Lambda Bacteriophage and Life Cycle Regulation - Video Tutorials & Practice Problems

Bacteriophage Life Cycle

Hi in this video I'm gonna be talking to to you about lambda bacteriophage and its life cycle regulation. Okay so bacterial pages. Um these are just viruses means virus. So bacterial fage is going to be a virus that infects bacteria. So bacteria fage is bacterial infecting virus. So we are interested in this because it really gives a great example of how viruses can regulate their life cycle. So we're going to focus on a particular bacteriophage. That's lambda back bacterial page. So bacteria pages in general, they have two life cycles. One that we call the lipstick cycle. And we call it that because what happens during the cycle is that the virus is actively replicating. It's producing more of itself and it actually fills up the cell to the point where the cell can't contain it anymore and it burst. Okay so that another word for burst is license. So we call that the analytic cycle now. The second life cycle is the lice a genic cycle. And the lice a genic cycle is where the virus is not actively replicating. What it does. Is it infects the cell and it says hey I don't really want to burst the cell. I don't really want to produce more of myself. So I'm just gonna hide in the so what it does is it integrates itself in the genome and sits there and does nothing. It just sits there and waits until it's ready to start producing more of itself which will eventually burst the cell. So we call it silent. It just sits in there. It's hiding in the genome now to control the entrance into the lyric or the lice a genic. The bacteriophage actually has two sets of genes. One that will control the lyrics cycle and one that will control the lice a genic cycle. And um how these genes are expressed and what regulates them determines which one the bacteriophage enters into. And we'll talk about this regulation and all those genes in much more detail in the next few videos. So just looking here quickly at this example we can see that we have a virus right here right? It's entering into the cell and it's infecting it's sticking its D. N. A. In there. And so it has two choices. It can continue with the lyrics cycle, right? So it integrates and it can continue with the lyrics cycle by producing more of itself and eventually it produces so much of itself that it has to leave the cell and it bursts or lice is the cell for little cycle. Or that's one choice. Or it can go through the lycee genic cycle where the D. N. A. Gets integrated into the genome and then it just sits there and it just keeps replicating. And as more cells replicate it as the D. N. A replicates and produce more of itself. But it does so in the genome it's not actively producing virus but it's just getting it DNA replicated and into a lot of different cells. So we are going to go into a really just detailed explanation on how all these genes work together. It's probably one of the more complicated things in this entire book this entire textbook. But I want to just give you the cheat sheet right now so that if you get confused later, you can always come back here and sort of reorient yourself to what's going on. So when the virus infects a cell and that cell is really it's filled with nutrients, the environment is really healthy. So it has good growth conditions, right? It's getting everything it needs. It's not deficient in anything that good growth conditions that will cause the virus to produce more of a protein called crow. And if there are good growth conditions and there's cro protein then that leads to the lyric cycle. Now the opposite. If the virus infects the cell and that cell is deficient in something, the environment isn't healthy, it has poor growth conditions. Then what will happen is that virus will produce more lambda protein. You can also call this the C. One protein. It's the same thing. And if they're poor growth conditions and there's more land A or C. One protein, then that will lead to the lice a genic cycle. So this is going to become much more complicated as we follow the pathway of each of these proteins. But just for your cheat sheet, this is what you need to know, good growth conditions is crow and that's lyric poor growth conditions is lambda and that slice a genic. So with that, let's move on and talk more about the nitty gritty of this process.

