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1. CHAPTER 7 1. CONTROL OF GENE EXPRESSION 1. Differentiation is require to regulate the expression of genes 2. Housekeeping genes = constitutive genes: genes that expressed in all cells of an organism 1. Ex: actin, ribosomal proteins, enzymes of glycolysis 3. Cell types can express small number of genes but at very low level 2. Transcription is the major point of regulation of most genes 1. 6 regulations sites 1. The 1 st (transcriptional control) site is the most important (DNA RNA transcript) 2. mRNA leaves the nucleus between site 2 (RNA processing control) and site 3 (RNA transport and localization control) 1. at which point, the mRNA splits: 1. mRNA inactive mRNA, through site 5 (mRNA degradation control) 2. mRNA protein, through site 4 (translation control) 1. protein active/inactive protein, through site 6 (protein activity control) 3. Bacterial genes are commonly organized in operons 1. In operons, a group of genes share a single promoter ( regulation by repressor ), transcribed as a single polycistronic mRNA 1. The genes also share a common function – biosynthesis of molecule or degradation of lactose 2. The promoter regulation by repressor 2. Eukaryotic organisms DO NOT contain operons. 1. Each eukaryotic gene has its own promoter monocistronic mRNA 4. Repressors turn off gene expression in 1. When Tryptophan (TRP) is high , repressor protein binds Tryptophan and binds operators (allosteric), turning the genes off 1. This conformation makes the active repressor blocks the binding of RNA polymerase 2. For other promoters, ligand binding may switch on expression by inhibiting repressor binding 3. This is negative regulation at operator 15bp: 1. TRP repressor binds TRP binds operator DNA blocks RNA pol binding
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4. So, expression of prokaryotic genes for biosynthesis is turned off when [tryptophan] is high because tryptophan binds to the trp reporessor, causing a conformational change so that the activated repressor binds RNA polymerase, blocking the binding of RNA pol to the trp promoter 2. When Tryptophan is low , the repressor does not bind tryptophan and cannot bind the operator, turning the genes on 5. Prokaryotic activator proteins turn on gene pression 1. An activator binds to a regulatory sequence near the promoter, increasing the rate of RNA polymerase binding 2. Activator proteins often bind an allosteric ligand that causes a conformational change in DNA binding site 1. Depending on the ligand and activator, ligand binding may inhibit DNA binding, or be required for DNA binding 2. Example: the activator CAP must bind cAMP before it can bind DNA sequence in the lac operons 3. So, positive regulation , binding cAMP to large domain triggers conformational change in small domain, binds DNA, is dimer, these genes bind RNA pol poorly, cAMP signal that glucose is low, switch on 1. So, prokaryotic activators such as cAMP-CAP increase the rate of RNA pol binding 6. In both eukaryotic and prokaryotic cells, activators and repressors bind tightly to specific sequences
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This note was uploaded on 12/05/2009 for the course BIO 341 taught by Professor Noris during the Fall '09 term at Rhode Island.

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