Figure 2. The generation of diverse mRNA repertoires.

a, Human KCNMA1 pre-mRNA is depicted, with constitutive exons in blue and alternative exons in yellow and red. The possible splicing patterns for each exon are indicated above and below the pre-mRNA (black lines).The KCNMA1 pre-mRNA contains multiple alternative 5′ splice sites, alternative 3′ splice sites and sequence corresponding to cassette exons. Together, these allow more than 500 mRNA isoforms to be synthesized from a single pre-mRNA. b, Drosophila melanogaster Dscam pre-mRNA is depicted, with constitutive exons in blue and alternative exons in the exon 4, 6, 9 and 17 clusters shown in yellow, orange, red and purple, respectively. The splicing pattern for one mRNA isoform is shown as an example. The Dscam gene contains 95 alternative exons organized into four clusters of mutually exclusive exons (that is, only exon from each cluster is transcribed). These four clusters are at exon 4, exon 6, exon 9 and exon 17, which contain 12, 48, 33 and 2 variable exons, respectively. In combination with the 20 constitutive exons of Dscam, this structure allows 38,016 mRNAs to be synthesized from a single pre- mRNA. c, The organization of the D. melanogaster gene mod(mdg4) is depicted, with common exons in blue and variable exons in purple. The common exons and some of the variable exons are present on one strand of the DNA and are transcribed from left to right as shown, whereas another set of the variable exons is present on the opposite strand and is transcribed from right to left as illustrated. The precise number, and the precise positions of the beginnings and ends, of the common exon pre-mRNA and the variable exon pre-mRNAs is unknown. The possible splicing patterns for each exon in the corresponding pre-mRNA are indicated by black lines, with the possible positions of poly(A)+ tails indicated by green wavy lines. A total of 28 mod(mdg4) mRNAs are synthesized, and this occurs through the trans-splicing of the common exons to one set of variable exons.