What is "Micro RNA" (as compared to "ordinary" RNA) and what is its function?

micro RNA

In genetics, micro RNAs (miRNA) are a form of single -stranded RNA which is typically 21-23 nucleotides long, and is thought to regulate the expression of other genes. miRNAs are RNA genes which are transcribed from DNA, but are not translated into protein. The DNA sequence that codes for an miRNA gene is longer than the miRNA. This DNA sequence includes the miRNA sequence and an approximate reverse complement. When this DNA sequence is transcribed into a single-stranded RNA molecule, the miRNA sequence and its reverse-complement base pair to form a double stranded RNA hairpin loop; this forms a primary miRNA structure (pri-miRNA). In animals, the nuclear enzyme Drosha cleaves the base of the hairpin to form pre-miRNA. The pre-miRNA molecule is then actively transported out of the nucleus into the cytoplasm by Exportin 5, a carrier protein. The Dicer enzyme then cuts 20-25 nucleotides from the base of the hairpin to release the mature miRNA. In plants, which lack Drosha homologues, pri- and pre-miRNA processing by Dicer probably takes place in the nucleus, and mature miRNA duplexes are exported to the cytosol by Exportin 5.

The function of miRNAs appears to be in gene regulation. For that purpose, 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. The annealing of the miRNA to the mRNA then inhibits protein translation, but sometimes facilitates cleavage of the mRNA. This is thought to be the primary mode of action of plant miRNAs. In such cases, the formation of the double-stranded RNA through the binding of the miRNA triggers the degradation of the mRNA transcript through a process similar to RNA interference (RNAi), though in other cases it is believed that the miRNA complex blocks the protein translation machinery or otherwise prevents protein translation without causing the mRNA to be degraded. miRNAs may also target methylation of genomic sites which correspond to targeted mRNAs. miRNAs function in association with a complement of proteins collectively termed the miRNP.

n genetics, microRNAs (miRNA) are single-stranded RNA molecules of about 21-23 nucleotides in length thought to regulate the expression of other genes. miRNAs are encoded by RNA genes that are transcribed from DNA but not translated into protein; instead they are processed from primary transcripts known as pri-miRNA to short stem-loop structures called pre-miRNA and finally to functional miRNA. Mature miRNA molecules are complementary to regions in one or more messenger RNA (mRNA) molecules, which they target for degradation.

OK this isn't an answer but actually another question. Are we saying that miRNA is effectively siRNA (short interfering) by another name?

http://www.biology-online.org/dictionary...

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