Many genes in higher organisms contain introns—regions that do not code for protein. These introns are interspersed with exons, or coding regions. If the introns are not spliced out properly, the protein produced from the message will contain regions of polypeptide nonsense. The protein won’t fold properly, its function will be diminished or destroyed, and you’ll get sick.

Still, you’d think the cell would have evolved genes that don’t need splicing. But introns add a new level of control to gene expression. Think about this: when Scorcese goes to the cutting room, he can splice his raw film in many different ways. Heck, he could walk out of there with two different movies. It’s the same with introns. Consider a length of mRNA with four regions. If you consider regions 2 and 4 introns, and you splice them out, then you get a finished message composed of regions 1 and 2, which will yield a polypeptide we’ll call A. If instead you consider region 2 the only intron, and splice it out, you get a finished message containing regions 1, 3 and 4, which yields a different protein, B. A and B are different proteins, but they come from the same gene!

Figure 18. Cutting Genes to Fit. Transcription yields a messenger RNA containing introns (green) and exons (red). Alternative splicing yields two alternative mRNA's, which code for either protein a or protein b.