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Subviral particles: viroids and prions

Get to know nonliving infectious agents like viruses and subviral particles. Understand the traits of viruses, viroids, and prions, and their methods of reproduction and infection. Learn to distinguish between viroids and virions, and grasp the concept of prions, the proteins that can infect.

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  • blobby green style avatar for user poulinn91
    I have never heard of protein aggregates being cleaned up and then causing 'holes in the brain' which further leads to disease. This is not very clear. Would you care to explain this more?
    (7 votes)
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    • marcimus pink style avatar for user Scarlett Varney
      Like in Creutzfeldt-Jakob Disease. From the National Institute of Neurological Disorders and Stroke (from NIH) website: "What Causes Creutzfeldt-Jakob Disease?
      Some researchers believe an unusual "slow virus" or another organism causes CJD. However, they have never been able to isolate a virus or other organism in people with the disease. Furthermore, the agent that causes CJD has several characteristics that are unusual for known organisms such as viruses and bacteria. It is difficult to kill, it does not appear to contain any genetic information in the form of nucleic acids (DNA or RNA), and it usually has a long incubation period before symptoms appear. In some cases, the incubation period may be as long as 50 years. The leading scientific theory at this time maintains that CJD and the other TSEs are caused by a type of protein called a prion.

      Prion proteins occur in both a normal form, which is a harmless protein found in the body’s cells, and in an infectious form, which causes disease. The harmless and infectious forms of the prion protein have the same sequence of amino acids (the "building blocks" of proteins) but the infectious form of the protein takes a different folded shape than the normal protein. Sporadic CJD may develop because some of a person’s normal prions spontaneously change into the infectious form of the protein and then alter the prions in other cells in a chain reaction.

      Once they appear, abnormal prion proteins aggregate, or clump together. Investigators think these protein aggregates may lead to the neuron loss and other brain damage seen in CJD. However, they do not know exactly how this damage occurs."
      (26 votes)
  • blobby green style avatar for user eileen.zhou.12
    Since the viroid is self-cleaving into more viroids, wouldn't it reach a point where the circular RNA would become too small to keep cleaving?
    (4 votes)
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    • leaf orange style avatar for user Tony Wang
      This will not happen because it is circular. The reason behind why linear DNA/RNA shrinks (i.e. our DNA) is that the lagging strand would never be able to start at the 3' end of the sequence because even if we lie down a primer at this end there would be no primer for polymerase to synthesize anything more upstream to it. Hence once the the primer is removed the section that the primer encompasses cannot be filled in and is lost in the next iteration of replicated DNA. For circular DNA/RNA this certainly won't be a problem. As you can probably fathom, a polymerase going linearly can go around and replicate every inch of the genetic material. This will apply for both the lagging and leading strand because for polymerase replicating either one, their tragetory is circular, covers the entire sequence and, this is most important, does not meet an edge where there is just a 3' end hanging off into nothingness. Thus it would never get shorter. It might help to check out the video on telomerase. There's only so much of that process I can put into words adequately. All the best.
      (10 votes)
  • blobby green style avatar for user Eileen Jager
    This video covers Viriods and Prions - but my textbook mentions a third: virusoids (aka Satellites). And, sure I've read my textbook. I still don't really understand what a virusoid is supposed to be. Can you help me on this one?
    (5 votes)
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  • marcimus pink style avatar for user Jaibun
    what z the difference btw virus and virion?
    (2 votes)
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    • leaf green style avatar for user northrupmina
      A VIRUS is a non-cellular, obligately parasitic (i.e. host dependent), self-replicating genetic element consisting of DNA or RNA with no metabolic capability, but capable of transmission from host cell to host cell via a vector stage called a VIRION, in which the nucleic acid is encapsulated in a protein coat.
      (6 votes)
  • piceratops seed style avatar for user LukeS_MM
    Total idiot here. What does "self-cleave" refer to? If there's a video link, please link me! I got here because my text book mentioned an infectious protein called a prion. Now I'm totally lost in the world of biology (again).
    (4 votes)
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  • leaf green style avatar for user christie2taylor
    Why did she say that "normal" proteins were generally alpha sheets ()? I thought beta sheets existed a lot in nature? You always hear about silk made from beta sheets and beta barrel membrane proteins...
    (3 votes)
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    • piceratops tree style avatar for user Jen
      You're right in that beta sheets certainly aren't uncommon. It sounds like she's referring to the beta amyloid plaques that are a hallmark of Alzheimer's disease when she talks about the deposits at , so my guess is she meant to emphasize the fact that it's beta sheets, not alpha helices, that are the primary component of these plaques.
      (2 votes)
  • piceratops tree style avatar for user jonbrandonmullholand
    Is it not true that the correct identification for the protein, not as a prion, would be PrP and then then the protein as a prion would be PrPSC. Or placing it in [PrP], I am asking just in case I read this in any other form so that I can understand what form the literature is referring to.
    (2 votes)
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  • blobby green style avatar for user binitufa
    how does a retrovirus communicate with a host cell ?
    (1 vote)
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    • leafers seed style avatar for user Aaron Richardson
      Retroviruses do not communicate with the host cell through chemokines or hormones but merely insert a copy of its genome into the DNA of a host cell that it invades. The host cell then treats the viral DNA as part of its own genome, transcribing and translating the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus.
      (2 votes)
  • blobby green style avatar for user Zainab Malik
    prions are RNA OR DNA enveloped ??
    (1 vote)
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  • winston baby style avatar for user Amy De Lury
    At , why is a prion considered a sub viral particle? Isn't it its own infectious material i.e. infectious protein? If so, then why is it categorized with viroids and other sub viral particles if it is not part of a virus?
    (1 vote)
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Video transcript

- [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.