Practice question

Early spliceosome assembly begins with E complex formation where U1 snRNP small nuclear ribonucleoprotein particle recognizes five prime splice site through complementary base pairing between five prime end of U1 snRNA CAGGUAAGU motif and pre-mRNA splice site sequence. This interaction stabilized by U1C protein and SR proteins binding exonic splicing enhancers. Recognition defines exon-intron border and recruits U2AF heterodimer to downstream polypyrimidine tract and AG at three prime site. Subsequent ATP-dependent rearrangements replace U1 with U6 snRNA forming catalytic center but initial fidelity depends primarily on U1.