Viruses: Study with Video Lessons, Practice Problems & Examples

Viruses are non-living parasitic particles composed of a protein coat (capsid) that encases genetic material (DNA or RNA). They reproduce by infecting host cells through two main life cycles: the lytic cycle, which destroys the host to release new viral particles, and the lysogenic cycle, where viral DNA integrates into the host genome. Viroids are infectious RNA molecules affecting plants, while prions are misfolded proteins causing diseases like mad cow disease. Scientists utilize viruses in research, particularly retroviruses, for gene manipulation and studying cell biology.

Viruses

Hi. In this video, we are going to be talking about viruses, viroids, and prions. So first, I just want to point out that I personally think that this is the most interesting topic of all of zoology. I love me some viruses. So what are viruses? Well, viruses are small parasitic particles that rely on other organisms for their life cycle. So generally, they're very tiny, composed of only a few proteins. So, what I put here is a range of 4 to 200 proteins, so very small, but they do have very similar structures. So all viruses have a capsid, which is just a protein coat that encloses its genetic material, which is DNA or RNA. Now some viruses have an envelope, which is just the phospholipid bilayer, very similar to that of the plasma membrane in cells, that surrounds the viral capsids, and the viral capsids then surround the DNA. And I'm going to show you a picture of this in just a second. Now capsid proteins come in two main shapes, helical, which is exactly what it sounds like. It's a helix, or icosahedron. Now that's kind of a fancy, geometry term, but essentially what it means is that there are 20 identical faces on the surface of the capsid protein. So those are the two main shapes. Now viruses are really simple. They don't contain any organelles, they don't have any cytoplasm, and like I said before, they only have a few proteins. Now we know viruses because they're typically named after diseases that they cause, and they can cause diseases in plants, they can cause diseases in animals, and in bacteria, and that is how they typically get their name. Now viruses are unique in the fact that they are not living organisms, because they cannot reproduce independently; they have to have their host cell machinery in order to reproduce, and they don't have metabolism, and they don't have all these properties that we've talked about previously that describe life and cells. So they're not living. So to survive, viruses have to infect host cells and use that host cell machinery in order to reproduce itself. And then another unique fact is that viruses are typically very specific, and the fact that they only infect certain hosts. So for instance, one common one that you'll see a lot in your textbook is a bacteriophage, and this is just a type of virus that affects only bacteria. Now there are viruses that infect only humans, there are viruses that infect only mammals, but generally, they're very specific as to what organism they infect. So if we look at just the basic structure of a virus particle here, you can see some of the most all of the structures that I mentioned above. So one you have this envelope here that is surrounding the entire virus inside the envelope. Like I said, this is only present in some, not all, viruses. Now you also have inside of this membrane a capsid, and this is actually in the icosahedron shape. You can see there are these repeating faces. And if you were to go all the way around, there should be 20 of them. So I've put that here. Icosahedron. Sorry if I botched that spelling. And then inside of the capsid, the protein capsid, you can see the DNA and RNA genes, which are right in here. So now that we've talked about this, let's move on to the virus life cycle. Let's flip the page.

