Viruses are Very Small – Viruses

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.

00:24 It's magnified about a 100,000 times.

00:26 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.

00:35 You can see it's quite small compare to the bacterium itself.

00:38 Now, above the bacterium that first green line is a virus particle.

00:44 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.

01:17 At the bottom on the left is a poliovirus particle, and above it is a ribosomes.

01:23 So, a ribosome is cellular component that's used to make protein.

01:26 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.

02:05 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.

02:14 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.

02:45 So, let's answer that question.

02:47 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.

02:57 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.

03:16 And below the Ebolavirus now is a Rhinovirus.

03:20 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.