RNA Splicing

The other major type of modification that takes place in eukaryotic pre-mRNA is the removal of introns by RNA splicing. This occurs in the nucleus following transcription but before the RNA moves to the cytoplasm.

Consensus sequences and the spliceosome Splicing requires the presence of three sequences in the intron. One end of the intron is referred to as the 5' splice site, and the other end is the 3' splice site (I Figure 14.10); these splice sites possess short consensus sequences. Most introns in pre-mRNA begin with GU and end with AG, suggesting that these sequences play a crucial role in splicing. Changing a single nucleotide at either of these sites does indeed prevent splicing. A few introns in pre-mRNA begin with AU and end with AC. These introns are spliced by a process that is similar to that seen in GU...AG introns but utilizes a different set of splicing factors. This discussion will focus on splicing of the more common GU...AG introns.

The third sequence important for splicing is at the so-called branch point, which is an adenine nucleotide that lies from 18 to 40 nucleotides upstream of the 3' splice site (see Figure 14.10). The sequence surrounding the branch point does not have a strong consensus but usually takes the

Pre-mRNA 5'

Consensus sequence

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