The Discovery of Viruses

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Everyone its. Mr. Senti and today.
Id like to discuss viruses and in particular. Particular. Id like to talk to you about the discovery of viruses.
You know are not necessarily our most favorite things to discuss because they cause sometimes some horrific illnesses. But its important to know how they work and in order to totally understand how they work and what their structure is and how they replicate. I think.
Its relevant to look into the historical aspects of how they were discovered and its kind of interesting. And i hope you enjoyed this video on this the what i want to say at first is this particular bacteria right here is being infected by viruses. And so viruses dont really affect just animal cells.
They they can infect plants they can even infect bacteria and though bacteria are smallest living organisms viruses are not considered to be living things and so theyre not part of the normal classification scheme. Now thats something that i just wanted to say right out of the gate. I also wanted to acknowledge microbiologist because most of our understanding of the structure of dna how its been replicated steps involved in transcription and translation have all come from the study of bacteria and viruses and so just to give you a little perspective of what were discussing here bacteria.
Which shown right here are prokaryotic organisms in other words they do not possess a nucleus. A typical eukaryotic cell is so much larger and it possesses a nucleus. Its hard to understand sizes.
So if i were to give sort of a scale analogy. I would say if i were my whole body was the size of a eukaryotic cell. Then a bacteria that was infecting.
Me would be the size of a football relative to my whole body okay so theyre pretty small about the size of an organelle. But yet a virus is even smaller still as you can imagine so again. If im the eukaryotic cell.
And if footballs a bacterium. Then maybe a double a battery would be the size of a virus. And some are even smaller than that can be like the size of a pill like an aspirin.
Something like this and so viruses are very small. And you can imagine that their discovery was difficult to make of course. Theyre microscopic.
You need a microscope to see that and when i say microscope. Im talking about an electron microscope and so they lack most of the the machinery of a cell and so they lack of course. All organelles and they lack ribosomes and so if they lack ribosomes.
They have no ability to replicate on their own and so therefore by definition. If you will theyre not considered to be living things and so but they do replicate. But they replicate inside of a host cell.
So theyre considered to be sort of intercellular parasites. If you will and theyre very small. I dont you know again i can give you the number of nanometers here between 20.
And 300 nanometers bacteria as compared to the 500 and 1500 nanometers. Theyre small. But what really what were talking about is their biological particles and so theyre kind of an aggregate and let me try to illustrate this in other words.
A collection of nucleic acid. So it goes. Something like this nucleic acid.
Now one thing i want to say about nucleic acids. If youre familiar with this that it could be dna. Viruses or it could be rna.
Viruses. So you can have a choice. On this.
And basically. What we got is this nucleic acid surrounded by protein. Okay so picture this three dimensional sort of like a circle this being the protein.

who is credited with discovering the virus and when-0
who is credited with discovering the virus and when-0

So basically its sort of jeans. If you all segments of dna along here covered in a protein coat and so the only way that you could make more protein or make replicate the dna is if this thing is sitting inside of a host cell. Now this is very simplistic.
There are some viruses that are a little bit more complex in structure. But well look at that in another video the structure of viruses. But i will say this ill lou to it a little bit heres that internal nucleic acid structure like this and so heres the nucleic acid.
But oftentimes thats surrounded by protein and so if you went in this direction. This would be a protein coat surrounding the nucleic acid to this protein right here. Shown in blue would be this and then sometimes viruses are even surrounded still by a phospholipid bilayer or a viral envelope.
As its sometimes referred. And that would be the viral envelope or membrane would be this so. This would be outer.
And then with within. This is the protein and then the nucleic acid and so this video in particular is going to look at sort of the research. Just discovering viruses in the first place.
And i find it kind of interesting that one of the very first things that we were most interested in of course were interested in human diseases caused by viruses. But maybe thats the second most important thing. The humans is you know yeah somewhat sarcastically is tobacco and so tobacco farmers were noticing that their tobacco plants were becoming infected.
And they had a disease and they were sort of this is a picture of that they became a little bit yellow and spotted and became known as like a little mosaic of spots so tobacco mosaic disease and of course we cant have this we must look into what is hurting our tobacco plants and so the research begins pretty early on and when you look at this 1883. And so adolf meyer was sort of looking at the causal agent that causes stunted growth of tobacco and this sort of modelled coloration and that was hurtin the tobacco crops okay so the search is on now. What is it so mayor concluded that the disease was infectious and what what infectious means.
If you dont know is that that it is transmitted in other words you could take sap for example from an infected tobacco leaf and sort of grind it up mix it with water and then spray it on subsequent lees leaves on other plants and it would cause healthy plants to to catch tobacco mosaic disease. And so it was infectious okay. Its interesting and then he concluded that the disease must be some kind of really really small organism like very small bacterium.
Why is that because other scientists sort of demonstrated that the sap of the plant. When you take tobacco leaves and crush them up and you you strain them through filtration now picture. This as like a coffee filter.
Where you have your grind your grounds here of the coffee bean and then water going through and obviously. Its sort of like a colander separating small things from large things the best filters at the time were unable to filter out the bacteria and so that even though they tried this this juice still caused the disease. But that doesnt mean too much that just could mean that the bacterias is just an incredibly small and it goes right through the filter you might be familiar with some filters commonly like for example.
If youre going camping. And youre a little bit worried about bacteria or even protozoan in the water supply you can simply pump using this pump of water in the tube and it goes through the filter and then the water coming out is basically germ free. So thats pretty cool.
But this was not able to set too to filter out the bacteria and then another thing is because the sap from one generation of plant infected. Other plants and then another another plant and another plant. It was suggestive that the fact that the sap alone would eventually if you kept doing that eventually the sap would become further diluted.
If you will from plant to plant to plant and that wasnt the case. It seemed like that the infectious agent could reproduce so when he tried it vecten multiple plants it just kept going on and so it ended that the agent wasnt dead so it wasnt a toxin. It was some kind of thing that was capable of reproducing and so its sort of why others ruled out the disease was not due to a filterable toxin.
Because like a that a bacteria produced because it was capable of infecting disease and so therefore. It was over several generations therefore. It was reproductive and what was really odd about this is so if youre following us so its something that can reproduce.
Its something thats really really small because it couldnt be filtered. And it wasnt really producing a toxin because that itself would have been diluted and then when you try to grow. It you try to grow this out onto a nutrient augur like this a a gel an lb agar plate its not growing it wont grow.
But it will grow on a host and so this is something that will only grow on a host like for example. The leaves of the tobacco and so just a little diagram. What im trying to describe is that you can take the sap from the tobacco and filter.
It and then you could rub it and so its infectious on other tobacco plants and so its something that can reproduce. Something that doesnt thats not a toxin per se. Something thats really small and something that needs a host in order to reproduce.
And so jumping right to the end of the of the story in 1935. I would say somewhat recently professor stanley wendell stanley crystallized the pathogen that causes tobacco mosaic disease and it was caused by tobacco mosaic virus or tem v. So professor stanley researcher at the university of california.

