RNA interference (RNAi) is a mechanism of gene silencing that is mediated by the presence of double‑stranded RNA. Arrange the steps involved in gene silencing by RNAi.

Double-Stranded RNA (dsRNA) is introduced into a cell

A gene's expression is silenced.

a. RNA-induced silencing complex (RISC) binds to short dsRNA
b. Antisense RNA pairs with the target RNA
c. Long dsRNA is cleaved into short dsRNA
d. The sense strand is separated from the antisense strand and degraded.
e. Target RNA is not trandlated.

Double-stranded RNA is synthesized with a sequence complementary to a gene of interest and introduced into a cell or organism, where it is recognized as exogenous genetic material and activates the RNAi pathway.

Explanation:

During RNAi, long dsRNA is cut or "diced" into small fragments ~21 nucleotides long by an enzyme called "Dicer". These small fragments, referred to as small interfering RNAs (siRNA), bind to proteins from a special family: the Argonaute proteins. After binding to an Argonaute protein, one strand of the dsRNA is removed, leaving the remaining strand available to bind to messenger RNA target sequences according to the rules of base pairing: A binds U, G binds C, and vice versa. Once bound, the Argonaute protein can either cleave the messenger RNA, destroying it, or recruit accessory factors to regulate the target sequence in other ways.

RNAi is widely used by researchers to silence genes in order to learn something about their function. siRNAs can be designed to match any gene, can be manufactured cheaply, and can be readily administered to cells. One can now order commercially synthesized siRNAs to silence virtually any gene in a human or other organism's cell, dramatically accelerating the pace of biomedical research. Furthermore, the ability to turn off expression of a single gene makes RNAi an appealing therapeutic approach to treat infectious diseases or genetic disorders, such as those that result from the inappropriate and undesirable activity of a gene, as in many cancers and neurodegenerative diseases. There are currently several clinical trials testing the safety and effectiveness of siRNA drugs.

RNAi is much more than a research tool. RNAi encompasses an array of ancient and sophisticated cellular mechanisms that regulate a variety of biological functions. Argonaute proteins bind many naturally occurring small RNAs to defend against transposable elements, maintain chromosome structure and stability, and regulate developmental timing and differentiation. For example, microRNAs represent a natural form of developmentally-important siRNAs. Like siRNAs, microRNAs are made by Dicer, but microRNA derive from single-stranded RNAs that fold back on themselves to generate small regions of double-stranded RNA—so called "stem-loops"— instead of the long double-stranded RNA that produces siRNAs. microRNAs can guide Argonaute proteins to repress messenger RNAs that match the miRNA incompletely, allowing one microRNA to regulate hundreds of genes. Humans make more than 500 distinct microRNAs, and the inappropriate production of specific microRNAs has been linked to several diseases. Drugs to inhibit disease-causing microRNAs are now being tested as therapies for several human diseases.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.