In this video, we're going to talk about the second level of post-transcriptional regulation, which is mRNA protection in the cytoplasm. mRNA transcripts must be transported to the cytoplasm of the cell in order for the mRNA transcript to be translated by ribosomes and create a protein product. But the problem is that the cytoplasm actually has many RNA degrading enzymes. When mRNA transcripts enter the cytoplasm, they are at risk of being degraded by these RNA degrading enzymes. These enzymes help destroy foreign viral RNA molecules, acting as a defense mechanism against viruses.
To counteract this degradation, the mRNA is processed to have a 5' prime cap added, as well as a PolyA tail. The 5' prime cap and the PolyA tail, added to the 3' end of the molecule, protect the mRNA from degradation by these enzymes. This mechanism of degradation and protection is a method of regulating gene expression.
Let’s examine an image below to gain a better understanding. In it, mRNA is portrayed as protected from degradation by cytoplasmic enzymes with a 5' prime cap and a PolyA tail. RNA processing, which includes the addition of the 5' prime cap and PolyA tail, occurs within the nucleus. However, the RNA must be transported outside the nucleus into the cytoplasm of the cell.
If the mRNA is not protected with a 5' prime cap and a PolyA tail, it becomes unprotected and is likely to be degraded, turning off the gene. Conversely, if the RNA is properly protected, the protected mRNA will not be degraded and can thus be translated into a protein, creating the final gene product and turning on the gene.
This process of turning genes on or off is a form of regulating gene expression. This concludes our introduction to this second level of post-transcriptional regulation. As we move forward, we will get some practice applying these concepts, as well as discuss the third and final level of post-transcriptional regulation in the next video. I’ll see you all in that video.