If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:23:17

Video transcript

considering that I have a cold right now I can't imagine a more appropriate topic to make a video on than a virus I don't wanna make it that thick a virus or viruses viruses and in my opinion viruses are on some level the most fascinating thing in all of biology because they they really blur the boundary between what is an inanimate object and what is life what is life I mean if we look at ourselves or you know life is one of those things that you know it when you see it if you see something that it's born it grows it it's constantly changing maybe it moves around maybe it doesn't but it's metabolizing things around itself it reproduces and then it dies you say hey that's probably life and in this we throw most things that we see or you know we throw in us we throw in bacteria we throw in plants I mean I could I could I'm kind of butchering the taxonomy system here but we we tend to know life when we see it but all viruses are is there they're just a bunch of genetic information inside of a protein and set of a protein capsule so let me let me draw so let's say and they connect information can come in any form so it can be you know it could be an RNA it could be DNA it could be single-stranded RNA double-stranded RNA sometimes they'll write you know for single-stranded all right these two little SS in front of it for say they're talking about double-stranded DNA they'll put a des in front of it but the general idea and viruses can come in all of these forms is that they have some genetic information some new clay some chain of nucleic acids either as single or double stranded RNA or single or double stranded DNA and it's just in contained inside some type of protein structure which is called a capsid and kind of the classic the classic drawing is kind of an eye I saw so Hedren type looking thing let me see if I can do justice to it looks something like this and not all viruses have to look exactly like this there's thousands of types of viruses and we're really just scratching the surface and understanding even what virus is out there and all of the different ways that they can essentially get kind of replicate themselves and we'll talk more about that in the future and I and I would suspect that pretty much any possible way of replication probably does somehow exist in the virus world but they really are just these proteins these protein capsids is just made up by a bunch of little proteins put together and inside they have some genetic material which might be DNA or might be RNA so let me draw their genetic material a little you know the protein is not necessarily transparent but if it was you would see you would see some genetic material inside of there and so the question is is this is this thing life it seems pretty an adamant it doesn't grow it doesn't change it doesn't metabolize things this thing left to its own devices is just going to sit there it's just going to sit there the way a book on our table just sits there it doesn't it won't change anything but what happens is the I guess the the debate arises I mean you might say hey Sal when you define it that way just looks like a bunch of molecules put together that isn't life but it starts to seem like life all of a sudden when it comes in contact with the things that we normally consider life so what viruses do the classic the classic example is a virus will attach it to self to a cell so let's let me draw this thing a little bit smaller so let's say that this is my virus I'll draw it as a little hexagon and what it does is it'll attach itself to a cell it could be any type of cell could be a bacterial cell it could be a a plant cell it could be a human cell let me draw the cell here the cells are usually far larger than the virus and the case of cells that have soft membranes the virus figures out some way to enter it sometimes it can essentially fuse it's so I'll I don't want to I don't want to complicate the issue but sometimes viruses have their own little membranes and we'll talk about in a second where it gets their membrane so a virus might have its own membrane like that that's around its capsid and then these membranes will fuse and then the virus will be able to enter into the cell now that's one method another method and they're seldom all the same way but let's say another method would be the virus convinces just by based on some protein scepters on it or protein receptors on the cells and obviously this has to be kind of a Trojan horse type of thing the cell doesn't want viruses though so the virus has to somehow convince the cell that it's a non foreign particle but that we could do a we can do hundreds of videos on how viruses work and it's a continuing field of research but sometimes you might have a virus that just gets consumed by the cell maybe the cell just thinks it's something that it needs to consume so the cell wraps around it like this the cell will wrap around it like this and these sides will eventually merge and then the cell and the virus will go into it this is called endocytosis I'll just talk about that it brings it into its cytoplasm and doesn't happen just endocytosis doesn't happen just to viruses but this is one mechanism that can enter and then in cases where the where the cell in question for example in this situation with bacteria if the cell has a very hard shell if the cell is let me do it in good color so let's say that this is a bacteria right here it has a hard shell the viruses don't even enter the cell they just hang out outside of the cell like this not drawing the scale and they actually inject their genetic material so there's obviously a huge there's a wide variety of ways of how the viruses