who is credited with discovering the virus and when-1
who is credited with discovering the virus and when-1

So pretty pretty important this is now a present transmission electron micrograph of what the tobacco mosaic virus. Looks like its rod shaped. And so theres many of them here.
And heres a close up of one of them and so if youre following. What i was talking about earlier about the structure of the virus. Its interesting the protein is this outer rod shape.
But its hollow on the inside and the nucleic acid are spoiled strands of rna and so thats kind of the background story of the discovery of viruses. So let me direct you here to this short little video clip that i find very interesting about sort of a transition about the discovery of viruses and how they might replicate a virus is an intracellular parasite that can reproduce only by taking over a host cell a virus consists of a nucleic acid genome enclosed in a protein shell called the capsid and the virus shown here the genome consists of dna. But some viruses have rna some viruses are also covered by a membranous envelope that is derived from the membrane of the host cell.
There is usually a lock and key fit between the proteins of the capsid and receptors on a particular type of host cell. So if this was a virus attaches to a bhakta lin viral dna enters the cell the viral dna uses nucleotides and enzymes of the host cell to replicate itself the viral dna. Then commandeers.
Other host cell materials and machinery to transcribe. Its genes into messenger rna. And translate the rna message into capsid proteins.
So. The word. There.
I really like in this video is commandeer. So notice when the virus came in its unable to make more copies of its own nucleic acid. So it uses the host cell like for example.
This is a cell of a tobacco leaf. It uses the tobacco. Leafs nucleotides in order to make more viral nucleotides in that brutal and then it also uses the the tobacco plants amino acids to generate viral proteins.
Now it uses the ribosomes of the plant and its own amino acids to do this can you imagine and then what it does is that it uses the cell entirely in order to replicate and so pretty crazy viral dna and capsid proteins then assemble into new viruses mature virus. So its kind of like the the plant cell becomes sort of like if you will a pinata not filled with candy. But filled with viruses cap protein capsids nucleic.
Acids and so this is trouble for the cell leave. The host cell often destroying the cell in the process. The viruses can go on to infect other cells spreading the viral infection.
So though they dont possess their own ribosomes. And they arent they dont possess their own rna. Polymerase or dna.
Polymerase. They use the cells machinery in order to replicate so they can reproduce. But they could only reproduce inside of a host cell and then finally this is kind of a cool video on discussing that heres the virus particle with its envelope and proteins and genome.
And youre going to see this represented as being within a pipette thats going to be dropped onto a petri dish containing human cells that are going to be susceptible to infection. So whats whats not shown here in the petri dish is some human cells that are being cultured like some fibroblast cells. And so these viruses are going to attack them.
Here. They are heres a blow up of a single cell keep your eye. Here these are the cell surface receptors here comes the virus part thats going to bind.
So it enters the cell here comes. The disassembly stage. The genomes will in this case.
Enter. The nucleus not all viruses have to do that and now you see replication here comes. The assembly.
Reaction and these cells are now the capsids are going to now go to the cell surface and bun. So what weve just witnessed is a single site. So.
Whats interesting here is that you know whats fascinating is that the virus can theres a number of things and so this particular video isnt going to look at it. But when you written you look at replication the cells are going to either the virus is either going to destroy the cell or its going to use the cell to replicate and bud off. But yet not try not to destroy the cell quickly and so its kind of interesting and so i hope you enjoyed this brief look at the discovery of viruses.
I hope you stay tuned for other episodes of virus thank you for watching. .

who is credited with discovering the virus and when-2
who is credited with discovering the virus and when-2

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