Final answer:
MicroRNAs (miRNAs) are short ~21 nucleotide RNAs that disrupt the expression of complementary mRNAs, leading to their degradation or repression of translation, serving as a key post-transcriptional regulatory mechanism.
Step-by-step explanation:
The short ~21 nucleotide single-stranded RNAs that are cleaved from small hairpin-shaped precursors and function to block the expression of complementary or partially complementary mRNAs are known as microRNAs (miRNAs). These miRNAs are crucial regulatory molecules that can cause degradation of target mRNAs or repression of their translation, thereby silencing gene expression post-transcriptionally. miRNAs are synthesized in the nucleus as longer pre-miRNA molecules and are processed by the enzyme called Dicer into mature miRNAs, which then associate with the RNA-induced silencing complex (RISC). The RISC-miRNA complex can either degrade the mRNA if there is complete complementarity or impede translation if the complementarity is partial, playing a significant role in gene regulation within the cell.