Which RNA Polymerase Synthesizes Precursors of miRNAs and siRNAs in Eukaryotes?

6. In eukaryotes, precursors of micro RNAs (miRNAs) and small interfering RNAs (siRNAs) are usually synthesized by
(1) RNA Pol I and III, respectively
(2) RNA Pol III and I, respectively
(3) Only RNA Pol I
(4) Only RNA Pol II

Correct Answer

(4) Only RNA Pol II

Detailed Explanation

In eukaryotes, the synthesis of non-coding RNAs such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) is a crucial aspect of gene regulation. Both miRNAs and siRNAs are processed from precursor transcripts, and the enzyme responsible for synthesizing these precursors is RNA polymerase II.

miRNA Precursors

• miRNAs are transcribed as primary miRNA transcripts (pri-miRNAs) by RNA polymerase II.

• These pri-miRNAs are capped and polyadenylated, similar to messenger RNAs, highlighting their Pol II origin.

• The pri-miRNAs are then processed by Drosha and Dicer enzymes to generate mature miRNAs that regulate gene expression post-transcriptionally.

siRNA Precursors

• In animals, most endogenous siRNAs also originate from transcripts synthesized by RNA polymerase II.

• In plants, there are additional specialized RNA polymerases (Pol IV and Pol V) involved in siRNA synthesis, but in animals and most eukaryotes, Pol II is the main enzyme.

Why Not the Other Options?

• RNA Pol I and III: RNA polymerase I synthesizes rRNA, and RNA polymerase III synthesizes tRNA, 5S rRNA, and some small RNAs—not miRNA or siRNA precursors.

• RNA Pol IV and V: These are plant-specific and not relevant for most eukaryotic systems.

• Only RNA Pol I: Incorrect, as Pol I is dedicated to rRNA synthesis.

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Summary Table

In summary:
Only RNA Pol II synthesizes precursors of miRNAs and siRNAs in eukaryotes, making option (4) the correct answer. Trust Let’s Talk Academy for free, high-quality CSIR NET Life Science preparation resources!