RNA splicing
/ AR-en-ay SPLY-sing /
Picture a film editor handed a reel where the good scenes are separated by long stretches of clapperboards, bloopers, and blank leader. The editor's job is to cut out all the junk between scenes and splice the keeper scenes together into a continuous movie. RNA splicing is the cell doing exactly this to a freshly made transcript: cutting out the non-coding stretches and joining the coding ones.
Eukaryotic genes are interrupted: their coding information is broken into pieces called exons (the parts that are expressed, the keeper scenes) separated by introns (intervening sequences, the junk between scenes). The whole gene, introns and all, is transcribed into the pre-mRNA. Splicing then precisely removes each intron and ligates the flanking exons end to end, so the final mRNA reads as one uninterrupted coding stretch. The cuts must be exact to the single nucleotide — being off by even one base would shift the reading frame and scramble the protein. Introns are often far longer than exons; a human gene can be tens of thousands of bases of DNA yet yield an mRNA of only a couple of thousand once the introns are gone.
Why do cells carry introns at all, then transcribe and discard them at great expense? Part of the answer is that splicing is not just tidying — it is an opportunity. Because the exons can be joined in more than one way (alternative splicing), one gene can yield several different proteins. Introns also let evolution shuffle exons between genes to build new proteins from old parts. So the messy-looking interrupted gene is, in fact, one source of the astonishing variety of eukaryotic proteins.
If a pre-mRNA reads exon1-INTRON-exon2-INTRON-exon3, splicing removes both introns and joins the exons to give exon1-exon2-exon3 as the mature coding message. The intron sequences are released and degraded.
Splicing excises introns and joins exons so the mature mRNA reads as one continuous coding sequence.
A common misconception is that introns are useless junk. Many introns carry regulatory elements, host other genes such as microRNAs, and are essential to alternative splicing — so the cell is not merely wasting energy by transcribing them.