Why don't we couple translation and transcription in eukaryotic cells?

• A. Because introns must be spliced out of pre-mRNA to produce functional mRNA
• B. Because the nucleus blocks ribosome access
• C. Because DNA replication must precede transcription
• D. Because RNA polymerase translates proteins

Answer: (A)

The main idea is that in eukaryotes transcription and translation are separated both in space and in time, so the RNA must be processed before it can be translated. Transcription in the nucleus produces pre-mRNA that contains introns. These introns must be removed by splicing, and the exons joined, to create a continuous, functional coding sequence. Beyond splicing, the mRNA also undergoes capping and polyadenylation before it’s exported to the cytoplasm. Only after this maturation can ribosomes access the mRNA and translate it. If translation tried to begin on the unprocessed transcript, introns would disrupt the reading frame and yield nonfunctional products.

The nucleus barrier is related but not the specific reason; the crucial point is that introns must be removed to produce a proper mRNA template before translation can occur.