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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.

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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
The provirus is copied when the cell divides
Overview of the lytic and lysogenic cycle
lytic

Lesson Summary

So even if viruses aren’t tiny aliens, they still sound like stars in a science fiction movie. And even though they make you sick, they sure are fascinating! Remember, viruses are pieces of nucleic acid that are covered by a capsid and, sometimes, an envelope. They cannot replicate without a host, so they must infect a host cell where they can undergo the lytic or lysogenic cycle.

Both cycles are very similar, but in the latter, a provirus is formed that replicates along with the host cell. Eventually the lysogenic cycle will enter the lytic cycle and the host cell bursts, releasing more viruses.Viruses are classified based on several factors, including: the type of nucleic acid they have (either DNA or RNA), the shape of their capsid, the presence of an envelope, how they replicate and who they infect. And if you’re ever feeling picked on by a virus, realize everyone gets them: from the rose bush sitting outside of your house, to the bacteria that is growing on your skin!

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