virus and they are probably more that we remain
to discover as well.
In plants there are great examples of good
viruses. This is a grass called dichanthelium
lanuginosum which you've probably never heard
of, but it's really neat because it grows
in hot places. These grasses will grow near
the hotsprings in places like Yellowstone
Park, so they grow at temperatures of 55°C
or higher. You can bring these grasses into
a laboratory and grow them in hot conditions
and they will grow very well. Turns out that
the ability of these grasses to grow at high
temperatures is because they have within them
a fungal infection. This fungus is called
curvularia protuberata. It's in the leaves,
it doesn't hurt the plant, but it helps them
to grow at high temperatures. You can remove
the fungus from the plant, you can cure the
plant of this fungal infection and if you
try and grow those plants at 55°C, they won't
grow. Even more interesting maybe, is the
fact that it's not just the fungus that's
needed to grow at high temperatures. The fungus
is in turn infected with a virus and if that
virus is present in the fungus in the plant
then the plant can grow at high temperature.
Fungus alone is not good enough, you have
to have the fungus plus the virus. This is
one of many, many examples of good viruses
that we have in nature.
Viruses in short are amazing. There are many
different kinds and shapes and sizes. Let's
go through just a few of them starting with
a really big one, Ebola virus that has a highly
unusual shape which caused a recent outbreak
in West Africa. Variola viruses, the virus
that caused smallpox, this has been eradicated
from the face of the earth but it's also quite
a large virus. Herpes viruses, these are the
ones that infect all of us. We all have about
a dozen infections including many herpes viruses.
Rabies virus, they're bullet shaped viruses
and one of the diseases is rabies, transmitted
by dogs. Measles virus, once a common childhood
disease, eradicated nearly by vaccination.
HIV-1, which is in the middle of a pandemic
at the moment. The SARS coronavirus, which
emerged from China. Influenza virus, which
causes yearly outbreaks of respiratory disease.
Adenoviruses also causative agents of respiratory
as well as G.I. tract infections. Rotaviruses, big
causes of diarrhea in humans. The papillomaviruses,
which include those polyomaviruses that we
talked about, as well as viruses that cause
warts, hepatitis C and hepatitis B viruses,
agents of liver disease as well as hepatitis A
virus and the very, very small parvoviruses.
The viral array is astounding, these are just
16 members of this incredible array of viruses
that are present on the planet, they are truly
amazing, not only in size and shape, but in
what they can do.
Let's start I think by defining what a virus
is. And here is our definition: An infectious
obligate intracellular parasite that's made
up of genetic material, and it can be DNA
or RNA, so unlike us, we have DNAs as our
genetic material, viruses can also have RNA.
That genetic material is surrounded typically
by a protein coat and then in addition it
may also have a host cell membrane. Now what
does this mean, obligate intracellular.
Intracellular means that a virus, in order to multiply and
make more viruses, it has to get inside of a
cell. It cannot replicate outside of a cell,
so that's the intracellular part and obligate
simply means it's necessary to get in. So
infectious, it can go from cell to cell or
organism to organism, it has to get in a cell
to replicate and then the definition tells
us a little bit about the structure of the
virus. A really interesting question which