Viruses Come in All Shapes, Sizes, and Languages!
by Michael Dumas, Master's Student, University of Delaware
Figure 1. A model of bacteriophage T4.
When you think about viruses, what kind of an image comes to mind? For me, it was always the well-known virus that infects the equally famous bacteria, E.coli. This virus is called a bacteriophage (back-tear-ee-oh-fahge): T4.
Bacteriophage, or "phage" for short, is a fancy name for a virus that infects a bacteria. Phage T4 looks like it belongs in a science fiction movie with its geometric head and distinctive legs called tail fibers (see Fig. 1). This virus is just one of many different shapes that viruses can be.
Viruses are differ from one another in the following ways:
3) The type of host they infect
4) The chemicals that are the virus’s genes (also known as genetic material)
5) And the way they force the host cell machinery to make more copies of the virus.
While plants and animals are measured in meters and centimeters, viruses are measured in billionths of meters. Imagine dividing a one-meter measuring stick into one billion parts. Each one of these parts would be called a nanometer (nan-oh-meet-er).
Viruses can range from as small as 20 nanometers (nm) in length to as large as 600 nm! For additional resources for getting an idea of the size of a virus in comparison to cells and other organisms, click here. The large difference in the size of viruses has to do with how many genes each type of virus carries, which is highlighted in "Genetic Makeup" below.
Figure 2. Above, EM image of the rod-shaped Tobamovirus. Below, EM image from the family Reoviridae.
Viruses come in a variety of shapes, but there are basically two that make up most viruses. The first is an icosahedron (I-co-sah-heed-ron). Viewed from the side, an icosdedron looks a bit like a stop sign or a six-sided shape known as a hexagon. However, the icosahedron is actually more like a soccer ball, with small patches of interlocking hexagons and pentagons making up a ball. Inside this ball are the chemicals (genetic material) that are the virus’s genes. The icosahedron, which is also called the capsid, is made of protein. Some viruses place a layer of lipids (a type of fat) around their capsid. This lipid layer is called an envelope.
The second shape of a virus is known as a helix. Imagine the spiral cord that connects the telephone handset to the main unit of the telephone. The inside of the spiral cord is hollow. For helical viruses, the outside of the cord is wound such that there is no connection between the hollow inside and the outside. So the helix is a tube. It is in the hollow middle part of the tube that a helical virus places its genes or genetic material. There are many viruses that use only the icosahedral shape or only the helical shape. The viruses we study are like Phage T4, which is actually a combination of both shapes. The icosahedral capsid contains the genetic material of phage T4 while the tail of T4 is a helix. When T4 infects an E. coli cell, the genetic material of the phage moves through the hollow part of the helical tail and into the cell. Click here to see this animation.
Figure 3. Images of the Parvovirus. This type of virus can be as small as 18-26 nm and it infects dogs.
All forms of life and all cell types have been found to have viruses which infect them.
Even though viruses infect many different kinds of life, an individual viral species can usually only infect one specific type of cell. For example, Parvovirus (see Fig. 3) infects only the intestinal or heart cells of dogs. It cannot infect any of your cells or even those of your pet hamster
So remember there is a virus for every living thing. But each virus typically can infect only one specific host.
The chemicals that a virus uses to make its genes come in two basic forms: one is DNA and the other is RNA. It is the genetic material (DNA or RNA) that carries all of the instructions or genes needed to make more copies of a virus.
Some viruses use DNA to make their genes. Our genes are also made from DNA. Before the genes on a piece of DNA can be used as instructions to make a protein they have to be transcribed into RNA. This would be similar to changing a Spanish word to a English word so that someone who speaks only English could understand. Once the instructions are transcribed into RNA words, then they can be understood by the cell's protein-making machinery.
Other viruses use RNA to make their genes. These viruses have it easy as they do not need to transcribe their information for the cell to understand.
These examples are only two of the seven possible ways that viruses make their genes and use these genes to make more copies of themselves.