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.
