In eukaryotes, which RNA polymerase transcribes mRNA, and where does processing occur?

• A. RNA polymerase I; in the cytoplasm.
• B. RNA polymerase II; in the nucleus.
• C. RNA polymerase III; in the nucleolus.
• D. RNA polymerase IV; at the plasma membrane.

Answer: (B)

Transcribing mRNA in eukaryotes is performed by RNA polymerase II, and the processing of that mRNA occurs in the nucleus. As transcription proceeds, RNA Pol II adds a 5' cap, splices out introns, and then a poly-A tail is added at the 3' end; these processing steps are coordinated by the polymerase II machinery via its C-terminal domain. Once processing is complete, the mature mRNA is exported to the cytoplasm for translation.

The other options mix up which enzyme handles mRNA and where processing happens. RNA polymerase I makes rRNA in the nucleolus, and RNA polymerase III makes tRNA and some small RNAs in the nucleus. RNA polymerase IV isn’t used for mRNA in animals. Processing in the cytoplasm, the nucleolus, or at the plasma membrane does not reflect how eukaryotic mRNA is generated and matured.