If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:05
AP.BIO:
IST‑2 (EU)
,
IST‑2.A (LO)
,
IST‑2.A.1 (EK)
,
IST‑2.A.3 (EK)
,
IST‑2.B (LO)
,
IST‑2.B.1 (EK)
,
IST‑2.C (LO)
,
IST‑2.C.1 (EK)
,
IST‑2.C.2 (EK)

Video transcript

we're now going to talk about one of the most famous operons and this is the lac operon and it is part of eco lies genome and it is involved and the lac right over here is referring to lactose and so you can imagine that it codes for genes involved in the metabolism of lactose and the word lactose might already be familiar to you it is a sugar found in milk some of us including myself are lactose intolerant I have trouble digesting lactose so I have mixed feelings regarding this but in general for a cell to make use of it it needs to be able to absorb the lactose it needs to be able to split it up into simpler sugars that it can actually use for fuel and that is what the genes in the lac in the lac operon actually do code for so just as an example the lac z gene right over here this codes for an enzyme that helps cleave the lactose into simple sugars the lac y gene codes for an enzyme that allows for the absorption of lactose through cellular membranes Lac a is a little bit more interesting a little less understood but the general idea here is all three of these are involved in the metabolism and the absorption of lactose and it is an operon so we have our promoter here where our RNA polymerase would attach and I've also drawn some other sites I've drawn this the operator right over here where you can imagine a repressor and it is indeed the lac repressor will could bind and over here this cap site or CA p site cap stands for Khattab a lighter protein catabolized catabolize activator whoops acti activator protein and so this is you can imagine where a protein called the Khattab elect evader protein can bind and perhaps be an activator so with that out of the way let's think about different scenarios let's think about a scenario let's think about a scenario where the e coli is an environment where there is no lactose so what do you think should happen over here and a lot of these things are very logical if you just assume that a lot of biological organisms are quite stingy they don't want to just waste resources well if there's no lactose well why transcribe why transcribe the genes that can be translated into enzymes for the metabolism of lactose so there's no lactose you can almost view this as a default state right over here you actually have the lac repressor protein being bound to the operator so this is the lac repressor lac repressor right over there and so you won't be able to transcribe these things the RNA polymerase the RNA polymerase won't won't be able to get anything done it's not no transcription is going to occur so no lactose no transcription transcription which makes a lot of sense the bacteria or the bacterium singular that doesn't want to waste resources so what do you think should happen if there is lactose so I'll keep this up here so you can see it so lactose present lactose lactose present well you can imagine well you don't want that repressor around anymore and that is indeed what happens is that you have an isomer of lactose called a low lactose so if lactose is present you're going to have also a low lactose present right over here and so that is a low a low lactose which can act as an inducer of transcription and the way that it acts as an inducer is if it binds to the Lac repressor the lac repressor can no longer bind to the operator site so when this is when the a low lactose is present it will bind to the repressor and then the repressor is going to leave the operator site it's not going to be able to bind as well and so let me draw that so in this case the operator sorry the repressor I should say the operators where the repressor binds so this is the repressor right over here you have some allolactose let me do that in white you have some aloe lactose that has bound to it and because of that it's not going to bind to the operator and since it's not bound to the operator well now the RNA polymerase can actually transcribe these genes and that's valuable because by transcribing these genes we are going to be able to metabolize this lactose so lactose present you have transcription transcription transcription occurs now that's a very high-level simple view of the lac operon but there is more involved because there's other sugars in particular glucose which is preferred by the cell so whoops I'm moving the wrong part there you go so let's think about what will happen in the presence of glucose and not in the presence of glucose so let me write here so glucose glucose and no and no glucose actually let me do it let me do it well let me I'll do no glucose first let's say we have no glucose and remember glucose is preferred to lactose simpler sugar if you have glucose around why worry about the lactose and then here we have we have glucose we have glucose around and we could talk about both of these both of these situations in the presence of lactose or not in the presence of lactose but if we don't have if we don't have any lactose around then we're not going to have the a low lactose around and then you're just going to have the repressor sit on the operator and you're not going to have any transcription so you can and that's going to be whether or not we have glucose so I'm going to think about no glucose but we do have lactose plus lactose and in here you have glucose plus glucose plus lactose well the lactose part if we have lactose around then we're going to have the a low lactose around and we just covered this scenario the a low lactose binds binds to the lac repressor keeps the lac repressor from binding to the operator and so you have you have your RNA polymerase is able to actually perform the transcription but that's not it URF you in a situation with no glucose you actually are going to also involve the cap side you're going to have an activator that's going to make this happen even more because if you don't have glucose around man you really need that lactose and so what you have is something called so let me draw this the Khattab alight activator protein right over here the Khattab alight activator protein and in the presence of cyclic ANP Edina side ICD design monophosphate it's a derivative of ATP and so this is that right over there cyclic a MP you'll see that come up a lot in biology so this is the Khattab alight activator protein in the presence of CA MP and we'll talk about how cyclic a MP relates to glucose in a second in that presence it is going to bind to this the cap site and it is going to further activate the transcription so in this situation no glucose plus lactose you're going to have even more transcription so let me write this down lots of transcription lots of trans transcription lots of transcription now I know you're probably asking this is what I first asked myself when people told me about cyclic a and B well how does cyclical A&P relate to glucose well I'm not going to go into a huge amount of detail here but what you need to know here and it makes sense is that if you have glucose glue so let me write it this way if you have high glucose high glucose I'm having trouble writing high glucose then that's going to inhibit the production of cyclic a and P so low cyclic adenosine monophosphate and if you have low glucose or no glucose it's like a tongue twister if you have low glucose well you're not going to inhibit the creation of cyclic a and P and so you're going to have high cyclic a and P so if you have no glucose or low glucose we are in this scenario right over here you're going to have high constant or higher concentrations of cyclic a and P which can bind to the catabolized of a toe protein which then acts as an activator to allow even more transcription of the lac operon which once again why is it important well if there's no glucose or low glucose you're really going to need that lactose so you really want to transcribe these genes as much as possible now what about the situation where there is glucose and lactose well once again if there is lactose then you're going to have a low lactose which is going to be able to bind to the lac repressor and by it binding to the lac repressor the lac repressor is not going to be able to bind to the operator and so you do have once again the RNA polymerase is going to be able to transcribe but because you have glucose present because you have glucose present you're going to have low or I'll just write no all right low cyclic a MP and since you have low or no cyclic A&P around well that cyclic a and P isn't going to be able to bind to the Khattab 'light activator protein and so the Khattab later the Khattab light activator protein isn't going to be able to act as an activator I know this is I'm using a lot of words multiple times and so it's not going to bond to the to the activator side right here to the to the cap site and so you're going to have less transcription less transcription transcription which once again makes sense you've got glucose and lactose around the cell would prefer to use glucose simpler sugar why waste resources you have plenty of energy around just go straight to the glucose but if you don't have glucose around well then use more resources so that you can digest the lactose
Biology is brought to you with support from the Amgen Foundation
AP® is a registered trademark of the College Board, which has not reviewed this resource.