Bacteriophage Regulation

Okay, so now let's talk about some of the introduction to the mechanism of regulation. So the lyrics cycle and the license genic cycle genes are actually physically separated on the chromosome. Now keep in mind this is a circular chromosome that we're talking about. And so I'm showing it is linear. But just keep in mind that this is actually curved. Not that it really matters. Other than just like making sure you're aware that bacterial chromosomes are circular. So the little cycle that is promoting the lyrics cycle, It has the O. P. And Q. Jeans. And the license genic cycle has two genes called aunt and X. I. S. Or XS. And so um those are two important genes with the genes that are mainly going to be talking about our regulatory genes and they sit in between the lipstick and Lissa genic cycle. And here they are, each one of them have different functions and are super important and we'll go through each one individually. But this is what it looks like. You have the lice, a genic cycle, jeans over here, the little cycle jeans over here and you have all these regulatory genes here. Now keep in mind we've been talking about opera Ron's and remember what does every opera and have. It has promoter and operator. Um repressor and a and the genes. So although these this isn't technically and well some of these may be operator but the regulatory genes aren't operators, but each one of these genes, even though I'm not showing it on here has a promoter an operator and it's a gene itself, right? And and also all of them have terminators as well which I don't mention a lot but terminate transcription. So there's a lot of factors on here. So if I were to write all of these that would get very confusing right? Because there would be a promoter and operator and a terminator promoter and operator and a terminator for every single one of them. And sometimes it actually goes in the opposite direction depending on the gene. So I'm saving you this like super confusing graph of promoter operator terminator for each of these genes. But during the regulation some of them get activated or repressed. And so I'm going to be writing the ones we're talking about as we're going so let's just get started in Delvin. So the first two M RNA is that are transcribed are controlled through different promoters. So the first gene that's transcribed is the end and the second is the crow and crow. So the INN has its own promoter called pl and it has a terminator called tl and the crow has a promoter called pr and is terminated by a terminator called TR one and these genes are actually transcribed in reverse of the other. So um the other thing you need to know I'm gonna show you an image. But the other thing that you need to know is that the end protein is a special type of protein called the anti terminator protein. Now this isn't anti terminator structure when we talked about the trip opera and this is a protein. So it's completely different. But it has the same functions as an anti terminator. It allows transfer option to take place. And what it specifically does is it allows transcription to transcribe past the terminator. So remember the engine and the crow both have terminators and the end protein when there's a lot of it will actually bind to these terminators and block them and it allows transcription to take place. So what this looks like and this is how this really gets started. So you have these two M. RNA is they get transcribed first. Um and I don't know my pin isn't working anyways you can kind of see I don't know why it's not working anyway. So the end of the crow get transcribed first and each one of them have a promoter. Right? Can follow my thing. Each one have a promoter and terminator and I've labeled them different colors. So you can see now they're actually transcribed in different different directions. Right? Because the promoter starts transcription. So the crow is transcribed this way and the end is transcribed the other way which I've shown by these arrows Now there is in and cro protein both produced. Right? So I'm only showing in here. But if I were to write crow all down here, there's crow protein being produced. But really what happens is a lot of in protein gets produced and those come in and they bind to both terminators. So I've shown um here is the red and the big boulder read their binding to the terminators. And what happens is that allows for this arrow, this transcription to transcribe all the way to the end, so all the way to the little cycle genes and all the way to the likes of genic cycle genes. Um but in the meantime, what it does is it allows for the transcription of these two proteins here, C. Two and C. Three. And so I'm going to talk about and then over the next pages what C. Two and C. Three due to regulate about which one is chosen because right now both are being are being transcribed and that's not what we want. What we want is we want One chosen over the other. And so C2 and C3 is going to help us choose which cycle is going which genes are going to be creative. But essentially this is the first thing that happens and and crow are transcribed and a lot of it is made and that binds the terminator and that causes transcription to occur past the terminator. So with that let's now turn the page

