Spliceosome !

The spliceosome is a large RNA–protein machine that cuts introns out of pre-mRNA and joins the exons; its catalytic core is RNA.

Updated 2026-11-01

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Key facts

Five snRNPs (U1, U2, U4, U5, U6), each with an snRNA and Sm proteins, plus about 150 accessory proteins.
U1 recognises the 5' site (GU), U2 the branch point (A), U2AF the pyrimidine tract and the 3' AG.
U6 snRNA together with U2 positions the Mg²⁺ ions that catalyse both transesterifications: it is a ribozyme.
It is assembled anew for each intron and recycled; it uses ATP in the helicases that rearrange it.
A 'minor' spliceosome (U11, U12, U4atac, U6atac) handles a small group of introns.

SR proteins (activate) and hnRNPs (repress) bind splicing enhancers and silencers and decide which exons are kept.
SR protein phosphorylation and Pol II speed modulate alternative splicing.

Pladienolide B, E7107, H3B-8800 (SF3B1 modulators; investigational)
Splice-switching antisense oligonucleotides (nusinersen in spinal muscular atrophy; exon-skipping ones in Duchenne)

Somatic mutations in SF3B1 (myelodysplastic syndromes with ring sideroblasts), SRSF2 and U2AF1 are common in myeloid neoplasms.
Anti-Sm (lupus) and anti-U1 RNP (mixed connective tissue disease) antibodies target spliceosome proteins.
Mutations in PRPF8 and other factors cause retinitis pigmentosa.

Think of the answer, then tap the question to see it.

Which spliceosome component carries out catalysis?

RNA: U6 and U2 snRNAs, which position the Mg²⁺ ions; the spliceosome is a ribozyme.

Which snRNP recognises the 5' splice site?

U1 snRNP, which pairs with the GU at the start of the intron.

Which somatic spliceosome mutation is linked to ring sideroblasts?

SF3B1 mutations in myelodysplastic syndromes.

What does the 3D model 6EN4 show?

The human SF3b core (with SF3B1 and PHF5A) bound to pladienolide B (BGZ), a splicing modulator.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: SF3B1: Splicing factor 3B subunit 1 (human) (UniProt, 2026); 6EN4: SF3b core in complex with a splicing modulator (RCSB Protein Data Bank, 2026); mRNA Splicing (Reactome, 2026); Intron (National Human Genome Research Institute (NHGRI), 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.