Subviral particles: viroids and prions

- [Voiceover] Let's talk about subviral particles. They're called subviral particles because they're smaller than viruses. And one thing to keep in mind is that viruses and subviral particles together are all categorized as nonliving infectious agents. And this is a mouthful, but let's break it down again. The nonliving part should be review for you now, because viruses, as we know, are acellular. They're only made of a protein coat, or a capsid, which protects the genetic information which is DNA or RNA. And sometimes a lipid envelope, which I haven't drawn here because not all viruses have that envelope. And because they're acellular, they're so small that they don't have organelles. And that means they can't make their own energy, which means ATP, and they also can't divide or reproduce on their own. And in order to do these two things, it means that a virus needs a host. So we've re-summarized what we know already, and that's why it's called nonliving. And because they need a host, they're infectious agents. So you might be thinking, "Well I know that about viruses, "so what are subviral particles, "and how do they share those same traits?" Well, let's talk about them now. There are two types of subviral particles. Viroids, and prions. And viroids are smaller than viruses because they're only made of a single strand of circular RNA. Up until very recently, they were pretty much only found to infect plants, which I hope you can remember because I've written this in green, and I've drawn this funky little bush thing on top of some grass. And today, viroids have been found in humans, in the case of Hepatitis D. So how do viroids make more of themselves if they only are made of circular RNA? Well, you can think of this in terms of Cs. Because it's thought to be catalytic RNA. So C for catalytic and C for circular. And catalytic RNA means that it can make or break covalent bonds, another C. And because it can do that, it can self-cleave to create more viroids. Now, there's one thing to really pay attention to. You don't wanna confuse viroids with virions. Because virions are what we call whole viruses. That means the protein coat plus the RNA, or maybe an envelope. Because once a virus gets inside of a host, and the protein coat falls off, it's only genetic material at that point. DNA or RNA. So that helps distinguish between a virus outside of a cell, and a virus inside of a cell. So remember, viroids and virions are different. And now, let's talk about prions, which are kind of funky. They actually were very recently discovered. Because scientists have always argued about whether or not proteins can be infectious by themselves. Prions come from the word "proteinaceous infectious particles". And I know that pro in is not prion, but they jumbled up the o and the i to make prion. And prions have no genetic material. That means no RNA or DNA at all. They are only made of proteins. So a normal protein is generally in the shape of an alpha-helix. Well, a prion protein, which we'll call PrP, prion protein, tends to be in a beta-sheet conformation. So to be completely honest, we don't really know that much about prions. But it's thought that because these two proteins, the prion protein and the normal protein, are made of the same amino acids, they are the same protein, but in a different shape, when the beta-sheet comes in contact with the alpha-helix, it will change the alpha-helix to a beta-sheet. And as more and more of these alpha-helixes become beta-sheets, this creates protein deposits. Which is already a bad thing, but if it happens somewhere like the brain, normal cleanup still happens. So these protein deposits will be cleaned up, and that would actually leave huge holes in your brain as the proteins are removed, causing disease. And again, prions are different from viruses and viroids because as we know them now, they are only proteins, and do not have any genetic information.