In which organisms is transcription-translation coupling common?

• A. Archaea and bacteria
• B. In bacteria; in eukaryotes separation by nucleus prevents coupling.
• C. In eukaryotes only
• D. Mitochondria only

Answer: (B)

Transcription-translation coupling happens when ribosomes begin translating an mRNA while it’s still being synthesized. This can occur only when there’s no nuclear barrier between transcription in the cytoplasm and translation. In bacteria, there’s no nucleus, so as soon as a gene is transcribed, ribosomes can latch onto the growing RNA and start protein synthesis right away. This tight coupling speeds up gene expression and keeps transcription and translation tightly linked.

In eukaryotes, transcription happens in the nucleus and the resulting mRNA is processed and then exported to the cytoplasm for translation. The physical and temporal separation means they aren’t coupled in time or space, so translation starts only after transcription is finished and the mRNA has been processed.

That contrast explains why this coupling is common in bacteria and not in eukaryotes.