Question

RECALL What are small interfering RNAs?

Short answer

Small interfering RNAs (siRNAs) are short, double-stranded RNA molecules involved in the RNA interference pathway, primarily functioning to guide the RISC to degrade target mRNA.

Step-by-step solution

Definition

Small interfering RNAs (siRNAs) are short, double-stranded RNA molecules, typically 20-25 nucleotides in length, that play a key role in the RNA interference (RNAi) pathway.

Origin

siRNAs are often derived from longer double-stranded RNA precursors that are cleaved by an enzyme known as Dicer.

Function

The primary function of siRNAs is to guide the RNA-induced silencing complex (RISC) to complementary mRNA sequences, resulting in the cleavage and degradation of the target mRNA, thereby silencing gene expression.

Biological Role

siRNAs play a crucial role in defending against viral infections and in regulating gene expression in cells by silencing specific genes.

Key concepts

RNA interference

RNA interference, often abbreviated as RNAi, is a biological process where RNA molecules inhibit the expression or translation of certain genes. The process begins when double-stranded RNA molecules (dsRNA) are introduced into a cell. These dsRNAs are then recognized and processed by different enzymes.
One important enzyme, Dicer, cleaves long dsRNA into smaller fragments known as small interfering RNAs (siRNAs). These siRNAs are critical because they guide the RNA-induced silencing complex (RISC) to bind to target mRNA molecules. Once bound, the RISC can either degrade the mRNA or prevent it from being translated, effectively silencing the gene.
RNAi plays a vital role in protecting cells from viral infections and regulating normal cellular functions. This mechanism is used in research and therapeutic applications because it can be tailored to silence specific genes.

siRNAs function

The primary function of small interfering RNAs (siRNAs) is to mediate the RNA interference process. siRNAs are typically 20-25 nucleotides in length and are generated from longer double-stranded RNA molecules by the Dicer enzyme. Once formed, these siRNAs are incorporated into the RNA-induced silencing complex (RISC).
The role of siRNAs within RISC is to guide the complex to a complementary mRNA target. The siRNA sequence is complementary to a specific part of the mRNA, allowing precise targeting. This targeting leads to the cleavage and subsequent degradation of the mRNA, effectively silencing the gene from which the mRNA is transcribed. Thus, siRNAs are crucial in gene regulation by preventing the production of specific proteins.

gene silencing

Gene silencing refers to the regulation or suppression of gene expression at transcriptional or post-transcriptional levels. When gene silencing occurs, the gene's information is not translated into functional protein molecules.
There are multiple mechanisms for gene silencing, but RNA interference (RNAi) is one of the most well-known. During RNAi, small interfering RNAs (siRNAs) play a key role. After processing by the Dicer enzyme, siRNAs guide the RNA-induced silencing complex (RISC) to complementary mRNA targets.
Once the RISC-siRNA complex binds to these targets, the associated mRNA is either degraded or its translation is inhibited. This prevents the synthesis of the protein encoded by that mRNA, effectively silencing the gene. Gene silencing can regulate various cellular processes and defend against viral infections.

Dicer enzyme

Dicer is an essential enzyme in the RNA interference (RNAi) pathway. It belongs to a family of RNase III endonucleases, specialized in processing double-stranded RNA (dsRNA) precursors.
Dicer's main role is to cleave these dsRNA molecules into smaller fragments called small interfering RNAs (siRNAs). These siRNAs are usually 20-25 nucleotides in length and play a key role in gene silencing. After cleavage, the siRNAs are incorporated into the RNA-induced silencing complex (RISC).
Dicer recognizes and binds to the dsRNA, then makes precise cuts to form the siRNAs. This process is crucial because without Dicer, siRNAs would not be produced, and the RNAi pathway would be disrupted.

RNA-induced silencing complex

The RNA-induced silencing complex (RISC) is a multi-protein complex that plays a central role in the RNA interference (RNAi) pathway. RISC's primary function is to incorporate small interfering RNAs (siRNAs) and use them to target complementary mRNA sequences for gene silencing.
Here's how it works: After the Dicer enzyme processes long dsRNA into siRNAs, these siRNAs are loaded into the RISC. The RISC then uses the siRNA as a guide to find mRNA molecules with a complementary sequence. When the RISC-siRNA complex binds to its target mRNA, it can either cleave the mRNA, leading to its degradation, or inhibit its translation.
By doing so, RISC effectively silences the gene from which the mRNA sequence was transcribed, preventing the production of the protein encoded by that gene. This mechanism allows for precise regulation of gene expression and protects cells from potentially harmful RNA sequences.