 Today we're going to be talking about viruses. First let's talk about what a virus is not. A virus is not a living thing. So a virus is not made up of a cell. A virus cannot reproduce on its own. And a virus cannot make ATP because it doesn't have the enzymes and materials to do fermentation. It doesn't have the materials to do cellular respiration. So because of these three reasons, a virus, by the definition of living things, is excluded. So what is a virus? So a virus consists of genetic material, either DNA or RNA. And then it's surrounded by a protein coat, which is often called a capsid. Now also inside of that protein coat, sometimes you can have a couple of enzymes inside to help the virus get started when it first invades a cell. Now viruses can have many different appearances. So here we've got one virus that often infects bacteria. And you can see up here it's got this large part on the top that's called the head. And inside the head of that virus is where we contain the genetic material. And then down here we've got the tail, which almost acts like a syringe, where we can pump that genetic material into the cell. And then over here we've got a different kind of virus. It's a little bit more round in nature. And then down here we've got another kind of virus. And this guy has a little bit of cellular membrane on the outside. And so that often occurs with animal viruses. Now a virus is a lot like a flash drive or a thumb drive. So it's an inanimate object. It can't do a thing on its own. But once you take that flash drive and plug it into a computer, then you can infect that computer with a virus. And that virus can then spread to others. Now a real virus that we use in life, again, is kind of like an inanimate object. It cannot do a thing on its own. But once that virus can infect a cell, then it can copy itself. And it can also spread to other cells. So again, a virus requires a cell to infect. Now there's a lot of different types of viruses. We've got some viruses that infect just bacteria. And these guys are called bacteriophages. And these guys can include lambda phage, T4 phage. They have a lot of funny names. We also have plant viruses, such as the tobacco mosaic virus that can infect tobacco plants. And then we also have a lot of animal cell viruses, like the flu virus and the HIV virus. Now viruses have two reproductive cycles, the lytic cycle or the lysogenic cycle. Now with the lytic cycle, the virus is able to make many, many copies of itself within a bacteria cell. And then the bacteria cell lices or breaks open. Now those viruses that infect the bacteria can also have what's called as a lysogenic cycle. Lysogenic cycle, a little bit longer to say. Now these occur when the viral DNA inserts itself directly into the bacterial chromosome. And when it inserts itself directly into the bacterial chromosome, every time that bacteria reproduces itself, it also reproduces the virus. Every time those bacteria cells split, again, they also are copying and reproducing that virus with each cell division. So here's a picture of what I mean. So one cycle that certain viruses can go through is the lytic cycle. Now with the lytic cycle, we start up here where we've got our virus. And then our virus inserts almost like a syringe injects its genetic material into the cell. Now this genetic material doesn't interfere, doesn't react with the bacterial chromosome. But instead, it stays within the cytoplasm. And so you can see here, the next thing that that virus genome does is it makes many, many copies of itself. If it's made of DNA, it makes many DNA copies. If the virus genome is made up of RNA, again, it makes many copies of RNA. And also when it's doing that, it's going to make many copies of its proteins. So remember that capsid, that protein coating on the outside? It's got to duplicate that as well. So you can see it's here. It's made a lot of heads and a lot of tails. And then when a virus has all those materials waiting, it's got its genome and all those proteins, it can reassemble into a regular virus and then bust out or break out of that cell. And that's called lysis. So again, the bacteria is infected by that virus. The virus duplicates itself. And then the virus reassembles and then bursts out of that cell, killing that cell in the process. And that's the lytic cycle. Now, some viruses can go through a different cycle. It's called the lysogenic cycle. And when this occurs, again, we've got our virus up here and injects its genome into the cell. So that's the same. But then the next part is different. Instead of making many, many, many copies of itself and then bursting forth from the cell, instead it inserts itself directly into the chromosome of the bacteria. And so then the cell, whenever the cell divides, we not only copy the bacterial chromosome, we also copy the viral chromosome. When those cells divide, again, they not only copy the chromosomal genome, they also copy the viral genome. And so we can end up with a lot of cells that have virus present in them because of that. Now, in most cases, with the lysogenic cycle, the virus kind of lies dormant, not doing very much. But sometimes it can be triggered. And it can be triggered to go into the lytic cycle. Now, scientists are still trying to figure out what these triggers are. In some cases, it's UV light. Maybe from a sunburn. Other cases, it might be stress or a lowered immune system or some other chemical trigger to make a lysogenic