Considering that I have a cold
right now, I can't imagine a more appropriate topic to make
a video on than a virus. And I didn't want to
make it that thick. A virus, or viruses. And in my opinion, viruses are,
on some level, the most fascinating thing in
all of biology. Because they really blur the
boundary between what is an inanimate object and
what is life? I mean if we look at ourselves,
or life as one of those things that you know
it when you see it. If you see something that,
it's born, it grows, it's constantly changing. Maybe it moves around. Maybe it doesn't. But it's metabolizing things
around itself. It reproduces and
then it dies. You say, hey, that's
probably life. And in this, we throw most
things that we see-- or we throw in, us. We throw in bacteria. We throw in plants. I mean, I could-- I'm kind of
butchering the taxonomy system here, but we tend to know
life when we see it. But all viruses are, they're
just a bunch of genetic information inside
of a protein. Inside of a protein capsule. So let me draw. And the genetic information
can come in any form. So it can be an RNA, it could
be DNA, it could be single-stranded RNA,
double-stranded RNA. Sometimes for single stranded
they'll write these two little S's in front of it. Let's say they are talking about
double stranded DNA, they'll put a ds
in front of it. But the general idea-- and
viruses can come in all of these forms-- is that they have
some genetic information, some chain of nucleic acids. Either as single or double
stranded RNA or single or double stranded DNA. And it's just contained inside
some type of protein structure, which is
called the capsid. And kind of the classic
drawing is kind of an icosahedron type
looking thing. Let me see if I can
do justice to it. It looks something like this. And not all viruses have to
look exactly like this. There's thousands of
types of viruses. And we're really just scratching
the surface and understanding even what viruses
are out there and all of the different ways that they
can essentially replicate themselves. We'll talk more about
that in the future. And I would suspect that pretty
much any possible way of replication probably
does somehow exist in the virus world. But they really are just these
proteins, these protein capsids, are just made up
of a bunch of little proteins put together. And inside they have some
genetic material, which might be DNA or it might be RNA. So let me draw their
genetic material. The protein is not necessarily
transparent, but if it was, you would see some genetic
material inside of there. So the question is, is
this thing life? It seems pretty inanimate. It doesn't grow. It doesn't change. It doesn't metabolize things. This thing, left to its
own devices, is just going to sit there. It's just going to sit there the
way a book on a table just sits there. It won't change anything. But what happens is,
the debate arises. I mean you might say, hey Sal,
when you define it that way, just looks like a bunch of
molecules put together. That isn't life. But it starts to seem like life
all of a sudden when it comes in contact with the
things that we normally consider life. So what viruses do, the classic
example is, a virus will attach itself to a cell. So let me draw this thing
a little bit smaller. So let's say that this
is my virus. I'll draw it as a
little hexagon. And what it does is, it'll
attach itself to a cell. And it could be any
type of cell. It could be a bacteria cell, it
could be a plant cell, it could be a human cell. Let me draw the cell here. Cells are usually far larger
than the virus. In the case of cells that have
soft membranes, the virus figures out some way
to enter it. Sometimes it can essentially
fuse-- I don't want to complicate the issue-- but
sometimes viruses have their own little membranes. And we'll talk about
in a second where it gets their membranes. So a virus might have its
own membrane like that. That's around its capsid. And then these membranes
will fuse. And then the virus will be able
to enter into the cell. Now, that's one method. And another method,
and they're seldom all the same way. But let's say another method
would be, the virus convinces-- just based on some
protein receptors on it, or protein receptors on the cells--
and obviously this has to be kind of a Trojan
horse type of thing. The cell doesn't want viruses. So the virus has to somehow
convince the cell that it's a non-foreign particle. We could do hundreds of videos
on how viruses work and it's a continuing field of research. But sometimes you might have a
virus that just gets consumed by the cell. Maybe the cell just thinks it's
something that it needs to consume. So the cell wraps around
it like this. And these sides will
eventually merge. And then the cell and the
virus will go into it. This is called endocytosis. I'll just talk about that. It just brings it into
its cytoplasm. It doesn't happen
just to viruses. But this is one mechanism
that can enter. And then in cases where the cell
in question-- for example in the situation with bacteria--
if the cell has a very hard shell-- let me
do it in a good color. So let's say that this is
a bacteria right here. And it has a hard shell. The viruses don't even
enter the cell. They just hang out outside
