Q.12 The enzyme that transcribes the eukaryotic genes encoding precursor
ribosomal RNAs (pre-rRNAs) of 28S, 18S and 5.8S rRNAs is
(A) RNA polymerase I
(B) RNA polymerase II
(C) RNA polymerase III
(D) RNA polymerase IV
RNA Polymerase I is the enzyme responsible for transcribing eukaryotic genes encoding precursor ribosomal RNAs (pre-rRNAs) for 28S, 18S, and 5.8S rRNAs. The correct answer to the multiple-choice question is (A) RNA polymerase I.
Correct Answer
The enzyme that transcribes the eukaryotic genes encoding precursor ribosomal RNAs (pre-rRNAs) of 28S, 18S, and 5.8S rRNAs is (A) RNA polymerase I. These rRNAs are synthesized from a single 45S pre-rRNA transcript in the nucleolus, where RNA Polymerase I (Pol I) operates at high rates to support ribosome biogenesis.
Option Breakdown
Eukaryotic cells use distinct RNA polymerases for specific RNA types, enabling precise gene regulation.
-
(A) RNA polymerase I: Transcribes rDNA genes into 45S pre-rRNA, processed into mature 28S, 18S, and 5.8S rRNAs; accounts for over 60% of nuclear transcription.
-
(B) RNA polymerase II: Primarily transcribes protein-coding genes into mRNA, plus snRNAs and miRNAs; essential for gene expression but not rRNAs.
-
(C) RNA polymerase III: Synthesizes tRNAs, 5S rRNA, and small RNAs like U6 snRNA; handles short, abundant non-coding RNAs excluding major rRNAs.
-
(D) RNA polymerase IV: Found mainly in plants for siRNA production in RNA-directed DNA methylation; absent in most eukaryotes like animals and fungi.
Why RNA Polymerase I?
Pol I’s specialization supports massive rRNA demand for ribosomes, linking transcription directly to cell growth. Dysregulation causes diseases like cancer. This makes it critical for exams in molecular biology and genetics.
