Hi. It's Mr. Andersen and this is the third podcast in the Life on Earth
series. In this one I'm going to talk about things that aren't really alive. And those
are viruses. In 1982 Richard Skrenta, when we have a 15 year old high school student
was sick of people taking his disks and borrowing his disks. And so we basically wrote a program
called the Elk Cloner that would copy itself on to the computer. And then it would copy
itself on to disks that were injected into the computer. Basically it would boot up the
50th time it would give you this warning. This little poem. And it was really highly
infectious. In fact it was the first really infectious computer virus that was ever written.
And so the program was able to move from computer to computer to computer. If there's no computer,
then there is no computer virus. And so viruses work the same way. These right here are bacteriophages.
And basically what they do is they inject their DNA into a bacteria. They use the machinery
of the bacteria to make copies of themselves. In other words they're going to make copies
of the DNA. They're then going to take the DNA to make messenger RNA. And then make proteins.
And then those proteins are going to enclose the DNA. And then they're going to disperse.
And they're going to move to another bacteria. And so viruses spread using cells. If there's
no cell, just like if there's no computer, then there's no virus. And that's why we don't
classify them as alive. They definitely have elements of living things, but they don't
have metabolism. They don't have reproduction on their own without a cell to inject it in.
And so basically it's important to study viruses because they're one of the main sites of diseases
that we have. In other words millions of people were killed by the influenza outbreak on the
heels of World War I. The 1918 influenza virus. This is a reproduction of what that virus
looked like. Or herpes virus. Or polio virus. Or look at all these viruses over here that
infect us. And so basically if it weren't for our cells, they couldn't make copies of
themselves. And so where did viruses come from? Well, we'll probably never know the
right answer to this, but here's a few theories. There's the cellular theory that they used
to be part of the cells. And so a plasmid for example is given off by bacteria can be
picked up by other bacteria. And so maybe they started that way. Or maybe they were
full on cells that regress. So chlamydia for example is a type of cell that's lost the
ability to live on its own. And so now it's an obligate parasite. Or it could have been
co-evolution that these viruses were evolving with cells. Just like insects and plants were
evolving together, this is a little viroid which is a little bit of RNA plant virus.
And so there's evidence to suggest that any of these theories could have produced that
first virus. But nonetheless we eventually had viruses. So what are some characteristics
in the structure of a virus? Well all viruses are going to have nucleic acid. So they have
to have some kind of genetic material. Sometimes that is DNA. And sometimes that is RNA. Sometimes
that DNA is double stranded. And sometimes it's single stranded. And sometimes that RNA
is normal, single stranded. And sometimes it's double stranded. And so these are all
different types of genetic material that could be found inside a virus. But they're all genetic
material. So that's a thing all viruses have. The other thing that all viruses have is a
protein coat. In other words they have a coat that surrounds that virus. One of the first
viruses really understood and crystalized and seen was the tobacco mosaic virus. And
you can see here's the two parts of it. It's got RNA in a coiled pattern. And then it's
going to have these protein subunits that go around the outside. So they protect that
DNA. Now a lot of the time that whole protein is simply two different types of proteins.
We call those proteins subunits. And you can see here that just having those two proteins,
it can self assemble into this shape or this characteristic viral shape. And so all viruses
are going to have these two things. But a lot of viruses are also going to have an envelope.
So they're going to have a lipid bilayer. And so this right here would be human immunodeficiency
virus. Or the virus that causes AIDS. And you can see here that we've got RNA on the
inside. We've got a capsid or a protein coat around the outside. But now we've got this
lipid bilayer around the outside with this envelope. Now why is that important? If we
say this is a typical cell. Let's say this is a typical cell in your upper respiratory
track. And let's say you're being infected by a rhinovirus or a virus that causes a common
cold. Well around the outside of your cell is going to be an envelope. And so if we have
another virus with a genetic material and a capsid, but let's say that they have an
envelope around the outside, when it attaches to your cell, those two envelopes, if it looks
exactly the same is going to fuse with that. And it makes it easy for that virus to gain
entry. Likewise when it lysis the cell or when it explodes, if it takes a little bit
of that envelope with it, it's much more likely to be able to infect another cell. And so
a lot of these envelopes are gathered from the cell in which they infected. And which
they are leaving. So how do they reproduce? How does this actually work? Well the viral
life cycle is two parts. There's the lytic cycle. Lytic comes from the word lyse or to
break. And so let's kind of look right here at a bacteriophage. A bacteriophage remember
infect bacteria. And so what this one is doing is injecting it's genetic material, in this
case DNA, into a bacteria. So it injected its DNA into a bacteria. As that cell makes
copies of itself you can see it's making copies of the DNA. Next thing it's doing, so how
did it do that? It's using DNA polymerase. So it's using the machinery of the cell to
make a copy of the DNA. What's going next? Well right here you can see it's using that
DNA to make proteins. And so how does it do that? Well in this case it's using RNA polymerase.
And it's also using ribosomes. And so it's using the ribosomes inside the cell to make
proteins. And so this right here would be transcription and translation. Eventually
we have our genetic material. We have the proteins. And now we're going to lyse the
cell. And so what's happening is these viruses are actually erupting from the cell. Where
are they headed? They're headed to another cell to inject their DNA and make copies of
it. And so that's how you can get just a few viruses and you can get a huge viral outbreak
where pretty soon exponentially we're going to have billions and billions of viruses being
created. And so that's one part of the life cycle. If you look at the second part of the
life cycle, that is called the lysogenic cycle. In other words it can generate the lytic cycle.
What happens here? Well basically they inject their DNA in. It becomes part of the genetic
material of the cell. And then every time the cell makes a copy of itself, it's making
a copy of that virus. And so that's kind of insidious. It's almost like a stealth mode.
Where they're injecting their DNA. It's not actually making viruses. But it's using the
machinery of the cell to copy it over and over and over again. So quick example in humans.
Because this is in bacteria. So if you've ever had chicken pox, that's a virus. It's
infecting its DNA into the cells of your body. It's forming these pox which is releasing
more of the virus to infect more of the cells. And so it's going through this lytic cycle.
Eventually your immune system is going to shut it down. But this cell has injected its
DNA and it will actually sit in the DNA inside you. And so it can stay there. It can manifest
itself decades later in another form of chicken pox called shingles. And so it's this same
DNA that's just been hitch hiking around waiting and waiting. And so it's in the lysogenic
cycle. And a lot of viruses will jump between the two. And so they'll be in the lysogenic
cycle. Cold sores on the mouth would be an example of that. It's just hiding in your
neural cells. All of a sudden your immune system gets depleted. They jump into the lytic
cycle. Make a bunch of copies of that. And then they can eventually go back. And so those
are viruses. Are they alive? No. Are they important? Yes. And I hope that's helpful.