I keep seeing that a single gene can produce many different kinds of proteins but I’ve also learned that eukaryotes have monocistronic genes which means a gene encodes the information for one protein only?
In: Biology
Well, there isn’t really any such thing as a “polycistronic gene”. Rather, there’s polycistronic and monocistronic mRNA.
A typical eukaryotic gene contains the following: 1) an opening bit called a **promoter**, that says “hi, I’m a gene, please start reading me here”, 2) a bit that says “gene sequence starts here”, 3) the actual genetic code describing a protein, and 4) a final bit that says “stop reading here”. The machinery in the cell that reads DNA will latch on to the promoter, read through the sequence of the gene, and produce a matching strip of mRNA — which will then be translated into a protein by other machinery. So that’s **monocistronic mRNA**: it contains only a single gene and will produce only a single protein.
In bacteria, it’s common for several genes with a related function to be arranged one after another, sharing a single promoter. If three genes share a promoter, the DNA-reading machinery will read all three genes in one go, which makes a **polycistronic mRNA** that contains three genes. A classic example is when one gene makes a protein that takes a certain nutrient into the cell, and a second gene makes a protein that breaks down the nutrient. If the nutrient is around the cell will need both proteins, if the nutrient is absent the cell will need neither, so it’s handy for both genes to share a promoter, which means they’re activated or deactivated together.
A length of mRNA will pass through a ribosome (protein-making machine) like a piece of ticker tape, with the corresponding protein chain being spat out bit by bit, like in [this picture](https://image.shutterstock.com/image-illustration/ribosomes-palades-corpuscles-comprise-complex-260nw-1698956266.jpg). If it’s a monocistronic mRNA, a single protein chain is made. A polycistronic mRNA will just keep ticking through the ribosome after the first protein is made, and the ribosome will encounter another “gene starts here” signal, so another protein is made — and so on, until the mRNA ends.
Okay, but as you correctly say, in eukaryotes a single gene can also code for several different proteins. This is actually because of a completely different mechanism, called *alternative splicing*. The way it works is that a typical eukaryotic gene is broken up into a bunch of segments, called **exons** — so in addition to the four components I listed above, there are actually also tiny bits of DNA in the gene that say “exon starts here” / “exon ends here” / “okay, new exon starts here” / “exon ends here”, etc. Before mRNA is sent off to the ribosomes to make proteins, some additional machinery may cut out some of the exons. This way, a single gene can give rise to different versions of a protein, which do or do not contain certain bits, depending on which exons (if any) were left out of the finished mRNA.
**To summarize:**
Polycistronic mRNA is a trick bacteria use to activate or deactivate related genes in a coordinated way.
Alternative splicing is a trick eukaryotes use to make different variants of a protein from the same gene.
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