Final answer:
RNA-mediated repression is executed by small RNA molecules, mainly microRNAs (miRNAs) and small interfering RNAs (siRNAs), which become part of the RNA-induced silencing complex (RISC) and lead to the degradation or inhibition of target mRNA molecules.
Step-by-step explanation:
RNA-mediated repression, also known as RNA interference (RNAi), is a biological process in which RNA molecules inhibit gene expression by neutralizing targeted mRNA molecules. This repression is chiefly carried out by two types of small RNA molecules: microRNAs (miRNAs) and small interfering RNAs (siRNAs). These molecules, after being processed by enzymes such as DICER, become part of a complex known as the RNA-induced silencing complex (RISC). In the case of siRNAs, the siRNA-RISC complex binds to and cleaves the mRNA, leading to its degradation. miRNAs, which are not completely complementary to the mRNA of the gene of interest, can bind to RISC and repress the translation of the mRNA without necessarily cleaving it. This process plays a critical role in cellular defense mechanisms against viral infection and is also being explored for therapeutic applications, such as treatments for viral infections and cancer.