So viruses have 2 main life cycles. These are called the lytic cycle and the lysogenic cycle, and we'll go through each one of them individually. So the lytic cycle is responsible for creating it here. Now you may not necessarily need to know all of these steps, here. Now you may not necessarily need to know all of these steps. You'll have to just sort of check with your lecturer and your professor whether you need to know these exact names. But most of you probably won't need to know the exact names, just sort of understand the concept that this is how new viral particles are made. So how this happens is that the virus binds to the host cell, usually through some type of cell protein called a receptor, and then they are internalized into the cell. Then, the penetration that viruses make across the plasma membrane penetrates into the cell. Formed into a virus particle, and then you have finally release, in which viral particles are released, and this disrupts destroys the cell. You can also use the word lyse to say destroys the cell, and enveloped viruses bud, which is a specific term used to describe viral particles leaving the cell. They bud from the cell. Now this is different from the lysogenic cycle, and this happens when the virus integrates its genetic material, into the host genome. So this isn't a process where it's creating new viruses, but instead just making sure that its genetic material is internalized into the host cell. So what happens, in this step is the virus usually infects through the lytic cycle, but instead of going through a process of creating new viruses, it decides it's just going to sort of hang around in the cell for a little while, by maintaining its DNA. So what happens is, or a term for this is called a prophage. You may see this termed as a provirus, but they're essentially the same thing, and it's the term for the act of the DNA that's integrated into the host genome. So the prophage is the viral DNA that's integrated. Now, how so what after this viral DNA is integrated into the host genome, eventually the virus is going to want to start replicating and producing new viruses and enter back into the lytic cycle. And so usually how this happens in bacteria, some sort of DNA damage occurs, and this is causes this, viral DNA to be replicated and then go back to the lytic cycle. Now I know this is confusing, give me a second I'm going to show you a really clear image on, what I'm talking about on how this happens, but we'll get to that in just one second. So, whenever it switches back to the lytic cycle, virions, which are new viral particles, are created by the provirus, which again is the integrated DNA, in the eukaryotic cells. Now, this can be really damaging because you can imagine that if, a virus integrates its DNA somewhere in, say, a human cell. Well, it can integrate into really anywhere it wants, and that can cause some serious problems for your genetic material. And so a lot of times, these integrations can actually cause cancer and other types of disease by, you know, causing the cell to lose control over its cell division, growth, and DNA replication. So let's show you that I've gone over a lot of verbal explanations of these cycles, but I think looking at the picture is going to make everything a lot clearer. So remember we have the lytic cycle, which is the new particles. So let's actually start here. So what you can see is that here is a virus. This is actually a bacteriophage, and I know this because of its unique bacteriophages look like this and no other viruses do. You don't necessarily need to know that, but I just think it's neat. So this bacteriophage is, putting its, DNA, into this cell. This is a cell, this little green rectangle here. And what happens, you can see, is that this DNA is then integrated integrated into the host cell that it infected genome. Now in the lytic cycle, the lytic cycle what happens is that this section is then, some type of damage occurs or something that allows it to be removed from this area, or it's just transcribed, depending on the cell type and you don't necessarily need to know in what cases that happens. But eventually, this section of the viral genome actually goes through some processing steps and is transcribed and made into various proteins, so that new viral particles can be constructed. So these steps here, new viral particles are made. And then eventually, you can see this, this is called lysing, which destroys the cell and the viral particles are released. Free to do more infecting into other cells. Now for the lysogenic cycle, this is a little different because it only deals with the integration part. So what happens is we're going to start in the same place right here with the bacteriophage, and it injects its DNA, and it gets integrated very the same way. But instead of, making new viral particles, it just stays there. And it continues to copy and continues to divide as the cell divides and now you have 2 cells here, and you have 2 copies. And this can keep going on and on, this division, where you get all of these cells containing the viral copy, and then eventually, it'll go back here and go into the lytic cycle, the lytic cycle. And so these are the 2 life cycles of viruses. So hopefully, that's clear. If not, just review the video and just remember that the lytic cycle is about the virus infecting and producing new particles. Whereas, the lysogenic cycle is about viral, DNA or RNA integration. So now, let's move on to the next concept.

Viruses for Research

K. So in this video, we're going to be talking about how scientists use viruses for research. And so viruses are such a great tool that we use as scientists to study cell biology and especially cancer. And so before we can actually study viruses, we need to be able to count them. And because we can't see viruses with our eye and because we can't even see them with traditional light microscopes that you may be familiar with, we have to use specialized assays in order to measure how much virus we're working with.

The first thing, and really a main way that this is done, is through a plaque assay, which is used to calculate the amount of virus in a sample. How this is done is cells are infected with viruses, and those viruses then integrate their DNA into the cell. This causes the cells to just start growing, I mean, just insanely fast. When they do, they actually start growing on top of each other, and so when you stain them, which you can see here with this blue stain, the cells that are growing really fast, produce these things called plaques, which you actually can't see that color, because it's blue. Here are some plaques, so there's 2 plaques. Here's 1 plaque, 2 plaques. And so by counting these and using the amount of virus, the volume, so 1 microliter, 2 microliters, 1 mL, you can do some math, or scientists can do math, and then determine how much virus is in their sample. It's a significant assay that is used in cell biology to measure the amount of virus.

Another way that this is done is through retroviruses. Retroviruses are extremely important in scientific research. So, what is a retrovirus? It is an enveloped RNA virus that incorporates its genome into the host cell chromosome. This is really important because that means that scientists can manipulate what genes get put into which cells through using retroviruses.

