Virus structure and replication | Viruses | High school biology | Khan Academy

In this video, we're going to talk about viruses, which I think are maybe one of the most fascinating things in biology because they have some aspects of living organisms, but we don't consider them living.

But before we go into the details of it, I want you to appreciate what is going on in this picture here. This is a zoomed-in image of HIV, human immunodeficiency virus, emerging from a host cell. In this case, it's an immune cell; it's a white blood cell that it's emerging from. From the scale of a virus, you can see that the cell looks like a whole universe that it is emerging from. This is an important picture because it really gives you a clue about how viruses replicate.

They can't replicate on their own, which is one of the reasons why we don't consider them living. But they do have genetic material, either DNA or RNA, and what they do is they infect a host cell like the one we see here. They use that host cell's machinery to produce more of that virus, to replicate the DNA or the RNA, and to produce the protein that envelops that DNA or RNA to construct the virus. Then it emerges from that cell, and in many cases, it does damage to that cell in the process.

Now, I mentioned that these are non-living infectious particles, and there's a lot of things that you can think about when you think about infections. Sometimes, you've heard the term germs; people think bacteria, they think viruses. But to be very clear, viruses are non-living while bacteria are living. They are unicellular organisms. But let's go into a little bit more depth.

So, as I mentioned, you're going to have some genetic material inside the virus. Let's assume that that is DNA, but it could be RNA. There's a lot of variation in terms of how much genetic material there is. The simplest viruses have only a few genes, while a complex virus can have on the order of 200 genes.

Now, around that genetic material—and I’m really going to be drawing a cross-section here—but you can imagine it in three dimensions, you have protein that essentially makes the surrounding or you could even think about it as the basic structure of the virus. This protein is called the capsid, and all viruses are going to have a capsid. In certain cases, a virus can have an envelope made up of very similar things to a cellular membrane, and they're actually derived from host cells. They allow the virus to attach and penetrate host cells.

So, let’s imagine a host cell here, and let's imagine a viral particle that is able to attach to the host cell. It does so in small parts due to the fact that it has this envelope made up of very similar materials to the cellular membrane—these lipids and glycoproteins. So, it attaches to it, and that first stage where it's really infecting it is called entry, where the genetic material from the virus is able to enter into the cell.

Now, at that point, you have something called replication, where the virus is able to hijack the cellular machinery in order to replicate not just the genetic material. Let's assume that this is DNA, but it could have been RNA as well, but also the material that makes up the capsid, so that protein there. DNA gets transcribed to RNA, which then gets translated to protein at ribosomes. The viral DNA is able to do that to produce these proteins, and then you have assembly, where these things come together.

So, the protein starts to come together with the genetic material all the way until you get to release, where the virus starts to emerge from that cell. We saw a very scary picture of that when we started this video of HIV emerging from an immune cell. If it takes some of the material or if it has an envelope, it will maybe capture some of that from the cellular membrane, some of that material so then it can also have the ability to attach to a future similar type of cell.