exon–intron junctions (marked by “/”) that are required for proper splicing out of the
introns. The 5^0 exon–intron junction consensus sequence is AG/GURAGU, and the 3^0
exon–intron junction consensus sequence is YAG/RNNN (Y, pyrimidine; R, purine; N,
either purine or pyrimidine). Also, about 100 nucleotides upstream of the 3^0 exon–intron
junction there is a branchpoint conserved sequence. Asplicesomecomposed of small
nuclear RNAs (snRNAs—these RNAs are 100–300 bases in length) and various proteins
forms over the intron and helps in the splicing process. Most mRNAs contain a polyadeny-
lated 3^0 end consisting of 200 A residues. This polyA tail acts as a protective buffer against
RNAses that could digest the mRNA from the 3^0 end, and thus stabilizes the molecule.
Approximately 10–30bp upstream of this polyA tail is the invariant hexamer sequence
AAUAAA (Fig. 6.10).
Figure 6.10.Overview of mRNA processing. Three steps of processing must occur prior to export of
the mature mRNA out of the nucleus: 5^0 capping, 3^0 polyadenylation, and intron splicing.
6.3. TRANSCRIPTION 147