Viruses: Definition, Classification & Life Cycle

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

From the common cold to HIV, viruses can be an annoyance, and even have the potential to kill you. This lesson will define what a virus is, describe how scientists classify them and will give an overview of the virus life cycle.

What are Viruses?

Tiny aliens that enter your body, highjack your cells, and then force you to make more and more tiny aliens, who will, in turn, take over even more cells?! Although this sounds stranger like science fiction, it's real and it could be happening to you as you scroll through this lesson! Okay, maybe they're not tiny aliens entering your body and making you sick, but viruses aren't like any living organism on earth, so they might as well be thought of that way. To start with, scientists aren't even sure if viruses are alive, since they don't embody the characteristics used to define a living organism. Crazy, right?

So, what characteristics do these tiny aliens, oops, I mean viruses, all share? Excellent question! First off, viruses are tiny. And I mean really tiny! Like 100 times smaller than bacteria! They have nucleic acid (either DNA, which stands for deoxyribonucleic acid, or RNA, which stands for ribonucleic acid). Nucleic acid is found in all living things, and is the blueprint for making an organism. Next, viruses have a capsid, or a protein coat, that covers the nucleic acid. Finally, some viruses have an additional outer coating known as an envelope.

Classification of Viruses

Now that you know a little more about the characteristics viruses share, let's take a moment to go over how scientists classify them. Viruses are classified based on several factors including: whether they contain DNA or RNA, their size, who they infect, the shape of the capsid, how they replicate, and if they contain an envelope. For example, picornaviruses are a group of viruses that all contain RNA, are small, and have a capsid that is icosahedral-shaped. Wait, what is an icosahedral shape? It sounds a little complicated, but it is a shape that contains 30 edges, 20 faces and looks like it is made up of a bunch of triangles. And pico means small, so you can see how picornaviruses get their name! But how small are they? Usually they are 18-35 nanometers and, just so you can compare, some viruses can be as large as 400 nanometers, so yep, picornaviruses are on the small side!

You may be familiar with some of the viruses in this group (well, hopefully not too familiar): hepatitis A, a virus that attacks your liver; the rhinovirus, which causes the common cold; and the virus that causes hand-foot-and-mouth disease.

Viral capsids come in many shapes. Hepatitis A has a capsid that is icosahedral-shaped

Picornaviruses target vertebrates (like you), but believe it or not, viruses can target anything: from vertebrates to invertebrates to bacteria! In fact, viruses that attack bacteria are collectively known as bacteriophages.

Virus Life Cycle

So now you know what viruses are and how scientists classify them, but in order to be a virus expert, you should know a little about their life cycle. Viruses can undergo a lytic or a lysogenic life cycle. Let's start with the lytic cycle, which starts with a virus docking onto a host cell and inserting its nucleic acid into the host. Next, the viral nucleic acid (the DNA or RNA) highjacks the host's cell, forcing it to make more viruses. Finally, in a violent ending for the cell, the virus causes the it to lyse, or explode, thus releasing these newly made viruses (which will go and infect more host cells).

I told you viruses sound like tiny aliens! In this case, ones that burst out of their host! Terrifying! The lysogenic cycle is a lot like the lytic cycle, except the virus will 'hang out' with the host for a while before destroying it. Like the lytic cycle, the lysogenic cycle starts with the virus docking on a host cell and inserting its nucleic acid into it. Next, the viral nucleic acid integrates into the host cell's nucleic acid forming a provirus.

The provirus is copied when the cell divides

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