Which statement distinguishes pre-mRNA from mature mRNA?

• A. Pre-mRNA contains introns; mature mRNA contains only exons
• B. Pre-mRNA is translated in the cytoplasm; mature mRNA remains in the nucleus
• C. Pre-mRNA has a cap and poly-A tail; mature mRNA lacks these features
• D. Pre-mRNA is found in mitochondria; mature mRNA is in the cytoplasm

Answer: (A)

RNA processing in eukaryotes involves removing introns and adding protective features to produce a mature mRNA. The crucial difference is that the initial transcript, pre-mRNA, contains both introns and exons, while the mature mRNA contains only exons after splicing. Introns are chopped out and exons are joined, and the molecule also gets a 5' cap and a 3' poly-A tail so it can be translated efficiently in the cytoplasm. This makes the statement about pre-mRNA having introns and mature mRNA containing only exons the best description of their difference. The other statements conflict with how transcription and processing actually work: translation occurs for mature mRNA in the cytoplasm, not pre-mRNA; mature mRNA has the cap and poly-A tail, which pre-mRNA lacks; and pre-mRNA is produced in the nucleus, not in mitochondria.