Final answer:
U2 binds the adenine nucleotide at the branch site in RNA processing, resulting in the nucleotide bulging out, which is a key step in the splicing reaction enabling proper gene expression.
Step-by-step explanation:
After BBP is displaced by U2 in the branch site, U2 binds the adenine nucleotide, causing it to bulge out. The branch site is a crucial component of the splicing process in RNA processing. The adenine nucleotide that bulges out is important because it serves as the nucleophile that attacks the 5' splice site during the splicing reaction, allowing for the removal of introns and the ligation of exons to form mature mRNA. This process is essential for proper gene expression and the creation of functional proteins.