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What is the role of RNA polymerase III in the process of transcription in Eukaryotes?
Remember the mnemonic "r-m-t" for RNA Pol I, II, III: Pol I transcribes rRNAs; Pol II transcribes mRNA (hnRNA); Pol III transcribes tRNA, 5S rRNA, and snRNAs!
Unlike prokaryotes where a single RNA polymerase transcribes all classes of RNA, eukaryotes possess at least three distinct RNA polymerases in the nucleus:
NCERT states verbatim: "The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S), whereas the RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs). The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA)."
Notice that while 28S, 18S, and 5.8S rRNAs are transcribed by RNA Pol I as a single polycistronic pre-rRNA in the nucleolus, 5S rRNA is uniquely transcribed outside the nucleolus by RNA Pol III.
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Unduly restrictive: RNA Pol III transcribes tRNA and 5S rRNA in addition to snRNAs.
28S, 18S, and 5.8S rRNAs are transcribed exclusively by RNA Polymerase I.
Matches NCERT verbatim: RNA Pol III transcribes tRNA, 5S rRNA, and snRNAs.
The precursor of mRNA (hnRNA) is transcribed by RNA Polymerase II.
Students frequently assume all ribosomal RNAs (rRNAs) are synthesized by RNA Polymerase I. 5S rRNA is the key exception transcribed by RNA Polymerase III.
"RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs)."
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