🚀 go-pugleaf

Ken Muldrew (kmuldrew@acs.ucalgary.ca) wrote:
: toms@kaylor.ncifcrf.gov (Tom Schneider) wrote:
: >Ken Muldrew (kmuldrew@acs.ucalgary.ca) wrote:
: >: Another purpose for introns may be to signal the DNA repair or
: >: proof-reading enzymes to stop. I think that point mutations have
: >: accrued in the hypervariable regions of immunoglobulin genes at a rate
: >: that is higher than the replication error rate, implying that there is
: >: a mechanism to promote mutation. If there was a sequence which told
: >: the proof-reading enzymes to stop when they reached that particular
: >: region, then that might be a way of generating the higher mutation
: >: rate. 

: >Introns are (presumably!) only on the transcribed RNA copy not the DNA.  One
: >would generally imagine that repair enzymes do not distinguish the regions,
: >although Andrez Konopka has shown that the intron and exon have distinct
: >properties so maybe it is possible.  I'm pretty sure that mutations in
: >immunoglobulins occur in specific places as part of their rearrangement
: >mechanism.

: Huh? Introns *are* in the DNA, that's what we're talking about. Was
: that a typo? They're transcribed onto the primary RNA and then cut
: out. Here I was just talking about DNA replication.

I suppose some of this is semantics, we talk about codons in the DNA also.  But
in terms of the mechanism, the intron exists only on the RNA where its ends are
is recognized by the spliceosome.  (More precisely, the spliceosome recognizes
the exon and then clips out the regions between them.)  However, there are
endonucleases that apparently reside in some introns (coded in the DNA) that
allow the intron to become a mobile element... so the idea of the intron as a
DNA creature may not be so far off.

  Tom Schneider
  National Cancer Institute
  Laboratory of Mathematical Biology
  Frederick, Maryland  21702-1201
  toms@ncifcrf.gov
  http://www-lmmb.ncifcrf.gov/~toms/