Final answer:
The three U's that define intron boundaries are the 5' UTR, the protein-coding region, and the 3' UTR.
Step-by-step explanation:
The three U's that define intron boundaries are the 5' untranslated region (UTR), the protein-coding region, and the 3' UTR.
The 5' UTR is the region just upstream of the protein-coding region in an RNA molecule that is not translated. The protein-coding region contains the sequence of nucleotides that codes for the amino acid sequence of a protein. The 3' UTR is the region just downstream of the protein-coding region that is also not translated.