Which statements accurately describe basic splice-site signals?

• A. Introns are removed; exons joined; 5' GU and 3' AG are splice-site signals; branch point A.
• B. Exons removed; introns ligated; 5' AT and 3' CT signals; branch point G.
• C. Introns removed; exons joined; 5' GT and 3' AG signals; branch point C.
• D. Introns removed; exons joined; 5' GU and 3' AG signals; branch point U.

Answer: (A)

Splicing relies on specific sequence signals at the ends of introns and a key nucleotide inside the intron to cut and rejoin the transcript. The 5' end of an intron starts with GU in the RNA, and the 3' end ends with AG. Inside the intron, a branch-point adenine serves as the nucleophile to initiate the first cut, creating a lariat intermediate. With these signals, the intron is removed and the neighboring exons are joined together. That combination—introns removed, exons joined, plus the 5' GU and 3' AG signals and the branch-point adenine—matches the basic description of splice-site signals. The other options mix in incorrect signals (such as AT/CT or GT in RNA context) or the wrong branch-point nucleotide, or describe the wrong RNA segments being removed.