RNA Processing

The fact that most human genes are composed of many exons has some important consequences for the expression of genetic information. First, we now know that many genes are spliced in more than one way, a phenomenon known as alternative splicing. For example, some types of cells might leave out an exon from the final mRNA that is left in by other types of cells, giving it a slightly different function. This means that a single gene can code for more than one protein. Some complicated genes appear to be spliced to give hundreds of alternative forms. Alternative splicing, therefore, can increase the coding capacity of the genome without increasing the number of genes.

A second consequence of the exon/intron gene structure is that many human gene mutations affect the splicing pattern of that gene. For example, a mutation in the sequence at an intron/exon junction that is recognized by the spliceosome can cause the junction to be ignored. This causes splicing to occur to the next exon in line, leaving out the exon next to the mutation. This is called exon skipping and it usually results in an mRNA that codes for a nonfunctional protein. Exon skipping and other errors in splicing are seen in many human genetic diseases.