virus and then go into the lytic cycle. Now, in addition to bacteria viruses, we can have plant viruses. So these viruses can sometimes infect the entire plant. So leaf, to stem, to root. And these viruses, because they're infecting these cells, can stunt the growth of the plants. And also, because it can stunt the growth of these plants, it can diminish crop yields. So this is a very bad thing for farmers. Or if you do gardening, it can produce very ugly looking plants. Now, the downside of these viral plant diseases is they often have no cure. So the only thing that you can do is you can pull those plants out of the ground and either throw them away or set them on fire. Now, if you want to avoid having a virus infection of your plants, one thing that you can do is sometimes you can buy plants that are resistant to viruses. So you can buy a resistant breed and use that in your garden instead. Now, there's also animal viruses. So these animal viruses are a very common cause of disease in humans, such as the flu, chicken pox, measles, mumps. And these animal viruses can sometimes have either an RNA or a DNA genome. And another thing that's unique about these animal viruses compared to the bacteria viruses is sometimes when they're leaving the cell, they can pick up a bit of that cellular membrane. So here, again, we've got the genome of the virus. We've got some protein coat on the outside. And then we also have a bit of the cellular membrane that we stole from the cell. And so that's able to protect the animal virus cell, make it a little bit harder to find by the immune system. Now, one very well-known virus that can infect humans is HIV, the AIDS virus. HIV is a retrovirus. And what that means is that it starts off with an RNA genome, and then it makes DNA from that RNA genome. It produces a DNA molecule. This does not happen anywhere else in nature, except with these retroviruses. Now, the way that these retroviruses work is they use an enzyme that's called reverse transcriptase. So normally when we do transcription, we start off with DNA, and then we make RNA. But with reverse transcriptase, we're going backwards. We're going from RNA to DNA. Or we're synthesizing DNA from an RNA template. So here's how it works. We start off with an RNA genome from the virus, and then we're going to make DNA using the enzyme reverse transcriptase. Then we're going to insert that DNA into the cell's genome. So it's going to be permanently inserted into the cell's genome. And then it's going to start making many copies of RNA and make many copies of the viral proteins. And then those little new viruses are going to bud off of the original cell, taking a little bit of that membrane with it. And eventually, through this process of making many, many viruses, the cell eventually can be killed from this infection. So here's a picture of how it works. So we start off with HIV. And it's got the RNA genome inside of it. And it's also got those little green balls that represent reverse transcriptase. It brings those along. Then it's wrapped in a protein coat and also the cellular membrane. So that HIV virus, when it finds its target cell, fuses, it sticks down to it, and then dumps all of its contents into the cell. Then we start off with RNA. And we have to make DNA. So we use the reverse transcriptase to make DNA. And once we've got a nice DNA molecule, that can insert itself into our genome or insert itself into the genome of the cell that infected. And now that cell is kind of like a zombie cell. It's taken over by this virus. And it's starting to produce a lot of the viral RNA, a lot of the viral proteins. And when there's enough of this viral RNA and viral proteins, then we can make more of the virus. So here you can see a little piece of virus. It's going to bud off and leave. And then we can have another virus, bud off and leave. Another virus, bud off and leave from this particular cell until the cell is so sick that it then unfortunately dies. So this leads me to talking about acquired immune deficiency syndrome, which is short for AIDS. So AIDS is caused by HIV infection of immune cells. And when you infect these immune cells, then you can make them sick and start killing them off. And unfortunately, when that happens, you don't usually directly die from the AIDS virus, but you die from another infection that you get. So maybe the common cold, maybe pneumonia, maybe something else down the line, unfortunately, when your immune system is crippled. Now acquired immune deficiency syndrome, where the AIDS virus can be transferred by blood and other bodily fluids. So it's very important for anyone that's going into the medical field to be careful when they're working with needles, when they're working with other material that could contain blood, that they disinfect those materials properly before they are used on the next individual. Now again, whenever you have fewer healthy immune cells, because these are the ones that are getting infected by the HIV virus, it's much easier to get sick from other things. But the nice thing about HIV now is that it can be treated with drugs like AZT that interfere with the reproductive cycle of the virus. And so here's an example, a beautiful picture of the AZT drug when it's crystallized. And the use of AZT plus a cocktail of other drugs now enables people to live with the HIV virus for many decades. And so it's more of a chronic disease that's treated instead of a lethal disease.