Understanding Virus Structure and Types

Our next job is to think about the structures that make up viruses and how a virus is put together. And the good news is that compared to eukaryotic and prokaryotic cells, viruses are actually fairly simple. So let's start by talking about the different molecules that make up a virus. The first thing to know is that viruses contain nucleic acid, and unlike prokaryotic and eukaryotic cells, they contain either DNA or RNA, but they don't contain both. And you'll remember from talking about prokaryotic and eukaryotic structure that cells have both DNA and RNA and they serve different functions in the cell. So this is a major difference. In a virus surrounding the nucleic acid, there's a protein coat called a capsid. In this diagram you see a icosahedral capsid, but they come in different shapes, and I'm going to show you a bunch of different shapes in a moment. Now a virus without an envelope is called a naked virus or a nucleocapsid. Some viruses don't have envelopes, so this is their complete structure, and when this is the case, the capsid itself will have these keys or spike proteins. And these are really important because they are what is needed for the virus to recognize the host cell that it's going to infect. An example of a non-envelope virus is the papillomavirus that causes genital warts and various kinds of cancer. Many viruses also have a third component, which is an outer layer called an envelope. The envelope is made of lipid, and the lipid comes from the host cell membranes, often the plasma membrane, but it can actually be derived from the nuclear membrane, the endoplasmic reticulum, the Golgi complex as well. I'll show you how a lipid envelope is acquired by a virus in a little bit. Now if the virus is enveloped, then those viral spikes or keys are going to need to be on the outside of the envelope so the virus can recognize the host cell in order to infect it. A viral capsid is made of individual proteins called capsomeres. In this cute little cartoon you can see a helical capsid and how it's made of many capsomeres that repeat as the structure is formed. The capsid of course surrounds the nucleic acid, we said either DNA or RNA, and that makes up the virus's genetic material. Capsids come in some really cool shapes, so let's go to the next slide to see some examples. As you might expect, the number of capsomeres that a virus has is going to vary depending on the virus's size. So for tiny little poliovirus, there are only 32 capsomeres, while the larger adenovirus has about 252. Looking from left to right on this slide, you can see examples of various capsid shapes. The first illustrates a helical capsid made of individual capsomeres, and then below it you can see an electron micrograph of an actual virus with this structure. This is that tobacco mosaic virus that we talked about in the first video. Another virus that has a helical capsid structure is the coronaviruses, which we are all very familiar with these days. The second virus has an icosahedral capsid which is seen in adenoviruses. These cause the common cold in humans. The third example is another helical capsid. Now this one is influenza and you can see that the envelope encloses eight helical capsids and each one of these contains one of the eight genes that the influenza virus needs. Influenza viruses are RNA viruses so that means each one of these capsids contains a single strand of RNA. Influenza viruses are enveloped viruses, and you can see the lipid envelope with the spikes or keys that recognize the receptors on the outside of the host cell and allow the virus to sneak inside. If you look back at the adenovirus, which is a naked virus, you see those keys are on the surface of the capsid instead. Now my favorite capsid shape is the one on the far right. This is that bacteriophage or phage with the complex capsid. You can see it's got a phage head, which in this case contains DNA inside of it. And what this phage does when it lands on the bacterial cell is to inject its DNA right into the bacteria. I always find it really interesting that bacteria are such simple cells, but they've definitely got the coolest, most complex viruses. These phages look really sinister, the way they land on the bacterial cell surface and just inject their DNA into the cell. I couldn't help myself, so here's another picture of a bacteriophage. This is an electron micrograph. And again, I just find it so amazing that we can get pictures of these viruses. They're so tiny. The colors you see have been added to help you visualize the virus, which is in turquoise in this case. Not all viruses, but many viruses, contain an outer layer called an envelope. When a virus is released from a cell, if it's an envelope virus, it will leave the cell by a process called budding. As the virus leaves the cell, cell, it's going to wrap itself in some of that cell membrane and take it with it. Now before