My research studies a molecule called microRNA. Don’t feel bad if you’ve never heard of it, since microRNA is a fairly new discovery. The first microRNA was discovered in 1993, and the second one wasn’t discovered until 2000. We’ve discovered thousands of microRNAs by now, but they’re still not something all biologists are familiar with, let alone non-biologists. I know when my advisor initially suggested I study microRNAs, the first thing I had to do was go read the Wikipedia article. I knew nothing!
So what the heck is a microRNA? As the name implies, it’s a very short RNA found in plants and animals. Its function is a little more complicated, so let’s back up a bit. Most people have heard of the “Central Dogma” from their high school biology courses: DNA is transcribed into messenger RNA, which is then made into protein.
DNA serves as the “blueprint” for how to make an organism.The messenger RNA, which as the name suggests, serves as an intermediate messenger between the blueprints in the nucleus of a cell and the machinery out in the cyotplasm. Once in the cytoplasm, the messenger RNA is read by a ribosome, which produces a protein based on the instructions originally encoded by the DNA.
Messenger RNA can be made in varying quantities, and more messenger RNA leads to more proteins being made. The amount of proteins made is just as important as the type of protein being made. Genes are “off” if no protein is produced, and varying quantities of a protein can have profound effects on how an organism functions. This is why large chromosomal duplications are generally lethal or have major effects (like Down Syndrome) – with an extra chromosome contributing to protein production, protein levels are totally out of whack.
But the Central Dogma isn’t so dogmatic. This is where microRNA comes in. In animals, micoRNA functions as part of a protein complex called the RNA-induced Silencing Complex (RiSC). MicroRNA guides RiSC to a particular messenger RNA through complementary basepairing – the A in microRNA matches with a U in messenger RNA, the G with a C, etc. RiSC will then block from becoming a protein. RiSC can do this by directly degrading the messenger RNA, de-adenylating the messenger RNA’s poly-A tail to lead to degradation, or by recruiting other proteins to get in the way of translation into a protein. So when microRNA targets a messenger RNA, it results in that messenger RNA producing fewer proteins than usual. If enough microRNA is made, it may turn the gene off completely.
MicroRNA is especially important because one microRNA can have dozens to hundreds of messenger RNA targets. This means a single type of microRNA can have really profound effects on an organism. It’s one of the most important regulators of gene expression, and is involved in key biological processes like the differentiation of stem cells into specialized adult cells, cell proliferation, metabolism, and apoptosis (programmed cell death). Because it’s so important, most microRNAs are highly conserved across animals. This is also why microRNA has been heavily implicated in cancer – one small tweak can have drastic effects.
Stay tuned for more riveting information about microRNA evolution later!
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