Question

Describe the steps of RNA interference.

Short answer

RNA interference occurs in different steps, which can be summarized as follows:

1. Cleavage of dsRNA into siRNA by Dicer.
2. Binding of RISC to the siRNA, which makes it single-stranded.
3. Binding of siRNA to the target mRNA.
4. Cleavage of the target mRNA.

Step-by-step solution

RNA interference

RNA interference (RNAi) is also known as Post-Transcriptional Gene Silencing (PTGS). It is a conserved biological response to double-stranded RNA (dsRNA) that promotes resistance to the endogenous parasite. It also provides resistance to harmful foreign nucleic acids and controls the expression of protein-coding genes.

Apoptosis

The steps involved in RNAi are as follows:

1. The RNase III enzyme Dicer catalyzes the cleavage reaction and converts the RNA (dsRNA or miRNA primary transcript) into short, interfering RNA (siRNA) fragments of 21 to 23 nt long. Each fragment has a 5’-phosphate and a 2-nt overhang at its 3′ ends.
2. The RNA-induced silencing complex (RISC) is a 250–500 kD enzyme complex that binds siRNA. RISC is made of four different proteins, and one of the proteins is an ATP-dependent RNA helicase that divides the siRNA into its two strands. The RISC binds the guide RNA, whose 5′ end has the lowest binding free energy while cleaving and discarding the complementary passenger RNA.
3. The guide RNA binds the RISC complex to the target mRNA's complementary sequence.
4. The mRNA opposing the attached guide RNA is cut by the RNase III component of RISC called Argonaute. Cellular nucleases continue to break down the cleaved mRNA, inhibiting translation.

Hence, the RNA interference involves the degradation of the target DNA with the help of dsRNA cleaved by Dicer and guided by RISC.