43. siRNAs and miRNAs are used for achieving gene silencing. Although, major steps are similar there are distinct differences in the key players of the two processing pathways. Following statements relate to some characteristic features of gene silencing.
A) Both siRNAs and miRNAs are processed by cytoplasmic endonuclease Dicer.
B) ‘Drosha’ is needed for processing miRNAs and precursor siRNAs.
C) Both siRNAs and miRNAs show association with Argonaute protein.
D) Both the silencing pathways involve RISC complex.
Which of the following combinations is NOT correct?
(1) A and C (2) C and D
(3) B only (4) D and A
Introduction
Small interfering RNAs (siRNAs) and microRNAs (miRNAs) are central to RNA interference (RNAi), a gene silencing mechanism conserved across eukaryotes. While both share common components, such as Dicer processing and association with Argonaute proteins within the RNA-induced silencing complex (RISC), they differ in their biogenesis and some processing steps. Understanding these differences is crucial for interpreting gene regulation mechanisms and for therapeutic applications.
Key Features of siRNA and miRNA Pathways
A) Processing by Dicer
Both siRNAs and miRNAs are processed by the cytoplasmic endonuclease Dicer, which cleaves double-stranded RNA precursors into short RNA duplexes (~21-25 nucleotides) that guide gene silencing.
B) Role of Drosha
Drosha is specifically required for processing primary miRNA transcripts (pri-miRNAs) into precursor miRNAs (pre-miRNAs) in the nucleus. This step is unique to miRNA biogenesis. In contrast, siRNAs typically arise from long double-stranded RNAs in the cytoplasm and do not require Drosha processing.
C) Association with Argonaute Proteins
Both siRNAs and miRNAs associate with Argonaute proteins, which are core components of the RISC complex. This association is essential for target recognition and gene silencing.
D) Involvement of RISC Complex
Both siRNA and miRNA pathways utilize the RNA-induced silencing complex (RISC) to mediate gene silencing by either mRNA cleavage (siRNA) or translational repression/degradation (miRNA).
Evaluating the Statements
| Statement | Correctness | Explanation |
|---|---|---|
| A) Both siRNAs and miRNAs are processed by cytoplasmic endonuclease Dicer | True | Both require Dicer cleavage in the cytoplasm. |
| B) Drosha is needed for processing miRNAs and precursor siRNAs | False | Drosha processes only miRNAs; siRNAs do not require Drosha. |
| C) Both siRNAs and miRNAs show association with Argonaute protein | True | Argonaute proteins bind both types of small RNAs. |
| D) Both the silencing pathways involve RISC complex | True | RISC is central to both pathways. |
Conclusion
The incorrect statement is:
(3) B only
Drosha is required for miRNA processing but not for siRNA processing, making statement B false.
Answer:
The combination that is NOT correct is (3) B only.