of the cell like this. Not drawing to scale. And they actually inject
their genetic material. So there's obviously a huge--
there's a wide variety of ways of how the viruses
get into cells. But that's beside the point. The interesting thing is that
they do get into the cell. And once they do get into the
cell, they release their genetic material
into the cell. So their genetic material
will float around. If their genetic material is
already in the form of RNA-- and I could imagine almost every
possibility of different ways for viruses to work
probably do exist in nature. We just haven't found them. But the ones that we've already
found really do kind of do it in every
possible way. So if they have RNA, this RNA
can immediately start being used to essentially-- let's
say this is the nucleus of the cell. That's the nucleus of the cell
and it normally has the DNA in it like that. Maybe I'll do the DNA in
a different color. But DNA gets transcribed
into RNA, normally. So normally, the cell, this a
normal working cell, the RNA exits the nucleus, it goes to
the ribosomes, and then you have the RNA in conjunction with
the tRNA and it produces these proteins. The RNA codes for different
proteins. And I talk about that in
a different video. So these proteins get formed and
eventually, they can form the different structures
in a cell. But what a virus does is it
hijacks this process here. Hijacks this mechanism. This RNA will essentially go and
do what the cell's own RNA would have done. And it starts coding for
its own proteins. Obviously it's not
going to code for the same things there. And actually some of the first
proteins it codes for often start killing the DNA and the
RNA that might otherwise compete with it. So it codes its own proteins. And then those proteins start
making more viral shells. So those proteins just start
constructing more and more viral shells. At the same time, this
RNA is replicating. It's using the cell's own
mechanisms. Left to its own devices it would
just sit there. But once it enters into a cell
it can use all of the nice machinery that a cell has around
to replicate itself. And it's kind of amazing, just
the biochemistry of it. That these RNA molecules
then find themselves back in these capsids. And then once there's enough
of these and the cell has essentially all of its resources
have been depleted, the viruses, these individual
new viruses that have replicated themselves using all
of the cell's mechanisms, will find some way
to exit the cell. The most-- I don't want to
say, typical, because we haven't even discovered all the
different types of viruses there are-- but one that's, I
guess, talked about the most, is when there's enough of
these, they'll release proteins or they'll construct
proteins. Because they don't
make their own. That essentially cause the cell
to either kill itself or its membrane to dissolve. So the membrane dissolves. And essentially the
cell lyses. Let me write that down. The cell lyses. And lyses just means that
the cell's membrane just disappears. And then all of these guys can
emerge for themselves. Now I talked about before that
have some of these guys, that they have their own membrane. So how did they get
there, these kind of bilipid membranes? Well some of them, what they
do is, once they replicate inside of a cell, they exit
maybe not even killing-- they don't have to lyse. Everything I talk about, these
are specific ways that a virus might work. But viruses really kind of
explore-- well different types of viruses do almost every
different combination you could imagine of replicating
and coding for proteins and escaping from cells. Some of them just bud. And when they bud, they
essentially, you can kind of imagine that they push
against the cell wall, or the membrane. I shouldn't say cell wall. The cell's outer membrane. And then when they push against
it, they take some of the membrane with them. Until eventually the cell
will-- when this goes up enough, this'll pop together
and it'll take some of the membrane with it. And you could imagine why that
would be useful thing to have with you. Because now that you have this
membrane, you kind of look like this cell. So when you want to go infect
another cell like this, you're not going to necessarily look
like a foreign particle. So it's a very useful way to
look like something that you're not. And if you don't think that this
is creepy-crawly enough, that you're hijacking the DNA
of an organism, viruses can actually change the
DNA an organism. And actually one of the most
common examples is HIV virus. Let me write that down. HIV, which is a type of
retrovirus, which is fascinating. Because what they do is, so
they have RNA in them. And when they enter into a cell,
let's say that they got into the cell. So it's inside of the
cell like this. They actually bring along
with them a protein. And every time you say, where
do they get this protein? All of this stuff came from
a different cell. They use some other cell's amino
acids and ribosomes and nucleic acids and everything
to build themselves. So any proteins that they
have in them came from another cell. But they bring with them, this
protein reverse transcriptase. And the reverse transcriptase
takes their RNA and codes it into DNA. So its RNA to DNA. Which when it was first
