In this video, we're going to talk about the third level of post-transcriptional regulation in eukaryotes, and that is RNA interference. And so RNA interference is commonly abbreviated as just RNA I with a lowercase I here. And so RNA interference or RNAi is really just the process of small non-coding RNAs blocking translation of target mRNA molecules. And so these small non-coding RNAs are really just short strands of RNA that have a complementary sequence to an mRNA target. And so we'll be able to see more and learn more about them down below in our image.

Now really there are two possible scenarios that are going to turn gene expression off when it comes to RNA interference. And so the first possible scenario is mRNA is going to be degraded and targeted for degradation. And then, the second possible scenario is going to be that the ribosome is going to be blocked from binding and that's going to prevent translation. And so let's take a look at our image down below where you can see in our example, RNA interference can block ribosome binding or recruit cellular enzymes for mRNA degradation. And so over here on the left-hand side, notice that we're showing you our miniature version of the map, and you can see that again mRNA degradation and translational control is going to occur in the cytoplasm of the cell outside of the nucleus.

And so up here, notice in this image where this image we're focusing on RNA interference or RNAi. And, notice that it is going to require small non-coding RNAs, like this little short orange RNA molecule that you see at the top. And this short small non-coding RNA is complementary to a small sequence on the mRNA itself, the messenger RNA. And so, there are two levels here. There is the mRNA degradation scenario, and then there is the translational control scenario. So in the mRNA degradation scenario, what happens is the small noncoding RNA is going to complementary bind to the mRNA, and the mRNA is going to be degraded. It's going to be degraded by enzymes. And so the small noncoding RNA here is basically marking the mRNA for degradation. And so you can see that this enzyme over here is degrading the mRNA into small tiny pieces. And of course, the gene product will not be made if the mRNA is being degraded. And so this is a way of turning off the gene expression.

Now if we take a look at the translational control scenario, what happens is the mRNA, the small noncoding RNA complementary binds to the mRNA. But in this scenario, when the small noncoding RNA binds to the mRNA, the mRNA is not going to be degraded. Instead, the ribosome is not going to be able to bind to the mRNA as it normally would to translate it. And so in this scenario, the ribosome is blocked from binding to the mRNA. And of course, that is also going to prevent the gene product, the protein from being made. And so that is also going to be turning off gene expression. And so, this here RNA interference is basically interfering with the mRNA and turning off the expression of the mRNA. And so this here concludes our introduction to RNA interference, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.