Q.22 Which of the following enzymes is/are involved in the biogenesis of miRNA?
(A) Drosha
(B) Cas9
(C) XRCC4
(D) Dicer

Drosha and Dicer are key RNase III enzymes central to microRNA (miRNA) biogenesis, processing primary transcripts into mature miRNAs that regulate gene expression. The correct answer to the query is (A) Drosha and (D) Dicer, as both play essential roles in the canonical miRNA pathway. Cas9 and XRCC4 do not participate in this process.

Correct Answer

Answer: (A) Drosha and (D) Dicer
miRNA biogenesis starts with transcription of pri-miRNA, where Drosha in the nucleus cleaves it into pre-miRNA hairpin structures as part of the microprocessor complex. Dicer then processes pre-miRNA in the cytoplasm into mature miRNA duplexes, which load into the RISC complex for gene silencing.

Option Breakdown

Drosha (A) – Involved

Drosha, an RNase III enzyme, recognizes stem-loop structures in pri-miRNA and cleaves flanking regions to produce pre-miRNA in the nucleus. This initial processing step is essential for most canonical miRNAs, and its absence disrupts miRNA production.

Cas9 (B) – Not Involved

Cas9 functions as an RNA-guided endonuclease in the CRISPR-Cas9 system for genome editing, targeting specific DNA sequences rather than RNA processing in miRNA biogenesis.

XRCC4 (C) – Not Involved

XRCC4 is a DNA repair protein that ligates non-homologous end-joining breaks during double-strand break repair, unrelated to RNA metabolism or miRNA pathways.

Dicer (D) – Involved

Dicer, another RNase III enzyme, cleaves pre-miRNA loops in the cytoplasm to generate ~22-nucleotide miRNA duplexes, enabling Argonaute loading and target mRNA regulation.

miRNA Biogenesis Pathway

Canonical miRNA processing involves RNA polymerase II transcribing pri-miRNA, followed by Drosha/DGCR8 cleavage to pre-miRNA, export via Exportin-5, and Dicer trimming to mature miRNA. These enzymes ensure precise small RNA production for post-transcriptional gene regulation in eukaryotes.