What signals indicate the 5' and 3' splice sites?

• A. 5' AU; 3' GC; no branch point; linear excision
• B. 5' GU; 3' AG; no branch point; direct exon ligation
• C. 5' GU; 3' AG; branch point A; lariat formation
• D. 5' GA; 3' GU; branch point C; loop removal

Answer: (D)

Splicing relies on specific signals at the intron boundaries and a pivotal branch point inside the intron. The 5' splice site is typically GU in the RNA sequence, marking where the first cut occurs, and the 3' splice site is typically AG, marking where the second cut happens. Inside the intron lies a branch point, usually an adenine, which acts as the nucleophile in the first transesterification to form a lariat-shaped intron. This lariat intermediate is then removed and the adjacent exons are joined. So the defining signals are the 5' GU, the 3' AG, and a branch point A that enables lariat formation. The other options describe noncanonical dinucleotides, missing a branch point, or suggest direct exon ligation without the lariat intermediate, which doesn’t fit the real splicing mechanism.