Viruses are not as Small as we Once Thought! – Viruses

by Vincent Racaniello, PhD

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    00:00 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.

    00:50 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.

    01:17 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.

    01:33 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.

    01:48 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.

    About the Lecture

    The lecture Viruses are not as Small as we Once Thought! – Viruses by Vincent Racaniello, PhD is from the course Microbiology: Introduction.

    Included Quiz Questions

    1. A virus is an obligate, intracellular parasite.
    2. Viruses reproduce by binary fission.
    3. A cellular host is needed for viruses to reproduce.
    4. A viral genome can be composed of DNA or RNA.
    5. Viral particles are formed by de novo assembly of newly synthesized components.
    1. Light microscope
    2. Handheld magnifying lens
    3. Electron microscope
    4. Naked eye
    5. Mass spectrometer
    1. Assembly of preformed components
    2. Mitosis
    3. Binary fission
    4. Sexual reproduction
    5. Meiosis

    Author of lecture Viruses are not as Small as we Once Thought! – Viruses

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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