So how would introns work to facilitate the origin and/or spread of metazoan life? Let’s begin with one obvious example – alternative splicing. Below if figure to help you visualize this.
We have one gene with 4 exons, the nucleotides that code for an amino acid. Between them are three introns (in green). When the DNA is is transcribed into RNA, the introns will be cut out by the spliceosome. But we can see tfrom this figure, exon four was cut away along with the 3rd intron, making a protein with exons 1-2-3. On the other hand, the RNA was processed such that exon three and its upstream intron was cut out, giving us a different version of the protein with exons 1-2-4.
Now simply imagine a gene with 10 exons and 9 introns and all the possible proteins that could be produced.
When you grasp the beauty of alternative splicing, the supposed klugy, inefficiency of intons fades from view. Cutting away stretches of “junk” would appear needlessly wasteful by itself. But a more holistic perspective allows as to see that this junk allows genes to get “more bang for their buck.” Without introns, it’s basically one gene – one protein. With introns, one can spawn dozens of proteins, all variants on a theme. And this looks like a designed search strategy. More on that in a bit, but next, let’s see how this process would so beautifully facilitate the evolution of metazoan life forms.
For those interested, there is a nice youtube video below the fold to visualize the process whereby alternative splicing makes it possible to give the same gene two different fates.