In this video, we're going to begin our introduction to viruses by focusing specifically on the general structure of viruses. First, we need to recall from some of our previous lesson videos that we already defined viruses as obligate intracellular parasites, which just means that these viruses have to make their way into a host cell in order to replicate themselves. These viruses serve as parasites because they actually harm the host cell as the viruses benefit themselves by replicating. These viruses that serve as obligate intracellular parasites will contain either DNA or RNA packed into a protein coat. Sometimes, some viruses will also have a lipid envelope. The protein coat that a virus has is specifically referred to as a capsid. The capsid is the protein coat that a virus has and that protein coat helps to protect the nucleic acids, either DNA or RNA, from environmental toxins and enzymes.
The term nucleocapsid specifically refers to the combination of the nucleic acids of the virus, once again either DNA or RNA, as well as the capsid protein coat itself. When you're referring to both the nucleic acids and the protein coat, the capsid, we use the term nucleocapsid. Lastly, we have spikes. The spikes are surface proteins on the virus that allow the viruses to bind to a specific host cell receptor in order to initiate infection. The term virions here refers to a complete virus particle that has been released from the host cell into the extracellular environment or extracellular space, which is really the space on the outside of a cell. Occasionally you'll see this term virions which is just referring to viruses on the outside of a cell.
If we take a look at our image down below, we can get a better understanding of what a typical structure of a virus could look like. Although the shapes of viruses can vary, and we'll get to talk a little bit more about the different shapes viruses can take later in our course, here is an image that represents the general structure of a virus. What you'll notice is that the virus consists of some kind of nucleic acid, which is the green squiggly here. This nucleic acid could be either DNA or RNA. Around the nucleic acid is a capsid. The capsid refers to the protein coat. You can see the protein coat is surrounding the nucleic acid and helping to protect the nucleic acid. When you're referring to both the nucleic acid as well as the capsid, then we're referring specifically to the nucleocapsid. These yellow structures are specifically referring to the spike proteins. The spike proteins are the surface proteins that help the virus bind to specific host cell receptors, and it allows the virus to initiate an infection. These spikes play a crucial role in allowing viruses to enter into their host cells in order to replicate.
This here concludes our brief introduction to viruses and the general structure of viruses, including the capsid, nucleocapsid, spikes, and the term virions. We'll be able to get some practice applying these concepts and learn more about viruses as we move forward in our course. So I'll see you all in our next video.