Decision Between Lytic and Lysogenic Cycles

Okay, so now we're gonna talk about the decision to enter into either the lice a genic or the lyrics cycle. And like I said before this is kind of the summary sentence. But the C one protein which is also called the lambda protein which you'll notice is right here in the middle. The C one protein is actually what determines entrance into the lice a genic cycle. So how this happens is where we were before is that we've created a lot of end protein and it's bound to the terminators and that blocks termination. And so um the c. two and C3 proteins are being created or transcribed whatever you want to do. So the important one to know about is see to this one here and so the C2 approaching comes in and activates another promoter called the P. R. E. Promoter and it sits here. Let me disappear. So I can kind of show you this image at the same time. So the C. Two protein gets created and it binds to this promoter down here that I'm just gonna draw right now. Just to save you some confusion and the P. R. E. Is really close to this terminator. And what happens is that when the P. R. E. Is activated by the sea to protein, C one gets created because the transcriptions happen this way. Right? So that means that you're going to get two types of proteins, you're going to get anti crow because you're transcribing crow in the opposite direction and that's called anti crow. And then you also get the C one protein or the land of protein which is transcribed this way right here. And this protein is what's responsible for entering into the likes a genic cycle. And let me tell you why. So this is this is what happens, right? So we have our N. It's transcribing it's blocking the terminator. C. Two gets produced. You get lots of it. It binds to the promoter and it creates anti crow and see one. So what happens is when you get C. One, it binds to two operators called O. R. And O. L. And it inhibits them. And that means that it prohibits transcription of N. And crow. And the longer transcripts. So what happens is you where we were is we had like I said this is so confusing, right? But you had we already talked about the end blocking or blocking the terminator allowing transcription to occur. This created C two. C two came down here bound the P. R. E promoter created anti crow and see one. Now we have a lot of C. One which here in green we have anti crow and you can see see one is all around. So what happens is C1 does a couple things. C1 comes in, it binds to these operator regions. And what that does is it blocks transcription of this whole entire region. This whole regulatory region that we've talked about it blocks it all. So the only thing that's being produced now is anti crow. And see one then A C. One also does another thing and it comes over here and it binds the P. One promoter and this P one promoter activates the lice a genic cycle. So this is how the land of protein or the c. one, whichever one you want to call. It activates entrance into the lice a genic cycle. But what happens if you actually want to enter into the lyrics cycle? So what happens is um there's actually, so what happens is there ends up during this process right there ends up being more crow around. And when there's more crow around, crow can actually bind to two operators O. L. At O. R. And when they are inhibited, they inhibit the two promoters Pl and pr. And so what this does is it lowers the amount of ci or land the protein in the cell. And when there's a low amount of ci in the cell it will not inhibit anything and it won't activate the license genic cycle which I showed you up here right? If ci is here, it's not inhibiting this, it's not activating this. And so when ci isn't present, that means that this transcription continues and the light, the lyrics cycle genes are extremely uh are more activated than before. So um so what happens is if you have a lot of crow here this will block this uh this ci production. It will block these operators. And this results in uh so no ci equals transcription of the lyrics cycle genes and you might say, okay, like even if you followed me to this point and you understand how you know these different proteins regulate their cycles. You don't understand? Probably at this point, why would there be more crow? Why would there be more in? Right. Because those are the two proteins we said if there was more in something happens, if there's more crow, something else happens. Well the reason that there would be more crow is actually dependent on the conditions in the environment the bacteria is growing in and this is because the protein called proteus proteus is group of proteins they destroy um specifically in this case the C. Two protein and this the presence of proteus is destroying this protein is what controls entrance into either the little or the lycee genic cycle. So like I said before, in good growth conditions, you the bacteria favors the lyric cycle and it does this because in good growth conditions there are a lot of C. Two. Proteus is and they degrade C two. When you have less c. to the P1 promoter is not activated and therefore the lyrics cycle is promoted in poor growth conditions. There are few C. To proteus is meaning that there are high levels of C. Two. When there are high levels of C. Two, it activates C one and P one and this leads to the lycee genic cycle. So what this looks like. Let me back up. So when there are lots of protein cases right? So uh and lots of proteus is that's going to degrade this approaching here when this protein is degraded. That means that this one isn't created and if this one isn't created it can't activate this. Whereas let me reverse and talk about this in the opposite direction. Whereas in poor growth conditions you get a lot of this created right? This gets created. That means you get C. One created because that activates C. One and when you have C. One that's going to inhibit the lyrics cycle and activate the lice a genic cycle. And so that is how the growth conditions and these the levels of these proteins cases affect the ability of the bacterial page to enter into the lid and let the the lice a genic analytic cycle. So I really highly um I know this is confusing and you're probably very confused which I can, there was any way I can make it simpler but I do suggest maybe going back and reviewing this and making sure you understand you know okay good growth conditions that produces those protea aces. That means that there won't be a lot of C. Two. If there's not C. Two, C one won't be created and then see one can't activate the whereas if poor growth conditions there's not many of them C. Two is created that creates C. One and C. One activate, slice a genic. So if you can just like understand that pathway and really make sure you comprehend that pathway. This this will be a breeze, but you might need to go back and review some of this a few times in order to really get it down. Um So with that let's not move on.

In which of the following life cycles does a bacteriophage integrate itself into the host genome?

• A. Lysogenic cycle
• B. Integrative cycle
• C. Lytic cycle
• D. Subdue cycle

In good growth conditions the bacteriophage is more likely to enter into which life cycle?

• A. Lysogenic cycle
• B. Integrative cycle
• C. Lytic cycle
• D. Subdued cycle

Activation of which of the following genes leads to entrance into the lysogenic cycle?

• A. N, cro, and O genes
• B. O, P, and Q genes
• C. Int and xis genes
• D. cIII genes

The N protein is an anti-terminator. What does this mean?

• A. The N protein terminates transcription
• B. The N protein allows for transcription to occur
• C. The N protein terminates translation
• D. The N protein allows for translation to occur

Which of the following proteins is mainly responsible for entering the bacteriophage into the lysogenic cycle?

• A. N protein
• B. Cro protein
• C. cI (Lambda) protein
• D. cII proteases