get into cell but that's beside the point the interesting thing is that they do get into the cell and once they do get into the cell they release their genetic material into the cell so the genetic material will float around if their genetic material is already in the form of RNA and you know this almost I can imagine almost every possibility of different ways for viruses to work probably do exist in nature we just haven't found them but the ones that we've already found really do kind of do it in every possible way so if if they have RNA this RNA can immediately can immediately start being used to and essentially let's say this is the nucleus of the cell that's the nucleus of the cell and it normally has the DNA in it like that maybe I'll do the DNA a different color but DNA DNA gets gets Trent it gets transcribed into RNA normally so normally the cell just a normal working cell the RNA exits the nucleus it goes to the ribosomes and then you have the RNA in conjunction with the tRNA and it produces these proteins right proteins the RNA codes for different proteins and I talked about that in the in a different video so these proteins get formed and eventually you know they can form the different structures in a Cell but what a virus does is it hijacks this process here hijacks this mechanism this RNA will essentially go and do what the cells own RNA would have done and it starts coding for its own proteins obviously it's not going to code for the same things there and actually some of the first proteins that code for often start killing the DNA and the RNA that might otherwise compete with it so it's codes its own proteins and then those proteins start making more viral shells so those proteins just start constructing more and more viral shells and at the same time this RNA is replicating it's using the the cells own mechanisms left to its own devices it would just sit there but once it enters into a cell it can use all of the nice machinery that a cell has around to replicate itself and then I'm it's kind of amazing just with just the the biochemistry of it that these that these RNA molecules and find themselves back in these capsids and then once there's enough of these and the the cell has essentially all of its resources have been depleted the viruses these these individual new viruses that have replicated themselves using all of the all of the the cell's mechanisms will find some way to exit the cell the the kind of the most I don't want to say typical because we haven't even discovered all the different types of viruses there are but one that I guess talked about the most is that when there's enough of these they'll release proteins or they'll construct proteins because they don't make their own that essentially cause the cell to either kill itself or its membrane to dissolve so the membrane dissolves and essentially the cell lysis let me write that down the cell lysis and lysis just means that the cell's membrane just disappears and then all of these guys can emerge from them cell now I talked about before that you know you have some of these guys that they have their own membrane so how did they get there you know these these kind of vide lipid membranes or some of them what they do is once they replicate inside of a cell once they replicate inside of a cell the exit maybe not even killing they don't have to lyse you know everything I talked about these are specific ways that a virus might work but viruses really kind of explore every well the different types of viruses do almost every different combination you can imagine of replicating and and and and coding for proteins and then escaping from cells some of them just bud and when they bud they essentially you can kind of imagine that they push against the cell wall or the membrane I shouldn't say cell wall the the cells outer membrane and then when they push against it they take some of the membrane with them and so eventually the cell will you know when this goes up enough this will pop together and they'll take some of the membrane with it and you could you could imagine why that would be a useful thing to have with you because now that you have this membrane you kind of look like this cell so when you want to go infect another cell like this it's you're not going to necessarily look like a foreign particle so it's a very useful way to kind of look like something that you're not and if you didn't if you don't think that this is creepy-crawly enough that you know you're hijacking the DNA of an organism cells can actually change of irises can actually change the DNA of the Norgan ism and actually one of the most common examples is HIV virus let me write that down HIV which is a type of retrovirus retrovirus which is fascinating because what they do is so they have RNA in them they have RNA and them and when they enter into a cell let's say that they got into the cell so it's inside of the cell like this they they actually bring along with them a protein and you you know every time you say where did they get this protein King all of this stuff came from a different cell they use some other cells amino acids and ribosomes and nucleic acids and everything to build themselves and he saw any proteins that they have in them came from another cell but they use this they bring with them this protein reverse transcriptase and the reverse transcriptase takes their RNA and codes it encodes it into DNA so it's RNA to DNA which was first discovered was you know it kind of people always thought that you always went from DNA to RNA but this kind of broke that paradigm but it codes from RNA to DNA and if that's not bad enough it'll incorporate it'll incorporate that DNA into the DNA of the host cell so that DNA if this would that DNA will