How does RNA interference function in gene expression?

RNA interference (RNAi) is a biological process in which small RNA molecules inhibit gene expression by causing the degradation of specific mRNA transcripts. The primary mechanism involves a complex known as the RISC (RNA-induced silencing complex), which incorporates small interfering RNAs (siRNAs) or microRNAs (miRNAs). These small RNAs then guide the RISC to complementary mRNA sequences, leading to the mRNA's cleavage and degradation, effectively silencing the gene.

This silencing action is crucial for various cellular processes, including regulating development, defending against viral infections, and maintaining cellular homeostasis. The role of miRNA in gene silencing through the RISC highlights how RNA interference can precisely control gene expression levels by blocking the translation of targeted mRNAs.

In contrast, the other provided options focus on different aspects of RNA functioning that do not directly relate to the silencing role of RNA interference. For example, promoting mRNA stability relates to keeping mRNA intact within the cell, while enhancing transcription rates involves increasing the production of mRNA from DNA. Synthesizing new RNA strands pertains to transcription, not to the RNAi process, which instead focuses on the degradation of existing mRNA to prevent translation into proteins.