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Lecture 7

Cell Biology - Lecture 7 - Video 3.1 - Notes

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Boston University
CAS BI 203
Martin Steffen

Lesson 7 – Video 3a [00:00:00.00] 3079 [00:00:00.49] SPEAKER: Hi. In this video, we'll continue our discussion on gene specific 3080 regulation.And up until now, we've been talking mostly about how individual genes are 3081 regulated. Now we'll go up a level in the hierarchy and talk about circuits of genes. In particular, 3082 three different types of circuits; positive feedback, a toggle switch, and a feed forward loop.And 3083 see how clusters of genes are regulated. 3084 [00:00:23.32] We'll talk a little bit about epigenetic regulation, which is a mechanism we haven't 3085 discussed previously.And lastly, we will talk about experimental methods for studying gene 3086 circuits; both DNAsequences and DNAbinding proteins. We've discussed negative feedback 85 two times already in this class. And those were examples 3087 of shutting things down when there's 3088 too much of a certain material. In here, we're going to look at a positive feedback loop. And this 3089 is thought of as a memory device, in the sense. What we're going to see is if you make a little of 3090 protein A, you will continue to make it. So that's where the memory comes in. 3091 [00:01:09.79] Now in the initial cell, we're not making proteinA. Because it doesn't have the 3092 activator protein that it needs to make proteinA, which is proteinA itself. It's an auto activator, 3093 that means positive feedback loop. 3094 [00:01:26.94] Something is going to occur, which we will not specify right now.A transient 3095 signal, a mistake, somehow a little ofAgets made.And asAgets made, it binds to its own 3096 promoter.And now you've got a steady stream of the production of proteinA. 3097 [00:01:44.59] Even after cell division with both cells getting parts of proteinA, it will continue 3098 to make protein Abecause it will continue to bind. So both cells inherit some proteinA. Both 3099 cells will inherit the state where the gene is turned on.And continue to make a very simple one 3100 element circuit, but nevertheless, it starts to control genes in different ways. 3101 [00:02:13.78] On this slide, we see perhaps the single most famous synthetic genetics circuit that 3102 has been created.And it was in fact created here in 1999 by Jim Collins and Tim Gardner.And 3103 this is called a genetic toggle switch.And this is one of the very few circuits that have 3104 established the entire field of synthetic biology. So this is really landmark work done here at 3105 Boston University. 3106 [00:02:49.74] Now, I first need to describe how most of the genes have been regulated by 3107 activators and then we'll talk about a toggle switch. I guess it's easier to start with a toggle 3108 switch. Think of a light switch on a wall. 3109 [00:03:03.12] To turn the light on, you flip the switch once, and then you walk away.As long as 3110 that switch is up, the light is on. Whenever you want to turn the light switch off you go over to 3111 the wall, and flip the switch, and then the light is off. 3112 [00:03:18.79] In contrast, most of the examples that we've been giving in biology would be that 3113 you go over to the light switch, you flip the switch to turn the light on, and then you stand there, 3114 and hold your thumb on the switch for as long as you want the light on.Applying continual 3115 pressure on it. And for cells that can work-- but think about how much simpler drugs could be if 3116 you could incorporate toggle switches. 3117 [00:03:47.42] Aperson who has high blood pressure has to continually take blood pressure 3118 medication every day to maintain their blood pressure. What if it were possible to design a toggle 3119 switch such that you took a drug once, the circuit switched, and put it into a low blood pressure 3120 state, and that was it? Well, the drug companies might make less money. But if they could do 3121 that, they will. And people are trying very hard. 3122 [00:04:16.92] But now let's talk about the details of the toggle switch. First thing we have to 3123 realize is that there are two states basically. And that is either promoter 1 will be active and make 86 this protein. Or promoter 2 will be active and make this protein. The 3124 reasons that a toggle switch 3125 is-- let's say for example, promoter 2 is active. It's going to make two proteins; repressor 1 and a 3126 reporter protein, which is a green fluorescent prote
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