I know at the beginning of the lesson, I was going on and on about how small viruses are. Well, it turns out they're actually not the smallest infectious pathogens. There are actually smaller infectious agents known as viroids. And these are actually the smallest pathogen known, and they consist of a short circular single-stranded piece of RNA pictured right here. This is an actual image of a viroid, and you can see it starts with this c base pair right here labeled 1. You can see the one right there. It goes all along this way. There is some base pair binding as it folds back in on itself. You can see there are these stretches of base pair binding. I'm kind of just running my pen through those bonds. It's almost like I'm cutting them, and so it folds back in on itself and ends with base 359. So this is not even 500 bases long. This is a tiny stretch of RNA. And yet, this is a pathogen. It infects viroids infect mostly plants, and they tend to disrupt plant growth. So, plants infected with viroids tend to be short or stubby or malformed because their growth is somehow disrupted. Now, viroids do not actually encode proteins like viruses. They simply replicate themselves using the host's enzymes, and this self-replication ultimately leads to problems for the host and propagation of the viroids. So kind of a similar idea to viruses there.
Now, there are actually other tiny infectious agents called prions. Now these are bigger than viroids. They're actually proteins, and these instead of affecting plants, tend to affect animals, specifically, specifically the brain tissue of animals. The way prions work, you can kind of think of a prion as a weirdly folded protein. And essentially, these weirdly folded proteins can interact with normal proteins that have their proper folded form and cause them to get all misfolded and bent out of shape. So in that sense, they self-propagate misfolding in other proteins. You can see a little simplified diagram of that happening right here. So here, these green balls, that's the proper fold for the protein. It interacts with this weird fold in this protein seen in red with little spiky balls around the outside, and that causes the green folded protein to end up being misfolded as we see there. And this leads to an accumulation of these misfolded proteins and protein gunk buildup in nerve cells, which is actually very toxic and harmful to the nerve cells. It causes nerve cell death. In fact, protein-like garbage protein buildup is also implicated in diseases like Alzheimer's as the cause for cell death in those diseases. So, building up garbage protein might not sound like it's that bad, but it is actually very harmful to cells. So that's how these prions can actually cause a lot of cell death in the brain and lead to a variety of diseases. Now, that's all I have for this lesson. I'll see you guys next time.
