Simply speaking, DNA, the genetic material, contains about more than 20,000 protein codes. When a cell decides to produce a protein, it will first have the protein's DNA code copied (known as Transcription) to be a RNA (known as mRNA, or messenger RNA). The mRNA copies will go to protein factory (known as Ribosome), lining up 20 matching amino acids to form protein.

Some viruses take advantage of such mechanism by injecting their own viral RNA into the cell, and using the ribosome to produce virus' own protein. To counter this, the cell uses an enzyme called Dicer, to cut the invading viral RNA into many pieces. Subsequently, those pieces will adhere to the virus-induced mRNA, and disrupting the virus' protein making process inside the ribosome. This self-protection mechanism is called RNA interference (RNAi).

RNAi is a widely-found and very conservative mechanism in Eukaryotes' life cycle. It is a natural antiviral mechanism. RNAi mechanism can be summarized as "double-stranded RNA degrades mRNA, and thus blocks protein synthesis," The following graph describes the process:

First, endo- or exo-genous long double-stranded RNA is degraded by Dicer enzyme to be a 21-23bp (bp) long small double-stranded RNA (called small interfering RNA, siRNA), which is an ATP-dependent the energy-consuming process.

Then, siRNA combines ribonuclease complex to form RNA-induced Silencing Complex (RISC). The complex will rely on ATP energy to cut double-stranded siRNA into a single chain in order to activate RISC. The activated RISC, through complementing siRNA-determined base, cuts the gene transcripts with homolous sequence, leading to the gene silencing effect.

Meanwhile, the process of RNAi resulted in new dsRNA. After the siRNA antisense chain identifies and complement target mRNA, the complemented mRNA can be used as a template to synthesize new dsRNA, and then cut by Dicer to form new dsRNA. The new dsRNA then goes identifying another mRNA to form new siRNA. After several form-and-cut cycles, the silencing signals are amplified. This process, known as target-directed amplification, offers RNAi efficiency and sustainability.

siRNA Working Mechanism Flow Chart: