Step 1: Understanding the Concept:
Transcription is the biological process of creating an RNA copy of a DNA sequence.
In prokaryotic cells, a single type of RNA polymerase enzyme is responsible for transcribing all three major types of RNA (mRNA, tRNA, rRNA).
However, eukaryotic cells have a higher level of complexity and utilize specialized RNA polymerases for transcribing different classes of genes.
Step 2: Detailed Explanation:
In the nucleus of a eukaryotic cell, there are three distinct RNA polymerases:
1. RNA Polymerase I: This enzyme is localized in the nucleolus. Its primary function is to transcribe genes for ribosomal RNA (rRNA), specifically the 28S, 18S, and 5.8S subunits.
2. RNA Polymerase II: This is the most crucial enzyme for protein synthesis. It transcribes the precursors of messenger RNA (mRNA), which are called heterogeneous nuclear RNA (hnRNA). These transcripts undergo further processing like capping, tailing, and splicing before they exit the nucleus for translation. It also transcribes some small nuclear RNAs (snRNAs).
3. RNA Polymerase III: This enzyme transcribes transfer RNA (tRNA), the 5S rRNA subunit, and other small regulatory RNAs.
- DNA Polymerase (A): This enzyme is involved in DNA {replication}, not transcription.
Since the question specifically asks for the enzyme that synthesizes mRNA, the correct answer is RNA polymerase II.
Step 3: Final Answer:
In eukaryotes, the division of labor between RNA polymerases ensures that Pol II is dedicated to transcribing the protein-coding genes into mRNA.
Final Answer: (C)