MicroRNA
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MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. miRNAs base-pair to complementary sequences in mRNA molecules, then silence said mRNA molecules by one or more of the following processes:Cleavage of the mRNA strand into two pieces,
Destabilization of the mRNA by shortening its poly(A) tail, or
Reducing translation of the mRNA into proteins.
Examples of miRNA stem-loops, with the mature miRNAs shown in red
Interaction of microRNA with protein translation process. Several translation repression mechanisms are shown: M1) on the initiation process, preventing assembling of the initiation complex or recruiting the 40S ribosomal subunit; M2) on the ribosome assembly; M3) on the translation process; M7, M8) on the degradation of mRNA. 40S and 60S are light and heavy components of the ribosome, 80S is the assembled ribosome bound to mRNA, eIF4F is a translation initiation factor, PABC1 is the Poly-A binding protein, and "cap" is the mRNA cap structure needed for mRNA circularization (which can be the normal m7G-cap or modified A-cap). The initiation of mRNA can proceed in a cap-independent manner, through recruiting 40S to IRES (
Image: Mi RNA biogenesis
Non-coding RNA
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A non-coding RNA (ncRNA) is a functional RNA molecule that is not translated into a protein. The DNA sequence from which a functional non-coding RNA is transcribed is often called an RNA gene. Abundant and functionally important types of non-coding RNAs include transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), as well as small RNAs such as microRNAs, siRNAs, piRNAs, snoRNAs, snRNAs, exRNAs, scaRNAs and the long ncRNAs such as Xist and HOTAIR.
Electron microscopy images of the yeast spliceosome. Note the bulk of the complex is in fact ncRNA.