spliceosome
/ SPLY-see-oh-some /
Cutting introns out of an RNA and rejoining the exons sounds simple until you realize it must be done with single-letter precision, on a huge variety of genes, millions of times. The cell does not leave this to chance. It uses a large, self-assembling molecular machine that finds the boundaries of each intron, cuts, and seals. That machine is the spliceosome.
What makes the spliceosome unusual is what it is made of. It is not purely protein; its core working parts are small RNA molecules (called small nuclear RNAs, packaged with proteins into units called snRNPs, pronounced 'snurps'). These RNAs do the recognizing and even help catalyze the chemistry of cutting and joining. The spliceosome assembles fresh on each intron, recognizes the signposts at the intron's start and end, loops the intron out into a lariat shape, snips it free, and ligates the two flanking exons together — then disassembles, ready to do it again.
The spliceosome matters for two big reasons. First, it is one of the strongest pieces of evidence that RNA can act as an enzyme, not just as a passive message — a clue to how early life might have worked before proteins dominated. Second, by deciding which boundaries to use, it is the very machine that carries out alternative splicing, so errors in it or in the signals it reads cause real human diseases, from some forms of muscular atrophy to certain cancers.
In spinal muscular atrophy, a gene is mis-spliced so that a key exon gets skipped, yielding a broken protein. A modern drug called nusinersen works by nudging the spliceosome to include that exon again — fixing the editing rather than the gene.
The drug nusinersen treats spinal muscular atrophy by redirecting the spliceosome to keep a vital exon.
The spliceosome's catalytic core is RNA, not protein — it is a ribozyme. That is a key reason scientists think RNA could have run chemistry before proteins existed.