discovered was, kind of, people always thought that you
always went from DNA to RNA, but this kind of broke
that paradigm. But it codes from RNA to DNA. And if that's not bad enough,
it'll incorporate that DNA into the DNA of the host cell. So that DNA will incorporate
itself into the DNA of the host cell. Let's say the yellow is the
DNA of the host cell. And this is its nucleus. So it actually messes with
the genetic makeup of what it's infecting. And when I made the videos on
bacteria I said, hey for every one human cell we have twenty
bacteria cells. And they live with us and
they're useful and they're part of us and they're 10% of
our dry mass and all of that. But bacteria are kind of
along for the ride. They don't change who we are. But these retroviruses, they're
actually changing our genetic makeup. I mean, my genes, I take
very personally. They define who I am. But these guys will
actually go in and change my genetic makeup. And then once they're part of
the DNA, then just the natural DNA to RNA to protein
process will code their actual proteins. Or their-- what they need to--
so sometimes they'll lay dormant and do nothing. And sometimes-- let's say
sometimes in some type of environmental trigger,
they'll start coding for themselves again. And they'll start
producing more. But they're producing it
directly from the organism's cell's DNA. They become part of
the organism. I mean I can't imagine a more
intimate way to become part of an organism than to become
part of its DNA. I can't imagine any
other way to actually define an organism. And if this by itself is not
eerie enough, and just so you know, this notion right here,
when a virus becomes part of an organism's DNA, this
is called a provirus. But if this isn't eerie enough,
they estimate-- so if this infects a cell in my nose
or in my arm, as this cell experiences mitosis, all of
its offspring-- but its offspring are genetically
identical-- are going to have this viral DNA. And that might be fine,
but at least my children won't get it. You know, at least it won't
become part of my species. But it doesn't have to just
infect somatic cells, it could infect a germ cell. So it could go into
a germ cell. And the germ cells, we've
learned already, these are the ones that produce gametes. For men, that's sperm and
for women it's eggs. But you could imagine, once
you've infected a germ cell, once you become part of a germ
cell's DNA, then I'm passing on that viral DNA to my
son or my daughter. And they are going to pass
it on to their children. And just that idea by itself
is, at least to my mind. vaguely creepy. And people estimate that 5-8%--
and this kind of really blurs, it makes you think about
what we as humans really are-- but the estimate is 5-8%
of the human genome-- so when I talked about bacteria I just
talked about things that were along for the ride. But the current estimate, and
I looked up this a lot. I found 8% someplace,
5% someplace. It's all a guess. I mean people are doing it based
on just looking at the DNA and how similar it is to
DNA in other organisms. But the estimate is 5-8% of the
human genome is from viruses, is from ancient retroviruses
that incorporated themselves into the human germ line. So into the human DNA. So these are called endogenous
retroviruses. Which is mind blowing to me,
because it's not just saying these things are along for the
ride or that they might help us or hurt us. It's saying that we are--
5-8% of our DNA actually comes from viruses. And this is another thing
that speaks to just genetic variation. Because viruses do something--
I mean this is called horizontal transfer of DNA. And you could imagine, as a
virus goes from one species to the next, as it goes from
Species A to B, if it mutates to be able to infiltrate these
cells, it might take some-- it'll take the DNA that
it already has, that makes it, it with it. But sometimes, when it starts
coding for some of these other guys, so let's say that this
is a provirus right here. Where the blue part is
the original virus. The yellow is the organism's
historic DNA. Sometimes when it codes, it
takes up little sections of the other organism's DNA. So maybe most of it was the
viral DNA, but it might have, when it transcribed and
translated itself, it might have taken a little bit-- or at
least when it translated or replicated itself-- it might
take a little bit of the organism's previous DNA. So it's actually cutting parts
of DNA from one organism and bringing it to another
organism. Taking it from one member of a
species to another member of the species. But it can definitely
go cross-species. So you have this idea all of
a sudden that DNA can jump between species. It really kind of-- I don't
know, for me it makes me appreciate how interconnected--
as a species, we kind of imagine that we're
by ourselves and can only reproduce with each other and
have genetic variation within a population. But viruses introduce this
notion of horizontal transfer via transduction. Horizontal transduction is just
the idea of, look when I replicate this virus, I might
take a little bit of the organism that I'm freeloading
off of, I might take a little bit of their DNA with me. And infect that DNA into
the next organism. So you actually have this
DNA, this jumping, from organism to organism. So it kind of unifies
