<P> snRNPs (pronounced "snurps"), or small nuclear ribonucleoproteins, are RNA - protein complexes that combine with unmodified pre-mRNA and various other proteins to form a spliceosome, a large RNA - protein molecular complex upon which splicing of pre-mRNA occurs . The action of snRNPs is essential to the removal of introns from pre-mRNA, a critical aspect of post-transcriptional modification of RNA, occurring only in the nucleus of eukaryotic cells . Additionally, U7 snRNP is not involved in splicing at all, as U7 snRNP is responsible for processing the 3 ′ stem - loop of histone pre-mRNA . </P> <P> The two essential components of snRNPs are protein molecules and RNA . The RNA found within each snRNP particle is known as small nuclear RNA, or snRNA, and is usually about 150 nucleotides in length . The snRNA component of the snRNP gives specificity to individual introns by "recognizing" the sequences of critical splicing signals at the 5' and 3' ends and branch site of introns . The snRNA in snRNPs is similar to ribosomal RNA in that it directly incorporates both an enzymatic and a structural role . </P> <P> SnRNPs were discovered by Michael R. Lerner and Joan A. Steitz . Thomas R. Cech and Sidney Altman also played a role in the discovery, winning the Nobel Prize for Chemistry in 1989 for their independent discoveries that RNA can act as a catalyst in cell development . </P> <P> At least five different kinds of snRNPs join the spliceosome to participate in splicing . They can be visualized by gel electrophoresis and are known individually as: U1, U2, U4, U5, and U6 . Their snRNA components are known, respectively, as: U1 snRNA, U2 snRNA, U4 snRNA, U5 snRNA, and U6 snRNA . </P>

Snrnps are most closely associated with which of the following