Viruses are not as small as we once thought.
This is because in recent years, virologists
have been discovering what are called giant
viruses. They're bigger than any virus we've
ever seen before and amazingly, they're visible
in the light microscope. So here's an example
of one of these giant viruses, it's called
Pandoravirus. On the right, you can see a
schematic drawing of the Pandoravirus particle
and on the left is a drawing of a field in
a light microscope and you can see these particles.
They are about 2 microns in length. You may
be wondering why this is called Pandoravirus.
Well you know the Greek legend about Pandora,
she received the container and she opened
it and all kinds of spirits came out of it.
Well, Pandoraviruses are sort of like that.
They have lots of genes in them and we don't
know what most of them do, so that's where
the name came from. So these are visible in
the light microscope and they are certainly
far larger than the pore size that previously
defined viruses, that is a 0.2 micron pore
size. We will come back to this in a moment.
Another key property of viruses is that they
replicate, they make more viruses by assembling
preformed components into particles. You can
think of an assembly line, where you're making
a car and you're adding a component one step
at a time. This is very much how viruses replicate.
You make the parts and then you assemble a
final component. Now when viruses were first
discovered at the end of the 1800s, we already
knew about bacteria and we knew how they divided.
We knew that they divided by what is called
binary fission. One bacterium became two and
then four and eight and so forth, that's not
how viruses replicate. Now on the right in
this graph is a graph showing the growth of
bacteria with time. If you inoculate a broth
with a single bacterium, it divides into 2
and then 4 and then 8 and so forth, you get
a line of growth, this is binary fission.
And again the point I'm trying to make is
that viruses don't replicate like this. On
the left is a graph of the growth of viruses
with time. So the green line is the production
of viruses. When we inoculate a cell with
a virus for a short period of time, no new
viruses are detectable and then we suddenly
have a burst where is a period where new viruses
are detectable and then a plateau. This is
very different from the bacterial way of growth
and that is because viruses don't simply reproduce
by binary fission. They reproduce by making
the parts and assembling the new viruses,
so it takes a little bit of time to make a
new virus and that's why we don't see any
infectivity after first inoculating a cell.