology
Department of Genetics 

robison at mito.harvard.edu 


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