the virus leaves, the host proteins that are embedded in the membrane are actually replaced with the viral proteins. These are those spike proteins or keys that the virus will need to recognize and infect a new host cell. They need to be on the outside of the virus in order for infection to take place. And as we said a little bit ago, depending on the type of virus, the envelope can be derived from nuclear membrane, endoplasmic reticulum membrane, Golgi complex membrane, or from the plasma membrane. Let's look at an example where the envelope is acquired from the host cell plasma membrane. This picture nicely illustrates what happens as an envelope virus leaves the host cell. In step one, you can see the assembled capsid moving towards the plasma membrane. Note that the plasma membrane is now decorated with these virus spikes or keys. These were built and then moved to the plasma membrane while the rest of the virus was being built. Now in steps 2 and 3, as the virus begins to leave, you can see that the plasma membrane containing those spikes folds over onto the virus and it wraps the virus in that envelope. In steps 4 and 5, you can see the virus is now ready to go and infect another cell. We've said that viral genomes can be made of either DNA or RNA, but not both. Your cells contain both DNA and RNA, and DNA's role is to store genetic information. But in the case of an RNA virus, RNA is the only genetic material that they have, and so RNA will fill that role of being the genetic blueprint for the virus. Some viruses have only a few genes. For example, hepatitis B only has four genes. Influenza, which we mentioned earlier, has eight genes. But herpesviruses have hundreds of genes. And there are some very large viruses called Mimiviruses and Pandoraviruses that don't cause any diseases in humans that we know of yet, and they have thousands of genes. Just for reference, a bacteria like E. coli has about 4,000 genes, and complicated intelligent humans have about 23,000 genes. Before we sequenced the human genome, it was speculated that there might be as many as 100,000 genes to account for our complexity, but the actual number turned out to be much lower. You have to be kind of careful using genome size as a measure of complexity. There are some plant genomes that are huge, much, much larger than human genomes. And we don't think of plants as being particularly complex or intelligent compared to us. Now, when we think about cells, in cells, the DNA is double-stranded and the RNA is single-stranded. And what this slide is trying to show you is that viruses don't follow any of the same rules that cells do. In a DNA virus, the DNA can actually be double-stranded or single-stranded. And the same is true for RNA viruses. The RNA can be double-stranded or single-stranded. And then to add to the complexity, in RNA viruses, the RNA can be either positively or negatively stranded, depending on whether or not it has to be copied before it's translated. Don't worry right now about what positive and negative RNA means. We haven't talked about DNA and RNA structure yet. So this concept might not mean all that much, but I'm including it because it's often used as a way that we classify viruses into groups. Some viruses have all of their genes on one piece of nucleic acid, and some viruses, like influenza, have each gene on a separate piece. Some viruses are retroviruses. These are a class of RNA viruses that, when they get into the human cell, first convert the RNA to DNA. Then the DNA gets integrated into the host cell's DNA as part of the viral life cycle. HIV, the virus that causes AIDS, is a great example of a retrovirus. I've included some tables here from your book, and I don't want you to worry too much about memorizing all of this. The point is really just to get a sense of how viruses are classified. You can see the first group on this page are double-stranded DNA viruses with envelopes. Below that are double-stranded DNA viruses without envelopes, and then single-stranded DNA viruses without envelopes. On this slide, there are some common RNA viruses, and you can see that they are classified by whether or not they are double-stranded or single-stranded, whether they are positive or negatively stranded, and whether they have an envelope. You have definitely been infected with some of these. Rhinoviruses cause many common colds, for example. There's a vaccine for rotavirus now, but if you were born before 2006, you didn't get the vaccine, and you most likely got rotavirus. It causes severe diarrhea and while most kids are fine, some end up in the hospital with dehydration. In many places in the world, kids routinely die of dehydration from diarrheal diseases and some of these are definitely caused by viruses. All of the viruses in the tables that I showed you are pathogens that infect and cause disease in humans. In our next lecture video, we're going to focus on the viral life cycle.

