Today our topic is animal viruses. Viruses
represent the very smallest forms of life, and they are among the smallest pathogens
that can cause disease. The image on this slide is a transmission electron micrograph
taken of the influenza virus. So this is the same virus
that causes seasonal and pandemic flu. The colors that you see in
the image are false. So they're added to the image
just to make it easier to visualize the structures. It's a really gorgeous image. Viruses are tiny. So the fact that we're able
to make an image like this amazes me every time I see it. Another amazing fact is that Louis Pasteur created a vaccine for rabies,
which is caused by a virus, before we even knew what viruses were He'd been successful already
creating a vaccine for anthrax. So what he did with rabies was he infected
rabbits with the rabies virus. And then he sacrificed the animals. He dried out their spinal cords,
and he used that material to save the life of a nine year
old boy in 1885. It turns out we know now that drying out
the spinal cords inactivated the virus. Rabies was not uncommon,
and it's 100% fatal even now. So you can imagine that
this vaccine was very well received. The discovery of viruses is attributed to a Russian microbiologist
named Dimitri Ivanovsky, and he was exploring a mosaic disease of tobacco plants
occurring in Eastern Europe. People were losing a lot of money
to this particular disease. And so there was a lot of motivation
to try to identify the cause. We now know that this is caused
by the tobacco mosaic virus. And what Ivanovsky did was he cut up
infected tobacco plants. He filtered them
through a porcelain filter. He knew that the pores of these filters
were tiny enough that bacteria would be trapped. And that's where he expected
to find the pathogen causing the disease. He thought it would be a bacteria. But what he found instead
was that the material that was filtered out was actually not infectious. It was actually the fluid that went
through the filter that was infectious. And this was the first clue
that for some infections, something smaller than bacteria,
were responsible for the disease. So he showed that the cause of the tobacco disease
was due to this filterable virus. And the term virus
comes from the Latin meaning poison. This is a wonderful picture of Louis Pasteur
in his own laboratory. It sure looks a lot different
than our lab at CCSF. I'm always incredibly amazed at what
these early microbiologists were able to discover with the limited tools
that they had available to them. A microbiologist who specializes in the study of viruses
is called a virologist. And among virologists,
there is a big debate. So some virologists say that,
yes, viruses are living and they can direct the processes of life. they're able to get into a cell
and direct their own synthesis or copying. They say that you know,
a virus is like a Xerox machine, it causes a cell
to make millions of copies of itself. And they're clearly much more than inert,
lifeless, molecules. So then you've got another group
of virologists that says, nope, a virus can't multiply by itself. They need another life form to be copied. Humans don't need another life
form to be copied in order to multiply. Right. We don't have a host organism
that we have to get into. We can reproduce our own. Bacteria divide in half
they don't need other organisms to divide. But viruses cannot divide on their own. They don't have all the machinery
that a cell has, and they are unable to make copies of their proteins
and nucleic acids like a cell can. Viruses actually have to infect a cell,
take it over, and then use the cell as a manufacturing
plant for making a new virus. So the people who argue that viruses
are not living explained that they think of viruses as more of an infectious set of molecules
that can't multiply on their own. We're also going to talk about prions
in this lecture. Prions are just proteins. They're clearly infectious
and they're definitely not living. So we're going to lump
viruses and prions together since they both have this ambiguity
about their classification as life. I think what you're going to see
is that there is somewhat of a spectrum between things that are living
and things that are clearly not living, and that maybe viruses fall
into sort of a gray zone in between. One of the themes of this class is that bacteria that we interact
with are mostly beneficial. The ones that are pathogenic
are relatively rare. And this is also true of viruses
in many ways. We know that viral infections
can change the genetic makeup of a cell that's infected. And some of these changes are permanent and then are inherited
when that cell divides. In sequencing the human genome,
we find that about 8% of our DNA consists of remnants of ancient viruses,
and another 40% is made up of repetitive DNA sequences
