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.