Question

Which of the following enzymes synthesizes precursor mRNA ?

• A. DNA polymerase
• B. RNA polymerase I
• C. RNA polymerase II
• D. RNA polymerase III

Hint:

• Mnemonic (r-m-t): To remember the primary products of the three eukaryotic RNA polymerases in sequence, associate Polymerases I, II, and III with the letters r, m, and t respectively: itemize

• I \(\rightarrow\) rRNA

• II \(\rightarrow\) mRNA precursor (hnRNA)

• III \(\rightarrow\) tRNA
Prokaryotic Exception: In prokaryotic organisms, a single DNA-dependent RNA polymerase is responsible for the transcription of all functional classes of RNA (mRNA, tRNA, and rRNA). itemize

Answer 1

The Correct Option is C

Concept:

• Eukaryotic Transcription: Unlike prokaryotes, which utilize a single RNA polymerase to transcribe all types of RNA, eukaryotic cells exhibit a clear division of labor with three distinct RNA polymerase enzymes in their nucleus: itemize

• RNA Polymerase I: Transcribes ribosomal RNAs (28S, 18S, and 5.8S rRNAs), which are structural and catalytic components of ribosomes.

• RNA Polymerase II: Transcribes protein-coding genes to produce heterogeneous nuclear RNA (

hnRNA), which serves as the direct precursor of messenger RNA (mRNA).

• RNA Polymerase III: Transcribes transfer RNA (tRNA), 5S rRNA, and other small nuclear RNAs (snRNAs).
itemize

Step 1: Understand the nature of precursor mRNA in eukaryotes
In eukaryotic transcription of protein-coding genes, the immediate product is not a fully functional messenger RNA (mRNA). Instead, a primary transcript called heterogeneous nuclear RNA (hnRNA) or precursor mRNA (pre-mRNA) is synthesized. This pre-mRNA contains both coding sequences (exons) and non-coding sequences (introns) and must undergo post-transcriptional processing (capping, tailing, and splicing) to become mature, functional mRNA.

Step 2: Identify the specific transcribing enzyme
Based on the division of labor in eukaryotic nuclei:

• RNA Polymerase I is restricted to transcribing ribosomal RNA genes.

• RNA Polymerase III is restricted to smaller functional RNA genes, such as tRNAs and the 5S ribosomal subunit.

• RNA Polymerase II is responsible for transcribing structural, protein-coding genes into hnRNA (pre-mRNA).
Thus, the enzyme that synthesizes precursor mRNA is RNA Polymerase II, which corresponds to option (C).