exons and introns

Imagine recording a long voice memo and then editing it: you keep the useful sentences and delete the long pauses and false starts before you send it. Eukaryotic genes work like that. The cell first copies the whole gene into RNA, then cuts out chunks and stitches the keepers back together before the message is used.

The keepers are exons (the 'expressed' pieces that end up in the mature messenger RNA) and the removed pieces are introns (the 'intervening' sequences). The full gene is first transcribed into a pre-messenger-RNA that contains both, in the order exon-intron-exon-intron and so on. A molecular machine called the spliceosome then recognizes the boundaries (introns almost always begin with GU and end with AG), loops out each intron, removes it, and joins the flanking exons. Because genes are interrupted this way, they are called 'split genes'. Most prokaryotes lack introns in their protein-coding genes, while a typical human gene has several or many.

This organization matters for two big reasons. First, the same set of exons can be spliced together in different combinations — alternative splicing — so one gene can encode many distinct proteins, hugely expanding the protein repertoire from a modest gene count. Second, having genes broken into modular exons lets evolution shuffle and recombine functional pieces (exon shuffling) to build new proteins. Introns are not mere waste; some carry regulatory information and small RNAs, and the act of splicing itself is woven into how the cell controls and exports its messages.