00:00
Another key part of the definition of a virus is that they're small.
00:06
And years ago it use to be very small
but we've change this as we discover new viruses
because we're finding bigger and bigger viruses.
00:14
So, let's illustrate that by this slide.
00:17
Here we have a part of an E.coli bacteria
which a rod-like bacteria that are part of your intestinal tract.
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It's magnified about a 100,000 times.
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You can see some flagella on this slide.
00:29
And attached to the bacterium is a bacteriophage or a virus that infects the bacterium.
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You can see it's quite small compare to the bacterium itself.
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Now, above the bacterium that first green line is a virus particle.
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It's called tobacco mosaic virus. It's a virus of plants.
00:47
It's the first virus that was discovered.
00:50
And above that to the left its an HIV-1 virus particle.
00:54
So you can see in relation to the bacteria they're quite small
although they are visible at this magnification.
01:00
Now, in the little box there are other viruses as well as cellular components that I'd like to compare.
01:07
So you get an idea of how big things are, but they're too small to see in this magnification.
01:12
So, let's move up to a million fold magnification and now we can see them.
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At the bottom on the left is a poliovirus particle, and above it is a ribosomes.
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So, a ribosome is cellular component that's used to make protein.
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So, you can see polio is about the size of a ribosome.
01:29
And all the way above it the component mark A that is a carbon atom.
01:37
So, you can see viruses are on the atomic level.
01:41
Some of them are not much bigger than individual atoms.
01:44
Now, to the right of this slide are various cellular components.
01:47
They're acting in miles and fibrous in a variety of enzymes of different sorts.
01:52
So, viruses exists at the molecular level but they also get much bigger as well.
01:58
Here's another way of looking at the relationship between virus size and cell size.
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Here is a cell surrounded by four virus particles.
02:08
These happen to be herpesviruses.
02:10
And let's expand this and look at just the part of the membrane of the cell.
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You can see the plasma membrane and then beneath it are a variety of cellular components like ribosomes.
02:19
And there's the herpesvirus particle which is 200 nanometer in diameter.
02:24
Where you can also see now at this magnification there's some polioviruses
which happen to be about 10 times smaller than the herpesvirus.
02:32
So, we couldn't see them in the original picture.
02:35
So, viruses are much smaller in most cases than cells.
02:38
Maybe some of you are wondering how many viruses would fit on the head of a pin.
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So, let's answer that question.
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Here is a pin head and in the very center you can see something that's very tiny and red.
02:53
So, let's expand that. That happens to be a red blood cell.
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Now, what we could not see in the original pin but we can see now.
03:01
There's something green to the left of the red blood cell.
03:03
What is that? Those are E.coli cells.
03:07
And now, at this magnification we can see there's something below the E.coli.
03:12
Then if we magnify that even more we can see that's Ebolavirus.
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And below the Ebolavirus now is a Rhinovirus.
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So, the only thing we can see on the pin in the very first magnification is the red blood cell,
but you can see that viruses are much smaller.
03:28
So, the answer is about 500 million Rhinoviruses will fit on the head of a single pin.
03:35
That's a lot of viruses and when you sneeze you're exhaling droplets of viruses.
03:42
If you happen to have a common cold.
03:43
So, if you're infected with a Rhinovirus every time you sneeze
you're firing aerosol that contains thousands and thousands of viruses.
03:51
And this of course is part of the viral strategy for finding new host.
03:56
They make lots of progeny, most of them don't go anywhere
but you just need one to find a new host and start a new infection.