In this video, we're going to talk about the 3rd level of post-transcriptional regulation in eukaryotes, and that is RNA interference. RNA interference is commonly abbreviated as just RNAi with a lowercase 'i' here. RNA interference or RNAi is really just the process of small non-coding RNAs blocking the translation of target mRNA molecules. These small non-coding RNAs are short strands of RNA that have a complementary sequence to an mRNA target, and we'll be able to see more and learn more about them down below in our image.

There are 2 possible scenarios that are going to turn gene expression off when it comes to RNA interference. The first possible scenario is that mRNA is going to be degraded and targeted for degradation. The second possible scenario is that the ribosome is going to be blocked from binding, which will prevent translation. 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. 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 are the focuses, representing RNAi. Notice that this process requires small non-coding RNAs, like the little short orange RNA molecule seen at the top.

This short, small non-coding RNA is complementary to a small sequence on the mRNA itself, the messenger RNA. There are two levels here: the mRNA degradation scenario and the translational control scenario. 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 by enzymes. This small noncoding RNA here is essentially marking the mRNA for degradation. You can see that this enzyme over here is degrading the mRNA into small tiny pieces. Of course, the gene product will not be made if the mRNA is being degraded, turning off the gene expression.

If we look at the translational control scenario, what happens is the small noncoding RNA complementary binds to the mRNA. But in this scenario, when the small noncoding RNA binds to the mRNA, it is not going to be degraded. Instead, the ribosome is blocked from binding to the mRNA as it normally would to translate it. In this scenario, the ribosome is blocked from binding to the mRNA, and this also prevents the gene product, the protein, from being made, turning off gene expression. RNA interference is basically interfering with the mRNA and turning off the expression of the mRNA.

This 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.