Splicing Step One
In the first step of primary RNA transcript splicing, the 2' –OH on the branch
point A attacks the phosphoryl group of the G in the 5' splice site. This G is
the boundary between the 5' end of the intron and the 3' end of the adjacent
exon. The attack leads to the breaking of the phosphodiester bond between the
3' end of the exon and the 5' end of the intron and the formation of a new
phosphodiester bond between the branch point A and the 5' end of the intron.
Figure3.1 Primary RNA Transcript Splicing Reaction
As is illustrated in the figure above, the first splicing step breaks the
primary transcript into two pieces. One piece contains the 5' end of an exon,
with a new free 3' –OH. The other piece is a strange looking structure that
resembles a lasso and as a result is called a lariat structure. The lariat
contains the intron that is still connected to a second exon.
Splicing Step Two
In the second step of primary transcript splicing, the newly released 3' –OH of
exon 1 attacks the phosphoryl group on the 5'-most nucleotide of exon 2. The
resulting phosphodiester bond is the boundary between the intron and the second
exon. The final products of the reaction are the two joined exons and the
intron, which is still in the lariat structure.
Alternative Splicing
Alternative splicing is a way of introducing diversity to the gene products of
transcription by creating multiple protein products from the same DNA sequence.
Alternative splicing accomplishes this in two ways: 1) by splicing together
exons from two different primary RNA transcripts in a process called trans-
splicing; 2) by splicing out entire exons.
Figure 3.2: Exon-Exclusion Splicing
