(CTN NEWS) -A family of molecules known as microRNA helps cells regulate the types and quantities of proteins they produce. In other words, microRNAs help control gene expression in cells.
MicroRNA molecules are present in the circulation and cells. (MiRNA is the abbreviation for microRNA, but we’ll use “microRNA” throughout.)
When a gene is expressed, it determines whether it makes too much, too little, or the normal amount of its protein.
It’s important to understand how cells use a gene’s DNA to make a protein to understand gene expression and microRNA. Protein synthesis is a four-step process.
(Skip to “How microRNA controls gene expression” if you are already familiar with protein synthesis.)
The 4-step process of Protein Synthesis
Step 1: The Activation of Genes
- Proteins are made by activating the gene that contains the information for them. A protein activates the gene called a transcription factor.
Step 2 – Transcription of Genes
- An activated gene’s DNA sequence is exposed when its DNA opens up.
- RNA copies the DNA sequence of a gene.
- Messenger RNA, or mRNA, is an RNA copy of the gene.
Step 3 – Modification of the mRNA
- A portion of the mRNA that is not needed is removed.
Afterward, the mature mRNA molecule leaves the nucleus and enters the cytoplasm.
Step 4 – Translation
- MicroRNA binds to mRNA in the cytoplasm of a gene. Translation will be prevented or delayed as a result.
How microRNA controls gene expression
MicroRNAs control gene expression primarily by binding to the cytoplasm’s messenger RNA (mRNA). Marked mRNA will either be destroyed, its components recycled, or preserved and translated later instead of being translated quickly into a protein.
If the level of the microRNA is abnormally low, the protein it controls may be underexpressed; conversely, if the microRNA is overexpressed (its quantity is unusually high), the protein it controls may be overexpressed (its level will be unusually low).
Cancer and microRNA
In addition to genes that encode information for making proteins, cells also have genes that encode information for making microRNAs.
- Mutations can damage microRNA genes in cancer cells
- Mutations in microRNA genes can result in the cell losing that particular microRNA or reducing its level.
- An abnormally low level of a microRNA can lead to overexpression of genes that the microRNA regulates, resulting in cancer development and progression. (A particular microRNA can regulate hundreds or thousands of genes by binding to their mRNA.)
- Many human cancers have been associated with abnormal microRNA expression.
The Development and Evolution of a MicroRNA Molecule
As many genes code for microRNA as for proteins in the human genome, microRNA is similarly synthesized by cells for protein synthesis. As the microRNA gene is activated, the DNA strand opens up, and the RNA copies.
- An initial transcript of a gene is called a primary miRNA (pri-miRNA).
- The double-stranded precursor miRNA (pre-miRNA) is formed by cutting these hairpin-loop molecules in the nucleus.
- Pre-miRNA is transported to the cytoplasm. It is then further cut to form a mature miRNA (mature miRNA molecules are about 22 nucleotides long).
- RNA interference silencing complex, or RISC, binds the mature miRNA first.
- Upon binding with its target messenger RNA (mRNA), the miRNA blocks translation or degrades the mRNA.
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