Capsids are the protein coat that protects viral genomes. They are made from one or a few proteins that form repeating units that assemble around the genome. The three major types of capsid that protect viral genomes are:
- Complex (these are usually only found in non-animal viruses)
Figure 3: Helical Capsid
Helical capsids are usually formed from one protein that interlocks to form a helix-like structure around the viral genome. They are usually more common among viruses that infect plants, though influenza, measles, mumps and rabies viruses all have helical capsid structures. These viruses are also negative single-strand RNA enveloped viruses.
Figure 4: Icosahedral Capsid
Many animal viruses as well as viruses that infect bacteria (bacteriophages) have an icosahedral structure. If you're not an obsessed Dungeons and Dragons fan, you should know that an icosahedron is a twenty-sided object, that is almost spherical. If you don't have a friend who is a Black Mage or 3rd level Elven Raider, a traditional soccer ball looks like an icosahedron. If you do have such friends, maybe it's time to get out more. Icosahedral viruses are often formed by either one to three repeating subunits of capsid proteins.
Figure 5: Complex Capsid
It is quite intriguing that most virus capsids are either icosahedral or helical in structure. Presumably, these two structures were the ideal structures to protect a viral genome and accessory proteins within a virus particle. And this makes sense, as you have proteins that wrap around a genome in the helical structure, or proteins that form a sphere-like shell in the icosahedron. Things go haywire with the complex capsid, like that seen in Figure 5.
Complex capsids look sort of like virus spaceships. Fortunately no virus that infects humans has a complex capsid structure, or a spaceship. So, we're safe from the complex capsid virus overlords…for now. But, to be safe, you should take this time to choose which friend you'd eat first.