27. During pre-mRNA splicing, which molecular event directly generates the 2'-5' phosphodiester bond characteristic of the lariat intermediate? (A) ATP-dependent helicase activity ofthe spliceosome. (B) Nucleophilic attack by the 2'-OH ofthe branch-point adenosine on the 5' splice site. (C) Cleavage ofthe 3' splice site by U2AF. (D) Base pairing between U1 snRNA and the 5' splice site. (E) Dephosphorylation of the 5' cap.