Cpsf

14.8 Processing of the 3' end of pre-mRNA requires a consensus sequence and several factors.

Hairpin-

Pre-mRNA 5'

^^ Hairpin-binding protein Consensus sequence 3' cleavage site

^^ Hairpin-binding protein Consensus sequence 3' cleavage site

Hairpin-

Region of probable pairing

14.9 Eukaryotic mRNAs that lack a poly(A) tail depend on a different mechanism for 3' cleavage.

Cleavage requires the presence of U7 snRNA, which has bases complementary to a consensus sequence downstream of the 3' cleavage site. Cleavage depends on the formation of a hairpin structure near the 3' end of the pre-mRNA; base pairing probably takes place between the complementary regions of the pre-mRNA and the U7 snRNA.

form YNYYRAY (Y is any pyrimidine, N is any base, R is any purine, and A is adenine). The deletion or mutation of the adenine nucleotide at the branch point prevents splicing.

Splicing takes place within a large complex called the spliceosome, which consists of several RNA molecules and many proteins. The RNA components are small nuclear RNAs (Chapter 13); these snRNAs associate with proteins to form small ribonucleoprotein particles. Each snRNP contains a single snRNA molecule and multiple proteins. The spliceosome is composed of five snRNPs, named for the snRNAs that they contain (U1, U2, U4, U5, and U6), and some proteins not associated with an snRNA.

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