intron-exon novice: minimum exon size??

[Rock Pulak]

David L. Haviland, Ph.D. writes:

>If memory serves, the murine C5-anaphylatoxin receptor is comprised of
two 
>exons.  Exon one contains only the 5' flanking sequence and the
initiation 
>ATG - that's it.  The rest of the coding (ORF) is on exon 2 which is 7
kb 
>downstream.  Perhaps this "takes the cake" for the smallest exon, 3 bp!

Actually the first exon extends from the 5' end of the mRNA to the first 
splice site and not just the  nucleotides of the amino acid coding
region (often referred to as the coding sequence).  There are several
genes that have "untranslated 5' exons" which have none of the coding
sequence; the first AUG is in the second exon.  There are also a few
examples of genes that have final exons that are untranslated; translation
termination occurs in the penultimate exon (second last exon).  However,
these are still exons even though they do not contain any coding sequence.

Rock Pulak							rapulak at macc.wisc.edu
UW-Madison