الفهرس 
MicroRNAOnly 14 pages are availabe for public view
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Abstract
MicroRNAs are endogenous, single stranded, non coding small RNAs that can negatively regulate gene expression at the post-transcriptional level. About 3% of human genes encode for miRNAs, and up to 30% of human protein coding genes may be regulated by miRNAs. MicroRNA is transcribed by RNA polymerase II to produce the primary transcript (pri-miRNA). In the nucleus, pri-miRNA is processed by the RNAse III enzyme, Drosha, to produce pre-miRNA that is exported to the cytoplasm via exportin-5. In the cytoplasm, pre-miRNA is further processed via another RNAse III enzyme, Dicer, to produce the mature functional form of miRNA.
MicroRNAs negatively regulate gene expression by binding to the 3` UTR of target mRNA leading to degradation of target mRNA and/or translation inhibition, thus producing gene silencing. MicroRNAs are involved in a variety of biological functions, including cellular development, differentiation, neuronal patterning, apoptosis, metabolism, reproduction, immunity and host-viral interaction. Their deregulation is associated with a variety of diseases including cancer, diabetes and cardiovascular diseases.
Since miRNAs are transcribed by RNA polymerase II, their transcription can be regulated by many transcriptional factors including nuclear receptors. Also, epigenetic modifications such as cytosine methylation and histone modifications control the transcription of miRNA, and thus, epigenetics participate with miRNA in regulation of gene expression.
Accurate and quantitative estimation of miRNA profiles or specific miRNA expression levels and their correlation with a given condition is the key to fully understanding the biological functions of miRNAs. Altered miRNA expression is likely to contribute to human cancer development. MicroRNA expression is deregulated in cancer by a variety of mechanisms including amplification, deletion, mutation, and epigenetic silencing. miRNAs can act either as oncogenes (oncomirs) or as tumor suppressors depending on the tissue they expressed in.
MicroRNAs have recently been identified as attractive targets for therapeutic intervention. The rationale for developing miRNA therapeutics is based on the fact that aberrantly expressed miRNAs play key roles in the development of human disease, and that correcting these miRNA alterations by either antagonizing or restoring miRNA function may provide a therapeutic benefit.
Conclusion: Understanding the mechanism of action of miRNAs will increase our knowledge about the potential role of miRNAs and their involvement in regulation of gene expression. The loss of function of a miRNAs could be due to several mechanisms that lead to the development of cancer.