What is the function of small interfering RNA (siRNA)?

Small interfering RNA (siRNA) plays a crucial role in the regulation of gene expression through the process known as RNA interference. Specifically, siRNA functions by binding to complementary messenger RNA (mRNA) molecules, which leads to the cleavage of the mRNA strand. This cleavage prevents the mRNA from being translated into protein, effectively silencing the gene from which it originated.

The process begins when long double-stranded RNA is processed into shorter siRNA molecules by an enzyme called Dicer. These siRNAs are then incorporated into the RNA-induced silencing complex (RISC), where they guide the complex to target mRNA molecules. The RISC, with the help of the siRNA, recognizes and binds to the mRNA with a complementary sequence, resulting in the cleavage of the mRNA. As a result, the translation of that specific mRNA into protein is inhibited, reducing the expression of the corresponding protein.

This mechanism is critical in various biological processes, including the regulation of gene expression, defense against viral infections, and the suppression of transposable elements. Thus, the primary function of siRNA is to cut mRNA to prevent translation, making this answer the correct choice.