microRNAs (miRNAs) are short (20–24 nt) non-coding RNAs that are involved in post-transcriptional regulation of
gene expression in
multicellular organisms by affecting both the stability and translation of
mRNAs. miRNAs are transcribed by
RNA polymerase II as part of capped and
polyadenylated primary transcripts (pri-miRNAs) that can be either protein-coding or non-coding. The primary transcript is cleaved by the Drosha
ribonuclease III enzyme to produce an approximately 70-nt stem-loop precursor miRNA (pre-miRNA), which is further cleaved by the cytoplasmic
Dicer ribonuclease to generate the mature miRNA and antisense miRNA star (miRNA*) products. The mature miRNA is incorporated into a
RNA-induced silencing complex (RISC), which recognizes target mRNAs through imperfect base pairing with the miRNA and most commonly results in translational inhibition or destabilization of the target
mRNA. The RefSeq represents the predicted microRNA stem-loop.
Repair of DNA damage Expression of miR-34A is markedly up-regulated in
hematopoietic stem cells (HSCs) from mice subjected to
ionizing radiation. HSCs that are deficient in miR-34A have decreased expression of genes involved in the
DNA repair processes of
homologous recombination and
non-homologous end joining. These and other findings demonstrate that miR-34A contributes to the survival of HSCs after irradiation. == Clinical relevance ==