User:Kinkreet/microRNA

Gene expression is regulated at many levels - at transcription, post-transcription, and at translation; and involving cell signalling, mRNA splicing, mRNA polyadenylation, mRNA and protein localization modification and degradation. MicroRNAs (miRNAs) are a large class of non-coding ssRNA ~22 nucleotides in length, found in plants and animals, and which functions in the post-transcriptional regulation of gene expression. The presence of high level of complexity in the gene regulatory machinery correlates well with the complexity of the organism; thus miRNAs are only found in higher eukaryotes such as plants and animals. These miRNAs were originally thought to act entirely in a post-transcriptional and repressive manner, but has now been shown to be capable of gene activation also. The effect of miRNAs in gene expression is widespread, thought to regulate ~60% of all human genes.

Generation
miRNA begins life as a transcribed (from RNAPII ) primary miRNA molecule (pri-miRNA), which can be thousands of nucleotides long. pri-miRNA is then processed by Drosha in the nucleus to generate one or many ~70 nucleotide-long, hairpin-structured precursor miRNAs (pre-miRNAs) ; the pre-miRNAs are then exported from the nucleus into the cytoplasm and finally cleaved by the Dicer complex, and then the strands separated by helicases to give the mature miRNAs.

Variations
Plant miRNAs mature inside the nucleus. Some miRNAs are not transcribed directly, but is part of the intron of a protein-coding gene, and generated from the splicing of the RNA.

Repression
miRNAs repress expression by binding to partially complementary sequences, termed the miRNA binding sites, of their target mRNAs.

The strength of repression by miRNAs in animals is often much weaker than those mediated by transcription factors.

Regulation
The genes that encode for the pri-miRNA appears to have promoter and enhancer elements just like protein-coding genes