that we also think came from viruses. So those are really big numbers. If you look at bacteria,
they're about ten to 20% viral DNA So it's clear that viruses play
a role in the changes in DNA over time. And we know that's an important part
of the evolutionary process. This is one of my favorite cartoons. We know that bacteria can share DNA
with other bacteria using a process called lateral
or horizontal gene transfer. This is the sharing of DNA between
individuals in the same generation. Now, humans can't do that. I can't just hand you some of my DNA if you wanted to have a specific trait
that I have. All of the DNA you've gotten
is from your parents. And this is called vertical gene transfer. But a virus can transfer
gene between individuals. And that's evidenced
by how much viral DNA is in our genomes. So this is a cute little comic, but
it makes an important biological point, and that is that normally we think of humans as only being able
to pass genes on to our offspring. However,
because a virus can get into one person and sometimes grab a little bit of their DNA,
when that virus is then copied and infects a new person, it can actually transfer
some of that DNA to the new host. This is one of the ways
that genetic information can be passed in the same generation
between two individuals. This is also the basis for a lot of ideas
behind gene therapy. Scientists are working on creating viruses with healthy genes
that can replace disease genes. The idea is that you'd use a virus
to infect the person who's sick, and the virus would then replace
the disease gene with the healthy gene. We refer to viruses as obligate
intracellular parasites. They don't independently fulfill
the characteristics of life, as we discussed earlier. They require another cell, a host cell
as a manufacturing plant for new virus. If you take the molecules in a virus,
whether they're together or in pieces, they're completely inactive outside
a host cell. Viruses are tiny. They're only about 20 to 450 nanometers. In diameter. That's much, much smaller
than even a bacterial cell, which is on the order of one micron
to ten microns. Human cells vary in size but average about 50 microns in diameter. The basic structure of all viruses
includes genetic material, either DNA or RNA,
but not both. Remember that prokaryotic and eukaryotic
cells contain both DNA and RNA, so that's a big distinction
between cells and viruses. Around the genetic material is a protein
shell called a capsid. Many viruses also have an outer layer called an envelope,
which is a lipid structure that's derived from the cell membrane of the host cell
that was infected with the virus. And then all viruses have molecules
on their surface, either on the capsid or on the envelope that determine
what kinds of host cell they'll infect. Viruses
don't infect every cell in your body. They infect specific cells. So for example, influenza viruses and coronaviruses
infect the cells in your respiratory tract and those surface "spike" proteins or keys
or what recognize those cells
and allow the virus to get inside. By contrast,
the hepatitis virus infects liver cells because it's spike proteins or keys
will recognize liver cells. There are a lot of things that viruses
don't have. Viruses lack enzymes
for their metabolic processes. That's why they have to get inside of you. They don't have DNA polymerase. They don't have ribosomes
to make proteins. Not all viruses have lipid envelopes,
but those that do obtain them from the host cell,
the cell that they infected So a virus needs the host
cell manufacturing structures in order to make or synthesize
their biological molecules and build new viruses
from those molecules. Now, as I said, viruses are tiny. We said they were very small in comparison
to a cell. They are so tiny that you can't see them
with the light microscope, but you can use an electron microscope
that does have enough resolution to allow us to visualize
what a virus looks like. Just to give you a sense
of how tiny viruses can be 50 million polio viruses can fit inside
the average human cell. There's also a lot of size variation among viruses, and that's nicely
illustrated in this picture. For comparison, here's a red blood cell. And now here is an E coli bacterium. And if you zoom in on the E coli,
you can compare the size of a bacterium
to some common viruses. All cells,
no matter what type bacteria, archaea, and eukaryotes, they're all infected
by different kinds of viruses. We often refer to the viruses
that infect, bacteria as bacteriophages or phage Some phage have really complex structures. They look like a spaceship
landing on a planet, but actually it's just a virus landing
on a bacterial cell. OK, so that's our introduction to viruses. In our next set of slides
We're going to do a general overview of structures
that all viruses have in common.