Introns in nuclear genes contain three consensus sequences critical to splicing: a 5' splice site, a 3' splice site, and a branch point. Splicing of pre-mRNA takes place within a large complex called the spliceosome, which consists of snRNAs and proteins.

The process of splicing To illustrate the process of RNA splicing, we'll first consider the chemical reactions that take place. Then we'll see how these splicing reactions constitute a set of coordinated processes within the context of the spliceosome.

Before splicing takes place, an upstream exon (exon 1) and a downstream exon (exon 2) are separated by an intron (I Figure 14.11). Pre-mRNA is spliced in two distinct steps.

Exon 1

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