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REGULATING GENE EXPRESSION 13.5 Are all our diploid body cells genetically identical? Why or why not? What makes each different cell type behave differently? I. What is gene expression? The process by which a gene is transcribed into mRNA and then translated into a protein II. What is the regulation gene expression? involves: Activating a gene so that it can be transcribed and translated Inhibiting a gene so that it can’t be transcribed and translated When a cell needs to make a particular protein, the gene is activated and transcription and translation occurs. When a cell no longer needs the protein, the gene is inactivated and transcription and translation can’t occur Some genes are active in some cells but not in others. This allows cells to behave differently III. Gene expression in prokaryotes A. What is a prokaryote? Bacteria No nucleus No membrane bound organelles B. Structure of prokaryotic DNA Organized into units called operons Operon = segment of DNA that contains all of the genes for a particular pathway C. The Lac Operon Controls the breakdown of the sugar lactose by bacteria 1. Components of the lac operon Promoter = region of operon that binds RNA polymerase to start transcription Operator = region of the operon that binds repressor to prevent transcription Regulator gene = makes the repressor The 3 structural genes = when transcribed, they make the 3 enzymes needed to break down lactose 2. Regulating the lac operon Lac operon off: o Occurs when glucose is present and/or lactose is absent o Regulator gene makes repressor o Repressor sits on operator o RNA polymerase can’t sit on promoter o No transcription of the 3 structural genes Lac operon on: o When only lactose is around o Regulator gene makes repressor o Repressor binds with lactose o Repressor can’t sit on operator o RNA polymerase can sit on promoter o Transcription and translation of the 3 structural genes does occur http://www.sumanasinc.com/webcon tent/animations/content/lacoperon.ht ml Why do bacteria want the lac operon off when glucose is around? http://evolution.berkeley.edu/evolibra ry/news/070401_lactose IV. Gene Regulation in Eukaryotes Prokaryotes have no nucleus so as a gene is being transcribed, it can be translated at the same time Eukaryotes do have a nucleus so transcription and translation occur at different times Eukaryotic gene regulation can occur at transcription or after transcription Prokaryotic gene regulation can occur only at transcription Eukaryotic gene regulation can control how much transcription occurs Prokaryotic gene regulation can only turn transcription on or off A. Transcriptional Control of Gene Expression 1. Controlling how much transcription occurs Eukaryotic genes have special regions called enhancers and silencers Enhancers = segment of the gene that can increase the amount of transcription Silencers = segments of the gene that can decrease the amount of transcription How transcription is increased: o Enhancer binds activator protein o This increases the amount of transcription that RNA polymerase can do How transcription is decreased: o Silencer binds repressor o This decreases the amount of transcription RNA polymerase can do Ex: 2. Controlling whether or not transcription occurs Heterochromatin = highly condensed chromatin Euchromatin = loosely condensed chromatin In a given cell, genes that are being transcribed are in euchromatin and genes that are not being transcribed are in heterochromatin B. Posttranscriptional control Regulating gene expression AFTER transcription has occurred mRNA storage the longer the mRNA is stored in the nucleus, the less protein will be made mRNA halflife the longer the halflife of the mRNA the longer it will take for it to disappear and the more protein can be made from it rate of translation the faster the mRNA is translated, the more protein will be made http://highered.mcgrawhill.com/sites/007337797x/student_vi ew0/chapter13/animation_quiz__control_of_gene_expression_in_eu karyotes.html