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Shapes of a Virus: Helical, Icosahedral, Prolate, Complex & Enveloped

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  • 0:05 Viral Shapes
  • 0:40 Helical, Icosahedral,…
  • 3:06 Enveloped and Complex Viruses
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
Learn about the different shapes viruses can have, such as helical, icosahedral, prolate, complex and enveloped. Find out why one shape is particularly useful from an energetic standpoint.

Viral Shapes

When you're driving around on the road, you see all sorts of different types of vehicles. You've got SUVs, minivans, pickups, motorcycles and 18-wheelers. In the end, they are all a type of vehicle regardless of their shape, size and color. Likewise, viruses have many different shapes as well, and some of their shapes are unique to certain viruses and the things they infect. These shapes aren't just for looks. Like certain types of automobiles, they actually confer a function.

Helical, Icosahedral and Prolate Viral Shapes

Before we get to the nitty-gritty of this lesson, I'd like to point something out. The protective protein shell of each virus is called a 'capsid.' This capsid is made up of protein subunits called 'capsomeres,' which are in turn made of subunits called 'protomers.'

With that in mind, one type of car driving around in the viral world is known as a helical virus. This is a virus that has its capsid shaped into a filamentous, or rod-shaped structure. This type of shape has a central cavity that encloses its nucleic acid. Some of these viruses are short, like a two-door car, while others are very long, like an 18-wheeler. Furthermore, many allow for a lot of flexibility or a lot rigidity depending on how the capsomeres are arranged.

Another type of vehicle shape for transporting viral nucleic acids is called icosahedral. An icosahedral virus is a virus consisting of identical subunits that make up equilateral triangles that are, in turn, arranged in a symmetrical fashion. A special type of icosahedral shape, called a prolate, is a variant of the icosahedral viral shape and is found in bacteriophages.

A lot of viruses are either helical or icosahedral in shape. Many animal viruses, which include those that infect humans, are icosahedral in shape. The icosahedral shape has been shown to be the most optimal way of forming a viral capsid for numerous reasons, but namely due to the fact that it provides the virus with a very stable shape with a lot of room inside for the storage of its passenger, the nucleic acid.

In addition, because the protein subunits that make up the shape are identical, the virus doesn't have to waste a lot of its genome on encoding many different kinds of proteins for its capsid. This leads to conservation of energy and genetic economy. You can sort of liken this shape to the hybrid cars that have the highest fuel economy on the road and therefore save a lot of energy when driving about.

Enveloped and Complex Viruses

Some people apply car wax to the outside of their car for an added layer of protection. Likewise, some viruses like to give themselves an additional layer as well - although this additional layer is not so much for protection but more for ease of infection. These viruses are called enveloped and are viruses that have a lipid bilayer around their protein capsid.

This lipid bilayer is derived from the host cell's outer membrane or an internal membrane, like the endoplasmic reticulum or the nuclear membrane. The viral membrane isn't just made up of fat (the lipids); it also has special proteins called 'glycoproteins' coded for by the viral genome. The glycoproteins are really important in helping the virus infect another cell or host and in helping the virus avoid detection by your immune system, which is trying to kill the virus.

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