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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.

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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.

There are many different types of glycoproteins, and this type of specificity allows viruses to infect only the cells they need to. In addition, these glycoproteins can literally be shed by the viruses in order to throw the antibodies who are trying to kill them off course, kind of like when military planes use flares to throw off heat-seeking missiles!In addition to adding a layer of wax to the car, some people like to really spice things up and add all sorts of add-ons to give their cars some kind of edge. This is no different with our viruses, as some of them have a complex shape. This is a virus that has a combination of shapes arranged in a symmetrical or asymmetrical fashion. For example, some viruses may have a prolate head with a long tail or even multiple tails!

Lesson Summary

Well, now that it’s the tail end of this lesson, let’s simplify the main points.

A helical virus is a virus that has a capsid shaped in a filamentous or rod-shaped structure that has a central cavity that encloses its nucleic acid. 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. Some viruses, regardless of their protein capsid shape, are enveloped and are viruses that have a lipid bilayer around their capsid. Finally, some viruses have a complex shape.

This is when a virus has a combination of shapes arranged in a symmetrical or asymmetrical fashion.

Learning Outcomes

After viewing this lesson, you will be able to:

  • Describe the different shapes of viruses
  • Define capsid, capsomere and protomer
  • Explain why many animal viruses are icosahedral in shape
  • Summarize what it means when a virus is enveloped

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