Final answer:
miRNA and siRNA function in post-transcriptional gene regulation, with miRNAs degrading or inhibiting the translation of target mRNAs and siRNAs leading to mRNA cleavage. Both are integrated into RISC and are critical for gene expression control.
Step-by-step explanation:
The role of miRNA and siRNA in post-transcriptional modification of genes involves regulating gene expression by targeting specific mRNA molecules for degradation or translation inhibition. miRNAs are typically 21-24 nucleotides long and are processed from longer pre-miRNA molecules in the nucleus. The mature miRNAs are incorporated into the RNA-induced silencing complex (RISC), which binds to specific sequences on target mRNAs leading to their degradation or inhibition of translation. On the other hand, siRNAs are double-stranded RNAs that are perfect or nearly perfect matches to their target mRNA. They also associate with RISC, but typically lead to the cleavage and destruction of the perfectly complementary mRNA. Both miRNAs and siRNAs are essential for proper cellular function and have potential applications in therapeutic intervention for diseases such as cancer.