Another mechanism that explains how the genome can encode for such a wide array of proteins is alterative splicing. Normally RNA splicing removes the introns and then reattaches the exons in sequence. However, in alternative splicing some of the exons may not be reattached. Since the exons contain coding information for the polypeptide, the removal of some of the exons will produce a slightly different form of the protein. It is important to note that the order of the exons is usually not changed, some of them are just not included in the final mRNA. Alternative splicing is also sometimes called exon shuffling. From an evolutionary perspective, alternative splicing allows a species to produce proteins of related function from a single transcription event. Often these proteins are tissue specific, meaning that the pre-mRNA is edited one way in one tissue, but is alternatively spliced in another. There are many examples of alternative splicing in mammals, and it is believed to play a major role in explaining the size of the proteome.
