Final answer:
Small interfering RNA (siRNA) is used to direct the RISC complex to the target mRNA, leading to its degradation. This is part of the RNA interference mechanism by which cells regulate gene expression and defend against viruses.
Step-by-step explanation:
The molecule used to guide the RNA-induced silencing complex (RISC) to the target mRNA for degradation is small interfering RNA (siRNA). The process begins when cells recognize foreign double-stranded RNAs, typically of viral origin. An enzyme known as DICER cleaves the dsRNA into short fragments, producing siRNAs.
These siRNAs then pair with the RISC, with one strand of the siRNA serving as the guide. The antisense strand of the siRNA within the siRNA-RISC complex binds to complementary sequences on the target mRNA, resulting in its cleavage and degradation. This RNA interference mechanism is a crucial part of the cell's regulatory functions and defense against viral infections.