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:11:05

Virus structure and classification

Video transcript

Viruses are interesting because they are the robot hackers of microbiology, and in this video, we're gonna learn about what, exactly, makes them so good at being robot hackers. So let's think about the things that define viruses. There's four things we're going to look at. First, they're really, really small. So, size. The virus is about that big. Compared to that tiny virus, this would be the size of a typical bacterium. It is 100 times larger than a virus. If you can just imagine this being, well, I clearly didn't draw this large enough, but you can just imagine that there's 100 viruses across here, and this would be 100 times larger, and a typical eukaryotic cell, like our own human cells, would pretty much not fit on this page. You can kind of imagine, since it's 1000 times bigger, it would form a circle that just goes straight off on either end of this quarter circle that I've drawn, kind of forming a full circle, just going all the way around. So, we talked about size between viruses, bacteria, and our human cells, but, there's another aspect of size, which is, the size of viruses compared to each other, and of course, some viruses are larger than others, and that's one way to tell different viruses apart. Some are super small, and other ones are just small. I could have drawn bigger and smaller dots that represent viruses. Now, the next thing that you can tell viruses apart with is their shape. Just think about these tiny, little things being blown up. We're just gonna talk about why they look like they look, and what causes them to be the shapes that they are. So, all viruses have this capsid. It's a protein code, and, they're all very unique shapes. You can think of them as the legos of these viruses. Legos because they need these little building blocks called the capsomers to build their shape. So I'm just building them with these little blobs here that you can see on the screen, and, even though I haven't drawn this really well, they're actually all the same size, and all the same shape for that particular virus. So each of these little things would be called a capsomer, and these capsomers form these three really beautiful three-dimensional shapes, so this looks kind of 2D, but if you can imagine kind of like a six-pointed, three-dimensional looking... This kind of six-sided diamond-like shape is called the icosahedral configuration, and there's also something that, if you first look at it, it looks kind of helical, but again, it's not formed like this. It's actually lots of little monomers that wrap around, kind of like a helix, and it looks actually more like a cylinder, but, this, because it wraps around like that, is called a helical shape, and one other possibility is the spherical shape, so this gray line that I've drawn is an envelope, and it sometimes covers the capsid, and I say sometimes, because not all viruses have this envelope. It kind of gives it an advantage that we're gonna talk about later, so, any one of these options can be inside, and, if you can imagine, since it's an envelope, wrapping that protein code in a circle, this is the spherical shape, like a ball. So that's two of four things to distinguish viruses with. Now here's the third one. This is also pretty straight-forward. It's just the genetic information contained in viruses, the nucleic acid. So, there are actually four options. So viruses are really cool because they can contain one type of nucleic acid. In fact, they only contain that type. So you've seen double-stranded DNA before, which is in most of your human cells. You've also seen single-stranded RNA, kind of like your messenger RNA, but you probably haven't seen some single-stranded DNA, or double-stranded RNA, and this is pretty unique to viruses. They're special, because they contain one of these types of nucleic acids. This is one of the ways to distinguish them. So, a virus can be a single-stranded DNA virus, or a single-stranded RNA virus. They can not be both, and that's why nucleic acids are that third category. It distinguishes viruses from each other, and this genetic information can't just float around. It actually is kind of packaged. It's stored inside of the protein coat, and because this is called a capsid, and this is nucleic acid, when they're put together to form that virus, and I'm just going to simplify that icosahedral drawing into this kind of hexagon shape, and let's just pretend it's a single-stranded RNA virus, then this is called a nucleocapsid, and again, this might, or might not be envelope. This one here that I've drawn is non-envelope, because it doesn't have a gray dotted line surrounding it. So now that we've gone over these first three basic ways to tell viruses apart, size, shape, nucleic acid, we can now go back and figure out why I said that viruses are robot hackers. And that actually will give us the fourth way to tell viruses apart, right? So, I'm just gonna write here, robot hacker, because if you look back at what we just talked about, viruses are really small, and they're made up of proteins, and one type of nucleic acid, they don't have organelles, and that means, they can't make ATP, or energy for themselves, and they can't really replicate, then, because they don't have organelles, so that's one problem, because all living things metabolize, so that's the robot part, and they sneak in to larger cells that have organelles, that they can take over to make copies of themselves. So the official term for robot hackers in biology is obligate, it absolutely needs to be inside a cell, obligate intracellular parasite. Hacks onto other things to survive, and because it needs to do that, you can probably guess now, that the fourth way to tell these apart, is by the type of host. So one question people always ask is that, "Well, is a bacteriophage a virus, or what is it?" So, it's actually the name for viruses that infect bacteria, and the ones that infect eukaryotic cells, for example, us humans, they're all different enough in size, shape, nucleic acid, and disease that they cause, that they have some pretty famous names, like pox virus, or herpes virus, or parvovirus, and there's so many more, so, these robot hackers hack in using some special methods that I haven't mentioned yet, and they actually both have to do with shape adaptations, which makes sense, because, that's the outside part of the virus that comes in contact with the cells, because if they weren't good at getting into the cells, they would never make copies of themselves, so, as robot hackers, they must do something special to get in, and bacteriophages have this complex shape. They are not just icosahedral or helical. They might have that initial, that nucleocapsid at the top, with the head portion that contains the nucleic acid, but it also has a sheath acting like a needle that the nucleic acid can be shot down, and a tail that attaches to the host bacteria, so, even though I've drawn it this way, you can actually imagine it attaching to the bacteria like this, because the tail will bind it, and it will act, literally, like a needle to inject the bacteria. And I'm gonna use this eukaryotic cell as an example for this other shape quirk that lets viruses get in, because the eukaryotic cell that I've drawn here is so large, it's just got this giant line of membrane for me to draw on, and basically, if it can't inject its genetic material like the complex virus, then it sneaks in, and I keep saying that, but the reason I'm saying "it sneaks," is because every cell has receptors on its surface, and these usually are regular receptors that cells need to communicate information to and from, but viruses take advantage of that. These receptors can't really tell the difference between normal signals, or normal cells, and a viral cell, so, this icosahedral or helical thing will come along, and signal to these receptors, and it'll trick the receptors into forming this pit, and eventually, it will bud off into an endosome, and it just kind of sits happily inside, having sneaked in, and you might recognize as endocytosis, endocytosis entering the cell, so they made up this big, fancy name for a very simple, it entered the cell with receptors, receptor mediated endocytosis, receptors, endosome, and, the sneaky reason as to why some cells have these gray envelopes that I mentioned before to give them that spherical shape, that just gives them an extra way in, so they can also enter with the receptor mediated endocytosis, so I'm just gonna draw a gray dotted line around this one area, so you can kind of imagine that, yes, if it had an envelope, it could also enter this way, but it has an extra option, and that's because it already has this bubble, so it signals to the membrane, "Hey, I'm just going to fuse with you. "I'm gonna combine with you, "and let myself in," and they kind of got fancy with this name, too, and because it directly fuses with the membrane to let itself in, it's called direct fusion. And now you have a general idea of how to tell viruses apart, and how they really are the robot hackers of the microbiology world.