introns-exons starts and ends.

Louis van de Zande L.P.W.G.M.van.de.Zande at biol.rug.nl
Wed Aug 25 11:09:42 EST 1999

Brice Quenoville wrote:

> 1/Is the so called GT-AG rule stating that all eukaryotic
> nuclear introns begin with a GT and finish with a AG
> believed to be that universal or not? If yes would these GT
> and AG always be excised when going to cDNA or can they (or
> part of them) be part of it? 

GT-AG exon-intron junctions are consensus sequences. Therefore, 
not ALL introns start with GT and end with AG, but they nearly 
always do (see Mount S.M. Nucleic Acids Research 10 (1982), 
459-472). Splicing will always leave these splice donor and 
acceptor sites out of the mRNA (the cell will not make cDNA, that 
is what we do in our labs)

> 2/Is it possible that, searching for a reading frame, a
> codon triplet would begin at the end of one exon and finish
> at the beginning of the next or is each exon a complete
> succession of codons (I would guess so but)?

Yes that is very well possible. Why should an exon exist of 
complete triplets? So you may find

   ......AGG TTG Cgt--------intron-------agAT ATT.......

and not necessarily
    ...... AGG TTG gt--------intron-------ag CAT ATT...........

for the mRNA  ........ AGG UUG CAU AUU.........

Louis van de Zande
RU Groningen
The Netherlands

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