A mechanism for generating multiple protein isoforms from a single gene that involves the splicing together of nonconsecutive exons during the processing of some, but not all, transcripts of the gene. This is illustrated in the diagram, where a gene is made up of five exons joined by introns i1–i4. The exons may be spliced by the upper pathway shown by the dotted lines to generate a mature transcript containing all five exons. This type of splicing is termed constitutive. The alternative mode of splicing shown generates a mature transcript that lacks exon 4. If each exon encodes 20 amino acids, the constitutive splicing path would result in a polypeptide made up of 100 amino acids. The alternative path would produce a polypeptide only 80 amino acids long. If the amino acid sequences of the two proteins were determined, the first 60 and the last 20 would be identical. The premessenger RNAs (q.v.) of at least 40% of all human genes undergo alternative splicing. This removes the intron RNAs and joins the adjacent exon RNAs by phosphodiester linkages. The splicing takes place in spliceosomes (q.v.) that reside within the nucleus. Therefore the number of proteins encoded by the human genome is many times larger than the number of structural genes it contains. See Chronology, 1977, Weber et al.; adenovirus, DSCAM, fibronectin, Human Genome Project, isoforms, posttranscriptional processing, myosin genes, RNA splicing, tropomyosin.
Subjects: Genetics and Genomics.