In this video, we're going to briefly differentiate between monocistronic mRNA and polycistronic mRNA. Eukaryotes tend to only make monocistronic mRNA. Whereas prokaryotes, on the other hand, can either make monocistronic mRNA or they could make polycistronic mRNA. Now, monocistronic, as its name implies with the root 'mono', is referring to mRNA molecules that can only carry one single gene. Whereas polycistronic, on the other hand, as its name implies with the root 'poly', which means many or multiple, is referring to mRNA molecules carrying multiple genes.

These polycistronic mRNA molecules tend to contain what are known as spacers. These spacers are random noncoding sequences that are found between different genes. If we take a look at our image down below, you'll notice on the left-hand side, we're focusing on monocistronic mRNA molecules. The root 'mono' means that it's only going to contain one single gene. Notice that this monocistronic mRNA molecule only has one gene which is gene A, and this gene A is being flanked by a start codon and a stop codon. The gene A encodes for protein A, as you see here. Because there's only one single gene, gene A, this is a monocistronic mRNA molecule. Within gene A, if it is a eukaryotic monocistronic mRNA, there will be introns and exons. Introns get removed whereas exons are going to remain and get spliced together. Eukaryotic organisms tend to make monocistronic mRNA like what you see here containing only a single gene such as gene A.

Now on the right-hand side, what we're showing you is polycistronic mRNA, and the root 'poly' means many or multiple, so it's going to contain multiple genes. Notice that in the image, it not only contains gene A, which codes for protein A, but it also contains gene B, which codes for protein B. Because there are two genes within this mRNA molecule right here, gene A and gene B, this is an example of a polycistronic mRNA molecule. Notice that in between gene A and gene B, is this region here in pink. This region here in pink is what we call the spacer. Once again, the spacer region is going to be a random noncoding sequence that's found between different genes. Spacers are not to be confused with introns because introns are not found between different genes; introns are found within a single gene, and those introns eventually get removed. However, the spacers are not found inside a gene; the spacers are found between different genes.

Eukaryotes tend to only make monocistronic mRNA, like what you see on the left side. However, prokaryotes can make either monocistronic or polycistronic mRNA. Prokaryotes can do either of these. We'll be able to see some examples of polycistronic mRNA molecules later in our course when we start to talk about the lac operon and operons in prokaryotes. But for now, this here concludes our brief lesson on the difference between monocistronic mRNA and polycistronic mRNA, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.