How this happens is that, because it's an RNA virus, the genetic material is RNA. So, in order for it to be put into the host cell genome, it actually has to be turned into DNA. This process is called reverse transcription, which is RNA to DNA, which is different than transcription, which is the opposite of DNA to RNA. This process actually creates two identical DNA strands. And then, the DNA can actually integrate itself into the host chromosome, which, if you'll remember, is called a provirus or a prophage. Then, the viral gene that has been incorporated can be expressed when the cell replicates and transcribes that region of the host cell genome. So, scientists use this because they can take out, sort of remove the viral genome and replace it with the genes that they would like to put in there. It's a really cool way that viruses are really useful for studying cell biology. So now let's move on to the next concept.

Viroids and Prions

Okay. So we've been talking a lot about viruses, but I do want to briefly mention 2 other types of infectious particles, and that's prions and viroids. So, viroids are small circular infectious RNA molecules found mainly in plant cells. They can be transferred between damaged plant cells, but, generally, how they work is that the RNA doesn't necessarily have any function itself, but instead, it just binds to proteins and blocks their function. And they, like I said, are infectious, so they can be transferred between plant cells. Now prions are not RNA and instead, they are infectious particles made from abnormally folded cellular proteins. And so, we're familiar with prions, more than you might think. They cause some diseases that you've probably heard of, including mad cow disease, and they're unique in the fact that they're actually not destroyed by cooking which a lot of infectious particles and diseases can be prevented just by cooking food properly. But for prions, there's still a risk after the meat has been cooked, and that makes these diseases especially dangerous, especially mad cow disease. So let's just very briefly look at the basic viroid structure, which you can see here. Let me move out of the way. But it's just simply a circular structure, very bland. You can imagine it is RNA, so there are base pairs binding all throughout here, and even maybe a few in here that bind. And, this is just what it basically looks like when it binds to a protein. So, let's move on now.

Which of the following is true?

• A. Viruses, viroids, and prions are all living infectious agents
• B. Viroids have two main life cycles:the lytic and lysogenic cycle
• C. Viruses have two main shapes:icosahedron and circular
• D. Viruses, viroids, and prions are all non-living infectious agents

During which life cycle is the virus making more copies of itself?

• A. Lytic Cycle
• B. Lysogenic Cycle

Retroviruses are able to integrate their genome into the host cell?

• A. True
• B. False

Here’s what students ask on this topic:

What are the main structural components of a virus?

Viruses are composed of a few key structural components. The primary structure is the capsid, a protein coat that encases the virus's genetic material, which can be either DNA or RNA. Some viruses also have an envelope, a phospholipid bilayer similar to a cell's plasma membrane, surrounding the capsid. The capsid proteins can be helical or icosahedral in shape. Viruses lack organelles, cytoplasm, and other cellular structures, making them much simpler than living cells. They rely on host cells for reproduction and metabolism, which is why they are considered non-living entities.

How do viruses reproduce inside host cells?

Viruses reproduce through two main life cycles: the lytic cycle and the lysogenic cycle. In the lytic cycle, the virus attaches to a host cell, injects its genetic material, and hijacks the host's cellular machinery to produce new viral particles. These new particles are then released, often destroying the host cell in the process. In the lysogenic cycle, the viral DNA integrates into the host genome and replicates along with it. The virus can remain dormant for a period before eventually entering the lytic cycle to produce new viral particles.

What is the difference between a lytic and a lysogenic cycle?

The lytic cycle involves the active production of new viral particles, leading to the destruction of the host cell. The virus attaches to the host cell, injects its genetic material, and uses the host's machinery to replicate. The new viruses are then released, often lysing the host cell. In contrast, the lysogenic cycle involves the integration of viral DNA into the host genome, where it can remain dormant for an extended period. The viral DNA replicates along with the host's DNA until certain conditions trigger its reactivation, leading to the lytic cycle.

How are viruses used in scientific research?

Viruses are valuable tools in scientific research, particularly in cell biology and cancer studies. One common method is the plaque assay, which measures the amount of virus in a sample by infecting cells and counting the resulting plaques. Retroviruses are especially useful because they can integrate their RNA genome into the host cell's DNA through reverse transcription. Scientists can manipulate retroviruses to insert specific genes into host cells, allowing for controlled gene expression studies. This technique is crucial for understanding gene function and developing gene therapies.

What are viroids and prions, and how do they differ from viruses?

Viroids are small, circular RNA molecules that primarily infect plant cells. They do not encode proteins but interfere with host cell functions by binding to cellular proteins. Prions are misfolded proteins that cause infectious diseases like mad cow disease. Unlike viruses, prions do not contain nucleic acids. Both viroids and prions are simpler than viruses and cause disease through different mechanisms. Viroids disrupt cellular processes by binding to proteins, while prions induce normal proteins to misfold, leading to cellular damage.