- [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.