Q.8 Which one of the following is transcribed by RNA polymerase III in eukaryotes?
(A) 18S rRNA
(B) 28S rRNA
(C) miRNA
(D) tRNA

RNA polymerase III in eukaryotes specifically transcribes tRNA among the given options. This enzyme handles small non-coding RNAs essential for protein synthesis and other cellular processes.​

Option Analysis

18S rRNA (A): This ribosomal RNA forms part of the small ribosomal subunit and derives from a 45S pre-rRNA precursor. RNA polymerase I transcribes it in the nucleolus.​

28S rRNA (B): As a major component of the large ribosomal subunit, 28S rRNA also comes from the 45S pre-rRNA transcript produced by RNA polymerase I.​

miRNA (C): MicroRNAs regulate gene expression and originate from pri-miRNA transcripts. RNA polymerase II transcribes miRNA genes, similar to mRNA.​

tRNA (D): Transfer RNA carries amino acids during translation. RNA polymerase III transcribes all tRNA genes using internal promoters (A-box and B-box).​

RNA polymerase III transcribes essential small RNAs in eukaryotic cells, making it a key topic for CSIR NET Life Sciences aspirants preparing for molecular biology sections. This guide breaks down a common MCQ: “Which one of the following is transcribed by RNA polymerase III in eukaryotes? (A) 18S rRNA (B) 28S rRNA (C) miRNA (D) tRNA,” with in-depth analysis for better retention.​

Eukaryotic RNA Polymerases Overview

Eukaryotes use three nuclear RNA polymerases with specialized roles. RNA polymerase I handles most ribosomal RNAs for ribosome biogenesis. RNA polymerase II produces mRNA and regulatory RNAs like miRNA. RNA polymerase III transcribes short, abundant non-coding RNAs such as tRNA, 5S rRNA, U6 snRNA, and others.​

Why tRNA is the Correct Choice

tRNA genes feature internal promoters recognized by transcription factors TFIIIB and TFIIIC, recruiting RNA polymerase III. This polymerase ensures precise synthesis of mature tRNAs (~70-90 nucleotides) critical for translation. Unlike prokaryotes with one polymerase, eukaryotic specialization enhances efficiency.​

Common Exam Traps

• rRNAs (18S, 28S): These large rRNAs (from Pol I) confuse students since 5S rRNA (Pol III) is smaller but not listed.​

• miRNA: Pol II transcription includes capping and polyadenylation, absent in Pol III products.​

Mastering RNA polymerase III transcribes patterns boosts CSIR NET scores in genetics and transcription units.​