How Much Do You Know About MicroRNA?

A microRNA (or miRNA) is a small non-coding RNA molecule containing about 22 nucleotides found in plants, animals, and some viruses. It functions in RNA silencing and post-transcriptional regulation of gene expression. Encoded by eukaryotic nuclear DNA in plants and animals and by viral DNA in certain viruses whose genome is based on DNA, miRNAs perform its function by base-pairing with complementary sequences within mRNA molecules.

While core components of the microRNA pathway are conserved between plants and animals, miRNA repertoires in the two kingdoms appear to have emerged independently with different primary modes of action. Plant miRNAs usually have near-perfect pairing with their mRNA targets, which induces gene repression through cleavage of the target transcripts. In contrast, animal miRNAs recognize their target mRNAs by using as little as 6–8 nucleotides, which is not enough pairing to induce cleavage of the target mRNAs. Combinatorial regulation is a feature of miRNA regulation in animals. A given miRNA may have hundreds different mRNA targets, and a given target might be regulated by multiple miRNAs.

The first miRNA was discovered in the early 1990s. However, miRNAs were not recognized as a distinct class of biological regulators until the early 2000s, because at first it was believed to be some kind of unknown regulatory protein. Later, after another small RNA was discovered by scientist, a very large class of small RNAs was found in C. elegans, Drosophila and human cells. The many newly discovered RNAs of this class, except their expression patterns were usually inconsistent with a role in regulating the timing of development, which suggested that most might function in other types of regulatory pathways. At this time, researchers started using the term "microRNA" to refer to this class of small regulatory RNAs.

The function of miRNAs appears to be in gene regulation. A miRNA is complementary to a part of one or more messenger RNAs (mRNAs). Animal miRNAs are usually complementary to a site in the 3' UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs. Perfect or near perfect base pairing with the target RNA promotes cleavage of the RNA. This is the primary mode of plant miRNAs. In animals?miRNAs more often have only partly right sequence of nucleotides to bond with the target mRNA. Animal miRNAs inhibit protein translation of the target mRNA (this exists in plants as well but is less common). MicroRNAs that are partially complementary to a target can also speed up deadenylation, causing mRNAs to be degraded sooner.

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