What Are Viruses? - Definition, Structure & Function

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  • 0:05 Viruses
  • 0:44 What Is a Virus?
  • 1:46 A Viral History
  • 3:13 Viral Structure
  • 4:45 Viruses, Bacteria, and…
  • 7:28 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

In this lesson, you'll learn some historical facts about viruses and the material they are composed of. Find out more as we review the capsid, nucleic acids, the viral envelope, viruses, virions, bacteriophages and virology.


When we think about all the things that people are afraid of or should be afraid of, we can come up with a lot of different examples. Some say that lions are to fear, or guns or poison. However, the biggest danger to our species lies not with large teeth, bullets or toxins; it has to do with things you cannot even see! Tiny little bugs so vicious, they have killed millions of people. These bugs are called viruses and have all sorts of structures, like the teeth of a lion, that help them attack and survive.

What is a Virus?

A virus is an extremely small, infectious agent that is metabolically inert and only replicates in living hosts. Viruses are so small that you usually need an electron microscope, as opposed to a light microscope you may find in your school lab, to even see one! Basically, the diameters of different viruses fall somewhere in between that of a bacteria on the high end, and that of a ribosome on the low end, which is essentially a range of about 20-300 nanometers in diameter.

To put it into a better perspective for you, an average virus is about eight times bigger than a ribosome, whereas the average bacteria is about 30 times bigger than your average virus. That's like taking a small dog (the ribosome), putting it next to a human (the virus) and putting the human next to a rhinoceros (the bacteria)!

A Viral History

Therefore, it's amazing, given the limited technology of the time, that on February 12, 1892, in St. Petersburg, Russia, a scientist by the name of Dmitri Ivanovsky discovered the possibility of a viral organism. Ivanovsky is therefore credited with starting the modern day study of viruses and the diseases caused by them, something we term virology.

It's pretty cool that he was able to even discover the potential for the existence of viruses way before the electron microscope was invented. He did this by noticing that plants were infected by something we now call a virus, after he had ruled out any possibility of a bacterial agent getting through to the plants by using a series of filters designed to stop the bacteria.

It was a Dutch microbiologist, Martinus Beijerinck, who confirmed Ivanovsky's experiments six years later and coined the modern use of the word 'virus' to describe this new agent of disease. However, it wasn't until almost the middle of the 20th century that modern technology finally proved once and for all that the virus was indeed an actual entity, all thanks to the electron microscope.

Viral Structure

After many years of study by many brilliant individuals around the world, we have come to understand some of the basic nature of the viruses that threaten us, plants and even bacteria. For example, the infective form of a virus that exists outside of its host is known as a virion. Since a virion is out there in the environment and not under the protection of its unwitting host, it needs some of its very own protection.

Hence, each virion is composed of an outer protective protein coat called a capsid. This is kind of like the exoskeleton that insects have. This capsid is composed of protein subunits called capsomeres, which are themselves composed of smaller units called protomers. The proteins for the capsid are encoded by the nucleic acids that are housed within the capsid itself. The viral nucleic acid genome can be either DNA or RNA, the organic molecules essential for life.

In some cases, the viral capsid may also be surrounded by a viral envelope, which is a lipid bilayer derived from the host cell and one that increases the infectivity of a virus. This envelope, sometimes called a coat, can change rapidly in response to many different factors and allows the virus to avoid your immune system if necessary.

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