In mature mRNA, which segments are retained after RNA processing?

• A. Exon
• B. Intron
• C. Promoter
• D. Terminator

Answer: (A)

During RNA processing in eukaryotes, the transcript that becomes mature mRNA is edited so that only the coding, or exonic, portions remain. The spliceosome cuts out the introns—noncoding intervening sequences—and then joins the exons together, creating a continuous sequence that will be translated into protein (or function as RNA, if it’s a noncoding gene). Promoters and terminators are DNA elements that signal where transcription starts and ends; they are not included in the final RNA product. The mature mRNA may also gain a 5' cap and a 3' poly-A tail to help stability and export, but these additions don’t change which segments are retained.