splicing, introns, and exons
/ SPLY-sing, IN-tronz, EK-sonz /
Imagine a recorded speech where the speaker kept rambling off-topic between the good lines. To get a clean version, an editor cuts out every off-topic stretch and tapes the good lines back together in order. Eukaryotic genes are like that messy recording: the useful coding parts are interrupted by long pieces of filler, and the cell must edit them out. That editing is called splicing.
The two kinds of pieces have names. Exons are the segments that are EXpressed — they remain in the final message and carry the actual instructions. Introns are the INtervening segments — the filler that lies between exons and gets removed. A freshly made transcript contains both, alternating along its length. Splicing is the cutting-and-rejoining process that precisely snips out every intron and seals the exons together end to end, producing a continuous coding message. The cuts have to be exact to the single letter; being off by even one would scramble everything downstream.
Why carry introns at all, only to throw them away? It looks wasteful, but it buys flexibility. Because exons can be stitched together in more than one way (alternative splicing), a single gene can encode several different proteins. Introns also give genes room to evolve and can harbor regulatory signals. Splicing is mostly a eukaryotic affair; bacterial genes are usually intron-free, so their transcripts need no such editing.
The human dystrophin gene is enormous, mostly introns: its primary transcript is over two million letters long, but after splicing removes the introns, the mature mRNA is only about fourteen thousand letters — over 99 percent of the original transcript is spliced out as introns.
In the dystrophin gene, splicing discards over 99 percent of the transcript as introns.
An easy memory aid: EXons are EXpressed (kept), INtrons are INterrupting (removed). But beware — an exon is not the same as a protein-coding region; exons also include the untranslated ends of an mRNA.