all DNA-based life. Which is all the life that
we know on the planet. And if all of this isn't creepy
enough-- and actually maybe I'll save the creepiest
part for the end. But there's a whole-- we could
talk all about the different classes of viruses. But just so you're familiar with
some of the terminology, when a virus attacks bacteria,
which they often do. And we study these the most
because this might be a good alternative to antibiotics. Because viruses that attack
bacteria might-- sometimes the bacteria is far worse for the
virus-- but these are called bacteriaphages. And I've already talked to you
about how they have their DNA. But since bacteria have hard
walls, they will just inject the DNA inside of
the bacteria. And when you talk about DNA,
this idea of a provirus. So when a virus lyses it
like this, this is called the lytic cycle. This is just some terminology
that's good to know if you're going to take a biology
exam about this stuff. And when the virus incorporates
it into the DNA and lays dormant, incorporates
into the DNA of the host organism and lays dormant for
awhile, this is called the lysogenic cycle. And normally, a provirus is
essentially experiencing a lysogenic cycle in eurkaryotes,
in organisms that have a nuclear membrane. Normally when people talk about
the lysogenic cycle, they're talking about viral DNA
laying dormant in the DNA of bacteria. Or bacteriophage DNA
laying dormant in the DNA of bacteria. But just to kind of give you
an idea of what this, quote unquote, looks like,
right here. I got these two pictures
from Wikipedia. One is from the CDC. These little green dots you see
right here all over the surface, this big thing you
see here, this is a white blood cell. Part of the human
immune system. This is a white blood cell. And what you see emerging from
the surface, essentially budding from the surface of this
white blood cell-- and this gives you a sense
of scale too-- these are HIV-1 viruses. And so you're familiar with the
terminology, the HIV is a virus that infects white
blood cells. AIDS is the syndrome you get
once your immune system is weakened to the point. And then many people suffer
infections that people with a strong immune system normally
won't suffer from. But this is creepy. These things went inside this
huge cell, they used the cell's own mechanism to
reproduce its own DNA or its own RNA and these
protein capsids. And then they bud from the cell
and take a little bit of the membrane with it. And they can even leave some
of their DNA behind in this cell's own DNA. So they really change what
the cell is all about. This is another creepy
picture. These are bacteriaphages. And these show you what
I said before. This is a bacteria right here. This is its cell wall. And it's hard. So it's hard to just
emerge into it. Or you can't just merge,
fuse membranes with it. So they hang out on the outside
of this bacteria. And they are essentially
injecting their genetic material into the
bacteria itself. And you could imagine,
just looking at the size of these things. I mean, this is a cell. And it looks like a whole
planet or something. Or this is a bacteria and these things are so much smaller. Roughly 1/100 of a bacteria. And these are much less than
1/100 of this cell we're talking about. And they're extremely
hard to filter for. To kind of keep out. Because they are such,
such small particles. If you think that these are
exotic things that exist for things like HIV or Ebola , which
they do cause, or SARS, you're right. But they're also
common things. I mean, I said at the beginning
of this video that I have a cold. And I have a cold because some
viruses have infected the tissue in my nasal passage. And they're causing me to have
a runny nose and whatnot. And viruses also cause
the chicken pox. They cause the herpes
simplex virus. Causes cold sores. So they're with us all around. I can almost guarantee
you have some virus with you as you speak. They're all around you. But it's a very philosophically puzzling question. Because I started with, at the
beginning, are these life? And at first when I just showed
it to you, look they are just this protein
with some nucleic acid molecule in it. And it's not doing anything. And that doesn't look
like life to me. It's not moving around. It doesn't have a metabolism. It's not eating. It's not reproducing. But then all of a sudden, when
you think about what it's doing to cells and how it uses
cells to kind of reproduce. It kind of like-- in business
terms it's asset light. It doesn't need all of the
machinery because it can use other people's machinery
to replicate itself. You almost kind of want
to view it as a smarter form of life. Because it doesn't go through
all of the trouble of what every other form of life has. It makes you question what life
is, or even what we are. Are we these things that contain
DNA or are we just transport mechanisms
for the DNA? And these are kind of the
more important things. And these viral infections are
just battles between different forms of DNA and RNA
and whatnot. Anyway I don't want to get
too philosophical on you. But hopefully this gives you a
good idea of what viruses are and why they really are, in my
mind, the most fascinating pseudo organism in
all of biology.