The first thing to know is that viruses contain nucleic acid, and unlike prokaryotic and eukaryotic cells, they contain either DNA or RNA, but they don't contain both. And you'll remember from talking about prokaryotic and eukaryotic structure that cells have both DNA and RNA and they serve different functions in the cell. So this is a major difference.

In a virus surrounding the nucleic acid, there's a protein coat called a capsid. In this diagram you see a icosahedral capsid, but they come in different shapes, and I'm going to show you a bunch of different shapes in a moment. Now a virus without an envelope is called a naked virus or a nucleocapsid. Some viruses don't have envelopes, so this is their complete structure, and when this is the case, the capsid itself will have these keys or spike proteins. And these are really important because they are what is needed for the virus to recognize the host cell that it's going to infect.

An example of a non-envelope virus is the papillomavirus that causes genital warts and various kinds of cancer. Many viruses also have a third component, which is an outer layer called an envelope. The envelope is made of lipid, and the lipid comes from the host cell membranes, often the plasma membrane, but it can actually be derived from the nuclear membrane, the endoplasmic reticulum, the Golgi complex as well.

I'll show you how a lipid envelope is acquired by a virus in a little bit. Now if the virus is enveloped, then those viral spikes or keys are going to need to be on the outside of the envelope so the virus can recognize the host cell in order to infect it. A viral capsid is made of individual proteins called capsomeres. In this cute little cartoon you can see a helical capsid and how it's made of many capsomeres that repeat as the structure is formed. The capsid of course surrounds the nucleic acid, we said either DNA or RNA, and that makes up the virus's genetic material.

Capsids come in some really cool shapes, so let's go to the next slide to see some examples. As you might expect, the number of capsomeres that a virus has is going to vary depending on the virus's size. So for tiny little poliovirus, there are only 32 capsomeres, while the larger adenovirus has about 252. Looking from left to right on this slide, you can see examples of various capsid shapes. The first illustrates a helical capsid made of individual capsomeres, and then below it you can see an electron micrograph of an actual virus with this structure. This is that tobacco mosaic virus that we talked about in the first video.

Another virus that has a helical capsid structure is the coronaviruses, which we are all very familiar with these days. The second virus has an icosahedral capsid which is seen in adenoviruses. These cause the common cold in humans.

The third example is another helical capsid. Now this one is influenza and you can see that the envelope encloses eight helical capsids and each one of these contains one of the eight genes that the influenza virus needs. Influenza viruses are RNA viruses so that means each one of these capsids contains a single strand of RNA. Influenza viruses are enveloped viruses, and you can see the lipid envelope with the spikes or keys that recognize the receptors on the outside of the host cell and allow the virus to sneak inside.

If you look back at the adenovirus, which is a naked virus, you see those keys are on the surface of the capsid instead. Now my favorite capsid shape is the one on the far right. This is that bacteriophage or phage with the complex capsid.

You can see it's got a phage head, which in this case contains DNA inside of it. And what this phage does when it lands on the bacterial cell is to inject its DNA right into the bacteria. I always find it really interesting that bacteria are such simple cells, but they've definitely got the coolest, most complex viruses.

These phages look really sinister, the way they land on the bacterial cell surface and just inject their DNA into the cell. I couldn't help myself, so here's another picture of a bacteriophage. This is an electron micrograph. And again, I just find it so amazing that we can get pictures of these viruses.

They're so tiny. The colors you see have been added to help you visualize the virus, which is in turquoise in this case. Not all viruses, but many viruses, contain an outer layer called an envelope. When a virus is released from a cell, if it's an envelope virus, it will leave the cell by a process called budding. As the virus leaves the cell, cell, it's going to wrap itself in some of that cell membrane and take it with it.

Now before the virus leaves, the host proteins that are embedded in the membrane are actually replaced with the viral proteins. These are those spike proteins or keys that the virus will need to recognize and infect a new host cell. They need to be on the outside of the virus in order for infection to take place.

