Practice question

Branch point adenine, usually located eighteen to forty nucleotides upstream of three prime splice site within polypyrimidine tract, is recognized by U2 small nuclear RNP through RNA-RNA duplex formation between conserved GUAGUA region of U2 snRNA and branch site consensus, bulging adenine out of helix. Bulged adenine's two prime hydroxyl acts as nucleophile in first transesterification. Recognition involves SF1 protein initially binding branch site, then helicase Prp5 remodeling replacing SF1 with U2, stabilized by SF3B1 and SF3B3 proteins and U2AF65 interacting with polypyrimidine tract, ensuring accurate lariat formation.