Figure 1.

MicroRNA (miRNA) biogenesis and canonical actions of post-transcriptional gene regulation. Left: Canonical protein-coding genes embedded throughout the human chromosomal structure are transcribed, spliced, and transported from the nucleus to the cytoplasm as mature mRNAs. miRNAs are encoded as discrete single genes, as gene families, or embedded in introns of protein-coding genes (mirtrons). After transcription, primary miRNAs (pri-miRNAs) undergo nuclear and cytosolic processing steps to premature miRNAs (pre-miRNAs) before nuclear export and further maturation to a biologically active 19- to 24-nt molecule containing a 6-nt “seed” region. Driven primarily by seed region binding to complementary sites of target mRNAs, miRNAs repress gene expression via translational repression or mRNA degradation (bottom). Right: Technological advances have allowed more efficient and more comprehensive strategies for quantifying miRNA levels in human tissue and fluids. miRNA* duplex = antisense strand of the miRNA duplex; PCR = polymerase chain reaction; RISC = RNA-induced silencing complex; UTR = untranslated region.