What Are Micro RNA Essay

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What are micro RNAs?

Micro RNAs (miRNA) are small non-coding RNAs that are involved in regulating the translation of messenger RNA (mRNA). Over 1000 miRNAs have been identified which control approximately 60% of the protein coding genes1. The miRNAs are on average 23 nucleotides long, with nucleotides 2-7 acting as the seed region. The seed region is needed for specific mRNA interactions and mutations occur in this region it can disrupt the miRNA, mRNA Watson-Crick base pairing2. mRNA is stabilized in the cytoplasm by adding a 5’ cap and a 3’ poly adenine tail, which prevents degradation by ribonucleases. The binding of miRNA can cause 3 events to occur; deadenylation, decapping, and 5’ to 3’ degradation1. Often in the 3’
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After processing by Drosha, the pre-miRNA is then exported from the nucleus via Exportin5 (Exp5) which has Ran-GTP bound as co-factor. The binding of pre-miRNA is not a sequence specific event, the size of the double stranded stem and 3’ overhangs is the deciding factor to whether to transport the pre-miRNA into the cytoplasm3. This interaction with Exp5 protects the pre-miRNA from degradation, as when knocked out pre-miRNA levels do not accumulate in the nucleus3,8.
Once in the cytoplasm the pre-miRNA can then undergo the final cleavage step before becoming a mature miRNA. The RNase III Dicer cleaves the terminal loop of the pre-miRNA to leave a duplex, which is approximately 22 nucleotides in length3. Dicer is able to cleave without assistance, however the binding of TRBP (transactivating response RNA-binding protein) is thought to increase stability and improve the reactions efficiency9. In humans there is only one copy of a Dicer like protein, which shows how essential it is to processing3. When this is knocked out in mice it is lethal with death early in embryo developments10. TRBP thought to be required for the recruitment of Argonaute 2 (Ago2), which the miRNA is loaded onto11. Before the miRNA can be loaded onto Ago2, the duplex needs to be separated and this is carried out using a RNA helicase such as p68

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