Lytic and lysogenic cycles | Viruses | High school biology | Khan Academy

What we're going to do in this video is talk about two of the ways that a virus can leverage a cell to replicate the virus's DNA.

So the first is the lytic cycle, and this is what people often associate viruses doing. Let's imagine a cell. It's going to be a huge simplification. Right over here is the cell, and let's say that this is some DNA for that cell.

Now let's bring a virus into the picture. So this virus is right over here. It attaches to the cell and it has some DNA that, say, in that color. What is going to happen is that DNA is going to make its way into the cell, and then it's going to use the cell's machinery, namely the ribosomes. Let's just imagine that's a ribosome, that's a ribosome, that's a ribosome.

So it hijacks some of that cell's machinery, and it makes and replicates that DNA and the proteins needed for that virus. Then it's just able to keep replicating, not just the DNA, or it could be RNA depending on the type of virus, but it actually can, in many ways, construct the entire virus itself.

It does it so much that eventually that cell can't function. You have so much of this virus here that the cell is no longer functioning, and then it blows up. The cell dies, and all of that virus can now go out. I'm just showing it. I almost imagine that the cell almost bursts.

One way you can think about it is that the cell dies because it has all of that viral load inside of it. Then that virus goes out and moves on to potentially infect other cells. This is kind of horror movie-like if you think about it, but this is the lytic cycle right over here.

Now there's another cycle that, in some ways, is even creepier, and that is the lysogenic cycle. These aren't necessarily separate; you can go from the lysogenic cycle into the lytic cycle.

So, in the lysogenic cycle, let me draw the cell again. Let me draw the cell's DNA, and let me do that same orange color right over here. Let me draw that virus again. I'm making it a little bit smaller because I want to do some interesting things here.

In this case, the DNA can make its way in, but it doesn't immediately hijack the machinery of the cell to replicate the DNA and the proteins of the virus. Instead, that DNA actually incorporates itself into the organism's DNA. It becomes part of the organism's genetic code, so to speak.

When the organism itself replicates, or in this case, the cell replicates, there are now just more cells. It divides from one into two, from two into four, etc. Now all of them have some of that viral DNA in it.

It turns out that even human DNA, the human genome, has a lot of leftover viral DNA, is what we believe, from hundreds of millions, if not billions of years of this happening. Some of that is just sitting there dormant, or maybe it's even being put to use in some way.

But there are situations where, potentially during stress or some other environmental conditions, this, in some ways, wakes up. You can go from this mode back into the lytic mode, where all of a sudden this part gets activated, and then you start producing many, many more viruses. Then the cell bursts, the way that we talked about in the lytic cycle.