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United States (324,794)
BSC 219 (22)
mortimer (22)
Lecture 12

BSC 219 Lecture 12: Genetics Lec 12
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Department
Biological Sciences
Course
BSC 219
Professor
mortimer
Semester
Spring

Description
THE LAC OPERON 2) Structural genes à genes that make a specific RNA or protein not involved in the regulation of the expression of a gene Operons often include: 1) Regulatory sequences like promoters and operators 3) Regulatory genes à genes that make a specific RNA or protein involved in the regulation of the expression of a gene Example: Transcription factors Example: hemoglobin or enzymes that make nutrients GENE REGULATION IN PROKARYOTES Genes and operons can be under negative control Transcription is blocked or repressed by a regulatory molecule Genes and operons can be under positive control Transcription is stimulated or induced by a regulatory molecule This is called a repressible operon This is called an inducible operon THE LAC OPERON Metabolism is a general term describing most chemical reactions involved in living cells It can be divided into catabolism and anabolism Anabolism à synthesizing complex molecules Catabolism à breaking down large molecules THE LAC OPERON Lactose is a sugar, consisting of one molecule of glucose and one molecule of galactose Lactose must be catabolized into glucose and galactose to serve as a carbon source (i.e. “food”) THE LAC OPERON Genes for lactose catabolism are induced (expressed at higher levels) when lactose is present in the environment E. coli prefers glucose, but will use lactose if there is no glucose in its environment When lactose is high, the lac operon is expressed The purpose is to breakdown the lactose, so the bacteria can use it for food THE LAC OPERON The P region is the promoter The lac operon contains a regulatory region that can be divided into two smaller regions THE LAC OPERON The lac operon contains a regulatory region that can be divided into two smaller regions The O region is the operator THE LAC OPERON LacZ LacY LacA The lac operon also contains three structural genes THE LAC OPERON Lactose induces the transcription of the structural genes (LacZ, LacY, and LacA) in the lac operon When lactose levels are high, the lactose needs to be broken down so the lac operon is turned on When lactose levels are low, the lac operon needs to be turned off THE LAC OPERON In the absence of lactose, the lac operon is turned off This is done by Repressor protein encoded by the LacI gene THE LAC OPERON When there is low/no lactose, the LacI gene express a repressor protein that binds to the operator sequence This prevents the RNA polymerase from transcribing the lac operon à no transcription, no expression LacI RNA pol THE LAC OPERON When lactose levels are high, the repressor protein now binds to the lactose instead of the operator sequence LacI RNA pol lactose mRNA This allows the RNA polymerase to transcribe the lac operon MUTATIONS AND THE LAC OPERON THE LAC OPERON Constitutive mutations à gain of function à gene/protein is always active (on) or expressed LacI X RNA pol I - a mutation in lacI that prevents the repressor from binding to the operator This results in the lac operon always being turned on THE LAC OPERON Mutations in the operator region (lac OC) also causes constitutive expression X LacI RNA pol This results in the lac operon always being turned on THE LAC OPERON X P- à is a mutation in the promoter sequence that prevents the RNA polymerase from binding This results in the lac operon always being turned off RNA pol lactose THE LAC OPERON X Z- or Y- or A- à are mutations in one of the structural genes This results in the lac operon being regulated normally, but you don’t expression of that specific gene THE LAC OPERON If it is always on, then you have two possibilities: The repressor (I- ) or the operator (OC) is mutated If it is always off, then you have two possibilities: The repressor (IS) or the promoter (P- ) is mutated If one gene is off, then that gene is mutated (LacZ- , LacYor LacA- ) THE LAC OPERON 2)Now you need to think about rescuing the lac operon using an F’ plasmid The F’ plasmid is a circular piece of DNA that contains genes that the bacterial cell needs It is NOT a part of the bacterial chromosome THE LAC OPERON 2)Now you need to think about rescuing the lac operon using an F’ plasmid The plasmid can contain a fully functional lac operon Depending on the type of mutation, the plasmid lac operon can rescue or restore the phenotype back to normal THE LAC OPERON The plasmid made repressor can bind to the operator of the chromosomal lac operon LacI This would restore the regulation of the lac operon 1) The repressor (I-
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