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Lecture 7

Cell Biology - Lecture 7 - Video 2.2 - Notes

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Boston University
CAS BI 203
Martin Steffen

Lesson 7 – Video 2b [00:00:00.00] 81 [00:00:00.76] DR. MARTIN STEFFEN: OK, resuming 2940 our discussion about complex gene 2941 regulation-- now, in this slide, we're going to see how the cell controls sugar metabolism. And in 2942 particular, we're going to look how it regulates the gene expression based on the presence of two 2943 sugars-- glucose, which is its preferred sugar source, or lactose. 2944 [00:00:28.69] It can use lactose to grow. But it only likes to do so when no glucose is present. 2945 Because it's a less efficient sugar to be used. The genes we're going to turn on or off is the lac 2946 operon. Here, we see the lac Z gene. Plus, there are about three other lac genes at this operon. 2947 [00:00:50.86] And we have two proteins that are going to be involved-- a repressor and an 2948 activator, which will bind depending on whether glucose or lactose are present. The repressor 2949 protein binds in response to lactose. The blue protein binds and responds to a molecule called 2950 cyclicAMP, which is related to the glucose concentration. 2951 [00:01:19.81] And again, the important point is you only want to turn on these lactose genes if 2952 lactose is present but glucose is absent. When lactose is your only possible source of energy, you 2953 want to make the lactose genes so you can take advantage of the presence of lactose. If glucose is 2954 present, making these lactose genes is a waste of time and energy. You will be much better off 2955 just using the glucose directly. So that's why it shuts them off when glucose is present. 2956 [00:01:52.85] So I think it's best to start with this case number two. That means glucose is 2957 present, and there's no lactose in the environment. Then, for sure no lactose is present, you want 2958 to make sure the lactose genes are not going to be made.And because there's no lactose present, 2959 there's this repressor protein, who's normal state is to bind the DNA. It's normally repressing the 2960 lactose genes. 2961 [00:02:24.28] Now, in the situation when lactose gets added to the environment, just like in the 2962 trp repressor, the molecule lactose will bind to the repressor. It will cause a change in 2963 confirmation preventing it from binding to the DNA. 2964 [00:02:42.96] Now the repressor is not able to bind. So the repressor will not stop the synthesis 2965 of these genes. But you still don't want to make them. Because you'd rather use glucose. So the 2966 point is the default setting here is to not make the genes. You don't have enough to get these 2967 genes started. Because you still don't want them. Because you have glucose present. 2968 [00:03:08.98] Now, the third case is there's no glucose and there's no lactose. Well, you can't use 2969 glucose. Because it's not there. But you still don't want to make the lactose genes. So these stay 2970 off. 2971 [00:03:21.50] Since there's no lactose to bind to the repressor, it's bound to the DNA, which is 2972 the repressor's normal state. But now, since there's no glucose, there's increased amounts of 2973 cyclicAMP. CyclicAMP, that's that red triangle right here. I'll circle it. 2974 [00:03:39.86] It binds to the CAP protein, which changes the confirmation.And this is a signal 2975 to say, hey, we might need to use these genes. Because we don't have glucose. But the repressors 82 think, but we don't have lactose anyway. So let's not bother. It's 2976 only in the last case where 2977 there's no glucose, so you have high levels of cyclicAMP. 2978 [00:04:03.24] So the CAP protein is binding to the DNA, providing extra thermodynamic 2979 binding energy to help recruit the RNApolymerase to the promoter.And since there is lactose 2980 present, lactose is bound to the repressor. The repressor has fallen off.And now, you can make 2981 the genes that are going to help you metabolize lactose. So in this very simple way, using two 2982 proteins, one an activator, one a repressor, you can respond to a variety of environmental 2983 conditions and make the proper choices on gene synthesis. 2984 [00:04:39.90] Now, most of what we've been talking with the Trp repressor and the lac operon, 2985 have been for bacteria. In eukaryotes, things are a little more complex, a little more regulated. 2986 The default setting for most of human genes is to do nothing. 2987 [00:05:00.06] So in general, there's extra emphasis placed on activation.And in fact, RNA 2988 polymerase II, which is the RNApolymerase that makes messenger RNAin humans, or 2989 eukaryotes, use, generally, multiple transcription factors to begin transcription. Even though we 2990 just saw examples where a single protein could help recruit the polymerase, in eukaryotes, it's 2991 frequently the case that many proteins help recruit RNApolymerase. 2992 [00:05:29.18] Eukaryotic cells do not have operons. Each gene is regulated individually. The 2993 distances involved in regulation can be much larger. There can be certainly many tens of 2994 thousands of bases between transcription start site and these regulators bound in, and even 2995 hundreds of thousands of base pairs. 2996 [00:05:51.08] And lastly, you might recall that when we were talking about chromosomes, there 2997 are things like euchromatin and heterochromatin.And in general, the density of histone packing 2998 around genes provides another level at which genes can be regulated. 2999 [00:06:09.17] So in this slide, which is pretty general, this shows a possible eukaryotic gene. 3000 Here, we have RNApolymerase, the TATAbinding protein, the general transcription factors, 3001 which we called TAFs, or TranscriptionAccessory Factors. They've helped recruit the RNA 3002 polymerase to the promoter region.And it will transcribe this gene stopping. 3003 [00:06:34.36] Here is the implication. Right upstream-- again, this is at the minus 10 and minus 3004 35 spot-- this location where you have gene-specific regulatory proteins or transcription factors, 3005 this could be a few hundred bases from the promoter. These sequences up here could be several 3006 thousand.And here, we could have a protein t
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