What is miRNA

MicroRNAs, or miRNAs, are small non-coding RNA molecules that regulate gene expression. They are around 21 to 25 nucleotides long and originate from specific genes. miRNAs control gene expression by binding to messenger RNA molecules and inhibiting protein synthesis.

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miRNA Synthesis

miRNAs are synthesized from specific genes by RNA polymerase II or III. The primary miRNA transcripts (pri-miRNAs) are processed in the nucleus by the microprocessor complex, which consists of Drosha and DGCR8. 

The complex cleaves the pri-miRNA to release a hairpin shaped precursor miRNA (pre-miRNA). This pre-miRNA is transported to the cytoplasm by Exportin-5, where it is further processed by Dicer. One strand of the resulting duplex, the guide strand or mature miRNA, is incorporated into the RNA-induced silencing complex (RISC), while the other strand is degraded.

Once in the RISC, the mature miRNA binds to target mRNAs’ 3’ UTR through complementary base pairing.

The functions of miRNA 

miRNAs can regulate gene expression by affecting the stability and translation of target mRNAs. When a miRNA binds to a target mRNA, it can recruit ribonucleases that break down the mRNA, leading to its degradation. This reduces the amount of mRNA available for translation into protein.

In addition to promoting mRNA degradation, miRNAs can also inhibit the translation of target mRNAs into proteins. This can occur through several mechanisms. For example, miRNAs can block the ribosome’s access to the mRNA, preventing it from being translated. Alternatively, miRNAs can repress translation without degrading the mRNA by inhibiting the initiation or elongation steps of protein synthesis. This prevents the ribosome from assembling on the mRNA or from moving along it to synthesize a protein.

Related Questions & Answers

What are miRNAs?

miRNAs are a class of small noncoding RNAs that function in posttranscriptional regulation of gene expression.

What is the role of miRNAs in biology?

miRNAs are powerful regulators of various cellular activities including cell growth, differentiation, development, and apoptosis1. They play an important role in gene regulation.

How do miRNAs regulate gene expression?

miRNAs can regulate gene expression by affecting the stability and translation of target mRNAs. They can promote mRNA degradation or inhibit the translation of target mRNAs into proteins.

Are miRNAs associated with diseases?

Yes, miRNAs have been linked to many diseases and are expected to be useful in their detection and treatment.

Can miRNAs be used for therapeutic purposes?

Yes, currently miRNA-mediated clinical trials have shown promising results for the treatment of cancer and viral infections.

Are miRNAs conserved across species?

Yes, since their discovery in 1993, miRNAs have been found in all eukaryotic cells and are conserved across species.

What is the difference between miRNAs and siRNAs?

Both miRNAs and siRNAs are small noncoding RNAs that regulate gene expression. However, they differ in their biogenesis and mechanism of action. miRNAs are transcribed from endogenous genes, while siRNAs are derived from exogenous double-stranded RNA.

Frontiers | Overview of MicroRNA Biogenesis, Mechanisms of Actions, and Circulation
microRNA - What it is and How it Works | OSUCCC - James