MicroRNAs (miRNAs): Role, Structure & Steps

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll be exploring the world of microRNAs (miRNAs). Here, we'll explain the role of miRNAs, their structure and the steps needed to control gene expression.

What Are MicroRNAs?

Imagine the factory that makes the boxes of cereal you enjoy for breakfast. Inside the factory there are many levels of control. The factory isn't simply off or on, but rather the boss can control many levels of production. He might call for the workers to make the cereal boxes faster or slower, or speed up the filling station. Sometimes production might be slowed down or even suspended. These detailed levels of control allow for a smooth working factory that makes just the right amount of product at just the right time.

Cells are similar to factories in this way. Cells transcribe DNA to mRNA as a template for protein production. But this processes isn't just 'off' or 'on'. It can be regulated at many levels to make exactly the proteins the cell needs at exactly the right time. One method of controlling this process of gene expression is through microRNAs.

MicroRNAs (miRNAs) are small pieces of non-coding RNA about 20-25 nucleotides long. MicroRNAs are complementary, or matching, to the 3' untranslated region of the transcript they regulate. They bind to the complementary region and prevent the transcript from being translated to protein, or target the transcript for degradation. MicroRNAs act like a fine tuning switch for gene regulation, providing finer control than simply turning on or off the gene. You can think of miRNA like a dimmer switch on your lights. You can still just turn the lights on or off, but the dimmer switch allows for fine control.

Role in Living Things

MicroRNAs have been found to play a role in many processes in living cells including immune system function, metabolism, cell death and cell division, neurological development, and disease states such as cancer and diabetes. MicroRNAs are not only found in human cells, but are widely conserved across species of all living things, from plants to worms. This conservation indicates an important role for miRNAs in the development of living things.

Let's look at some examples that are especially relevant to humans. Cancer is a disease caused by uncontrolled cell growth. In this condition, our own cells turn against us, dividing out of control and crowding out healthy cells. Millions of people are affected by cancer, and consequently, lots of research has gone into finding treatments. One avenue is manipulating miRNAs.

MicroRNAs have been shown to be dysregulated during cancer. Changes in the amount of miRNAs present through transcription, loss of miRNA genes, the machinery needed to process miRNA, or changes to the gene structure itself have all been implicated in cancer growth and development. Researchers are currently investigating which types of miRNA transcription are changed during cancer and are looking for therapeutic ways to treat cancer using miRNAs.

Structure of MicroRNAs

So what do miRNAs look like anyway? Like most other RNA molecules, they start off in a larger form. MicroRNAs are interesting because their original transcript is usually embedded in the intron, or non-coding, portions of the mRNA molecule they ultimately regulate. As the intron is transcribed, it is further processed in the nucleus to form small stem-loop structures by the enzymes Drosha and Pasha. These structures look like an mRNA molecule folded over on itself, held together by base pairing. The stem-loop structure is then sent out of the nucleus to the cytoplasm.

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