In eukaryotic cells, which nuclear RNA polymerase is primarily responsible for transcribing messenger RNA (mRNA)? Select the most appropriate option.

Introduction / Context:
Eukaryotic transcription is partitioned among three major nuclear RNA polymerases, each specializing in different RNA classes. Knowing which polymerase makes mRNA is foundational for understanding gene expression, RNA processing, and the targets of transcriptional regulation and inhibitors.

Given Data / Assumptions:

• Eukaryotic nuclei contain RNA polymerase I, II, and III.
• mRNA refers to capped, polyadenylated transcripts that encode proteins.
• rRNA and tRNA are primarily noncoding functional RNAs.

Concept / Approach:
The canonical division of labor is: RNA polymerase I synthesizes the large rRNA precursor (45S rRNA in higher eukaryotes); RNA polymerase II synthesizes mRNA and many small nuclear RNAs; RNA polymerase III synthesizes tRNA, 5S rRNA, and other small RNAs. Therefore, the polymerase for mRNA is RNA polymerase II.

Step-by-Step Solution:
Match RNA class to polymerase: mRNA → RNA pol II.Exclude Pol I (mostly 45S rRNA precursor) and Pol III (tRNA and 5S rRNA).Confirm that nuclear mRNA synthesis requires Pol II and its general transcription factors.

Verification / Alternative check:
Experimental inhibitors such as alpha-amanitin selectively inhibit RNA polymerase II at low concentrations, blocking mRNA synthesis; this supports the assignment.

Why Other Options Are Wrong:
RNA polymerase I and III transcribe rRNA and tRNA classes, not mRNA. 'None of these' and mitochondrial RNA polymerase are incorrect because nuclear mRNA is synthesized by RNA polymerase II.

Common Pitfalls:
Confusing RNA polymerase II (mRNA) with RNA polymerase I (rRNA) or thinking that all RNA is made by a single polymerase as in bacteria.

Final Answer:
RNA polymerase II.