And as we said a little bit ago, depending on the type of virus, the envelope can be derived from nuclear membrane, endoplasmic reticulum membrane, Golgi complex membrane, or from the plasma membrane. Let's look at an example where the envelope is acquired from the host cell plasma membrane. This picture nicely illustrates what happens as an envelope virus leaves the host cell.

In step one, you can see the assembled capsid moving towards the plasma membrane. Note that the plasma membrane is now decorated with these virus spikes or keys. These were built and then moved to the plasma membrane while the rest of the virus was being built.

Now in steps 2 and 3, as the virus begins to leave, you can see that the plasma membrane containing those spikes folds over onto the virus and it wraps the virus in that envelope. In steps 4 and 5, you can see the virus is now ready to go and infect another cell. We've said that viral genomes can be made of either DNA or RNA, but not both.

Your cells contain both DNA and RNA, and DNA's role is to store genetic information. But in the case of an RNA virus, RNA is the only genetic material that they have, and so RNA will fill that role of being the genetic blueprint for the virus. Some viruses have only a few genes. For example, hepatitis B only has four genes.

Influenza, which we mentioned earlier, has eight genes. But herpesviruses have hundreds of genes. And there are some very large viruses called Mimiviruses and Pandoraviruses that don't cause any diseases in humans that we know of yet, and they have thousands of genes. Just for reference, a bacteria like E.

coli has about 4,000 genes, and complicated intelligent humans have about 23,000 genes. Before we sequenced the human genome, it was speculated that there might be as many as 100,000 genes to account for our complexity, but the actual number turned out to be much lower. You have to be kind of careful using genome size as a measure of complexity.

There are some plant genomes that are huge, much, much larger than human genomes. And we don't think of plants as being particularly complex or intelligent compared to us. Now, when we think about cells, in cells, the DNA is double-stranded and the RNA is single-stranded. And what this slide is trying to show you is that viruses don't follow any of the same rules that cells do. In a DNA virus, the DNA can actually be double-stranded or single-stranded.

And the same is true for RNA viruses. The RNA can be double-stranded or single-stranded. And then to add to the complexity, in RNA viruses, the RNA can be either positively or negatively stranded, depending on whether or not it has to be copied before it's translated. Don't worry right now about what positive and negative RNA means. We haven't talked about DNA and RNA structure yet.

So this concept might not mean all that much, but I'm including it because it's often used as a way that we classify viruses into groups. Some viruses have all of their genes on one piece of nucleic acid, and some viruses, like influenza, have each gene on a separate piece. Some viruses are retroviruses.

These are a class of RNA viruses that, when they get into the human cell, first convert the RNA to DNA. Then the DNA gets integrated into the host cell's DNA as part of the viral life cycle. HIV, the virus that causes AIDS, is a great example of a retrovirus. I've included some tables here from your book, and I don't want you to worry too much about memorizing all of this.

The point is really just to get a sense of how viruses are classified. You can see the first group on this page are double-stranded DNA viruses with envelopes. Below that are double-stranded DNA viruses without envelopes, and then single-stranded DNA viruses without envelopes. On this slide, there are some common RNA viruses, and you can see that they are classified by whether or not they are double-stranded or single-stranded, whether they are positive or negatively stranded, and whether they have an envelope.

You have definitely been infected with some of these. Rhinoviruses cause many common colds, for example. There's a vaccine for rotavirus now, but if you were born before 2006, you didn't get the vaccine, and you most likely got rotavirus. It causes severe diarrhea and while most kids are fine, some end up in the hospital with dehydration. In many places in the world, kids routinely die of dehydration from diarrheal diseases and some of these are definitely caused by viruses.

All of the viruses in the tables that I showed you are pathogens that infect and cause disease in humans. In our next lecture video, we're going to focus on the viral life cycle.

Transcript for:Understanding Virus Structure and Types

Transcript for:
Understanding Virus Structure and Types