Final answer:
Eukaryotic cells rely on small RNAs, such as microRNAs (miRNAs), which associate with the RNA-induced silencing complex (RISC) to regulate gene expression by targeting specific mRNA molecules for degradation or translation blockage.
Step-by-step explanation:
Regulation of Gene Expression by Small RNAs and RISC
Eukaryotic cells generate various types of small RNAs that regulate gene expression by engaging in complementary base pairing with specific mRNA molecules. Each class of small RNA accomplishes its function by forming specific associations with particular members of the RNA-induced silencing complex (RISC). MicroRNAs (miRNAs) and other non-coding RNAs like siRNAs and lncRNAs play significant roles in this regulatory mechanism, known as RNA interference (RNAi). These RNA molecules can block translation or lead to the degradation of target mRNA, effectively silencing specific genes.
In RNAi, after being transcribed in the nucleus, the miRNAs are processed into mature forms by a protein called Dicer. They then associate with RISC to guide it to complementary mRNA sequences. Once bound, RISC can degrade or prevent the translation of the mRNA, thus regulating gene expression post-transcriptionally. This system is not only crucial for normal cellular processes but also serves as a defense mechanism against viral infections by targeting and eliminating viral RNA.
This sophisticated level of gene regulation builds upon the primary RNA types like mRNA, tRNA, and rRNA, allowing for more intricate control over which proteins are produced in a cell at any given time. The eukaryotic evolution towards increased compartmentalization has permitted more elaborate gene expression control mechanisms, such as the involvement of miRNAs and RISC in post-transcriptional regulation.