incorporate itself to the DNA of the whole let's say the yellow is the DNA of the host cell and this is its nucleus so it actually messes with the genetic makeup of what it's infecting and you know when I made the videos on bacteria I said hey you know where we have for every one human cell we have 20 bacterial cell and they live with us and they're useful and they're part of us and they're 10% of our dry mass and all of that but bacteria are kind of along for the ride they don't change who we are but these retroviruses they're actually changing our genetic makeup I mean I my genes I take very personally they define who I am but these these guys will actually go in and change my genetic makeup and then once they're part of the DNA then just the natural DNA to RNA to protein process will code will code their actual proteins or their what they need to so you know sometimes they'll lay dormant and do nothing and sometimes well say you know sometimes it's some type of environmental trigger they'll start coding for themselves again and they'll start producing more but they're producing it directly from the organisms from the cell's DNA they become part of the organism I mean I can't imagine a more intimate way to become part of an organism than to become part of its DNA I can't imagine any other way to actually define and an organism and if this if this by itself is not URI enough and and just so you know this this notion right here when a virus becomes part of an organism's DNA this is called a pro virus Pro virus but if this isn't eerie enough they estimate so you know if this just if this a cell in my I don't know in my nose or in my arm it'll you know as the cell experiences mitosis all of its its offspring but this I off spring her genetic identical are going to have this viral DNA and that might be fine but at least my children won't get it you know at least it won't become part of my species but it doesn't have to just infect somatic cells it could infect a germ cell so it could it could go into a germ cell and the germ cells we've learned already these are the ones that produce gametes for men that's sperm and for women it's egg eggs but you can imagine once you've infected a germ cell once you become part of a germ cells DNA that I'm passing on that viral DNA to my son or my daughter and he they're going to pass it on to their children and you know just that idea by itself is at least to my mind vaguely creepy and people estimate that five to eight percent and this kind of really blurs you know it makes you think about what we as humans really are but the estimate is five to eight percent of the human genome of human genome so when I talked about bacteria I just thought talked about things that were along for the ride but the the current estimated I looked up this a lot I found eight percent someplace five percent someplace it's all I guess I mean people are doing it based on just looking at the DNA and how similar it is DNA and other organisms but the estimate is five to eight percent of the human genome is from viruses is from like ancient retroviruses that incorporated themselves into the into the human germline so into into human DNA so these are called endogenous retroviruses and dajin Asst retroviruses which is mind-blowing to me because it's not just saying these things are along for the ride or that they might help us or hurt us it's saying that we are out five to eight percent of our DNA actually comes from viruses and this is another thing that speaks to just genetic variation because viruses do sorry I mean this you can this is called you know I guess you could call it a horizontal transfer of DNA because you can imagine as a virus goes from one species to the next as it goes from you know species A to B if it mutates to be able to infiltrate B's cells it might take some but it'll take the DNA that it has that it already that makes it it with it but sometimes when it starts coding for some of these other guys so let's say that this is a pro virus right here where this is the blue part is the original virus the yellow is the is the organisms I guess historic DNA sometimes when it codes it takes up little sections of the other organisms DNA so you know maybe most of it was the viral DNA but it might have when it transcribed and translated itself it might have taken a little bit or at least when it translated or replicate itself it might take a little bit of the organisms previous DNA so it's actually cutting parts of DNA from one organism and bringing it to another organism taking it from one member of a species to another member of the species but it can definitely go cross species so you have this idea of all of a sudden that DNA can jump between species and it really kind of I don't know for me it makes me appreciate how interconnected you know we've species we kind of imagine that we're by ourselves and we can only reproduce with each other and have genetic variation within a population but viruses introduce this notion of horizontal transfer via transduction and trans transfer horizontal transduction is just the idea of look when I when I replicate this virus I might take a little bit of the the organism that I'm used that I'm kind of freeloading off of I might take a little bit of their DNA with me and infect that DNA into the next organism so you actually have this DNA that's jumping from organism to organism so it kind of unifies all you know all like it's a DNA base life which is all the life that we know on on the planet and if all of this all of this isn't creepy enough and actually maybe I didn't I'll save the creepiest part for the end but there's a whole class you could talk all about the different classes of viruses but just so that you're familiar with