Although chemically similar to siRNAs, miRNAs are made from hairpin-loop RNA precursors rather than long double-stranded RNAs. These precursors are transcribed from genes within the genome. The number of miRNA genes roughly corresponds to 0.5 – 1% of the total number of genes in typical genomes. Most miRNA genes are conserved between related species, and approximately 30% of miRNA genes are highly conserved, with orthologs found in vertebrate and invertebrate genomes, suggesting that a significant fraction have evolutionarily conserved biological functions. Biogenesis of miRNAs from primary transcripts involves several steps. Primary miRNA transcripts are processed by an RNase III enzyme called Drosha. The resulting hairpin-loop RNAs are then processed into 21 – 24 nucleotide miRNAs by Dicer. The mature miRNAs accumulate as a single-stranded species from one arm of each miRNA hairpin precursor, and they incorporate into a ribonucleoprotein complex that is similar, and possibly identical, to RISC.

Plant miRNAs generally trigger target mRNA degradation by base-pairing with near-perfect complementarity. Conversely, most miRNAs from animals repress gene expression by blocking the translation of mRNA transcripts into protein. They interact with their targets by imperfect base-pairing to mRNA sequences within 3’ untranslated regions. Although the functions of only a few miRNAs are known, evidence suggests that miRNAs play diverse but important roles in the development of organisms. Genetic studies suggest that miRNAs regulate stem cells, organ differentiation, and developmental timing. They appear to regulate events throughout the life cycle, including embryonic and post-embryonic stages. They regulate cellular activities as diverse as division, differentiation, and apoptosis.