some of the terminology when a virus when a virus attacks bacteria which they often do and we studied these often the most because this might be a good alternative to antibiotics because virus that attack bacteria might you know sometimes the bacteria is far worse for the virus but these are called bacteria phages bacteria phages and I've already talked to you about and you know they have their DNA but since bacteria of hard walls will just inject the DNA inside of the bacteria and you know when you talk about when you talk about DNA this idea of kind of a provirus so when a virus you know lysis it like this is called the lytic cycle this is just some terminology that's good to know if you're going to take a biology exam about this stuff and when the virus incorporates it into this DNA and lays dormant it incorporates into the DNA of the host organism and lays dormant for a while this is called the lysogenic cycle and normally although you know provirus is essentially experiencing a lysogenic cycle in eukaryotes in organisms that that that have a nuclear membrane normally when people talk about the lysogenic cycle they're talking about viral DNA laying dormant in the DNA of bacteria or bacteria phage DNA light laying dormant in the in the DNA of bacteria but just to kind of give you an idea of what this quote-unquote looks like what this looks like right here I got these two pictures from Wikipedia one is from the CDC this is actually so this is these little green dots and this was colored I think the original picture was in black and white these little green dots you see right all over the surface this this big thing you see here this is a white blood cell this is a white blood cell part of the human immune system so this is a white blood cell white blood cell and what you see emerging from the surface essentially budding from the surface of this white blood cell and this gives you a sense of scale - these are HIV one viruses and just so you're you're familiar and this is why it's so you're familiar with the terminology the eight is HIV is the virus that it infects I guess you can call it infects white blood cells AIDS is the syndrome you get once your immune system is weakened to the point and then many people you know they suffer infections that people with a strong immune system normally won't suffer from but this is creepy I mean these things went inside this huge cell they used the cells own or mechanism to reproduce its own DNA or its own RNA and these protein capsids and then they bud from the cell and they take a little bit of the membrane with it and they can even leave some of their DNA behind in this cells own DNA so they really change what the cell is all about this is another creepy picture these are right here or bacteria phages these are bacteria phages bacteria phages and these show you what I said before this is a bacteria right here this is this is its cell wall and it's hard so it's hard to just kind of just emerge into it or you can't just merge fuse membranes with it so they hang out on the outside they hang out on the outside of this bacteria and they're essentially injecting their genetic material into it into the bacteria itself and you could imagine just looking at the size of these things I mean this is a cell and it looks like a whole planet or something or not you know or this is a bacteria and these things are so much smaller roughly one hundredth of a bacteria and these are much less than one hundredth of this cell we're talking about they're extremely hard to kind of to filter for to kind of keep out because they are such such small particles but you know just if you think that you know these are only exotic things that exist for things like you know a HIV or or Ebola which which they do cause or SARS you're right but they are also common things I mean I said at the beginning of this video that I have a cold and I have a cold because some viruses have infected the tissue in my in my nasal passage and they're causing me to you know have a runny nose and whatnot and you know viruses also cause the chicken pox they call it the herpes simplex virus causes cold sores so they're with us all around you I can almost guarantee you have some virus with you as you speak they're all around you but they can but it's just a very I you know it's it's a very philosophically puzzling question because I started with at the beginning you know are these life and at first when I just showed it to you is it look they're just this you know protein with some nucleic acid molecule in it and it's not doing anything and that doesn't look like life to me it's not moving around it doesn't have a metabolism it's not eating it's not reproducing but then all of a sudden when you think about what it's doing to cells and how it uses cells to kind of reproduce it kind of like you know in the business terms its asset light it doesn't need all of the machinery because it can use other people's machinery to replicate itself it's almost you almost kind of want to view it as a a smarter form of life because it does it doesn't it doesn't go through all of the trouble of what you know every other form of life has and it makes you question what life is or even what we are you know are we these things that contain DNA or are we just are we just transport mechanisms for the the DNA and these are kind of the more important things and you know these viral infections are just battles between different forms of DNA and RNA whatnot anyway I don't want to get too philosophical on you but I hopefully this gives you a good idea of what viruses are and what why they really are in my mind the most fascinating pseudo organism in all of biology
Biology is brought to you with support from the Amgen Foundation