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Gene regulation
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Gene expression models Prokaryotes and Eukaryotes employ common and different methods of gene regulation Prokaryotic models 1. Trp operon 2. Lac operon Eukaryotic concept 1. Transcription factors
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Anabolic tryptophan pathway http://highered.mheducation.com/sites/0072995246/student_view0/chapter7/the_trp_operon.html Tryptophan is an amino acid that must be made by E. coli through an anabolic pathway involving several enzymes The trp operon encodes these enzymes Operon - a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter.
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Trp operon The trp operon consists of 5 genes
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Trp operon Upstream of these genes is the promoter region (eg. TATA box) Within the promoter is a sequence called the operator
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Trp operon A protein called repressor is able to bind the operator when conditions are correct
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In the absence of Trp Without tryptophan, the repressor protein cannot bind the operator
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In the absence of Trp With nothing in its way, RNA polymerase is free to initiate transcription
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In the absence of Trp With nothing in its way, RNA polymerase is free to initiate transcription This transcript will be translated into the enzymes capable of making tryptophan
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In the absence of Trp With nothing in its way, RNA polymerase is free to initiate transcription This transcript will be translated and the cell will begin making Trp
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In the presence of Trp When Trp is present it is able to bind to inactive repressor protein
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In the presence of Trp When Trp is present it is able to bind to inactive repressor protein
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In the presence of Trp The repressor is now able to bind to the operator
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In the presence of Trp The repressor is now able to bind to the operator This blocks RNA polymerase from transcribing the trp genes and the cell does not make Trp
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Catabolic lactose pathway http://highered.mheducation.com/sites/0072995246/student_view0/chapter7/the_lac_operon.html Lactose is used by E. coli in the absence of glucose, its favourite carbohydrate food source The enzymes responsible for breaking down lactose are under the influence of the lac operon
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lac operon The lac operon is a bit more complicated and involves a gene that is outside the operon
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lac operon The lac operon itself contains 5 genes but control over it is exerted by a regulatory gene called lacI
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In the absence of lactose lacI encodes the lac repressor protein and is always transcribed/translated into its active form
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In the absence of lactose The active repressor protein blocks transcription of the genes that encode enzymes for lactose metabolism
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In the presence of lactose When E. coli has lactose available, -galactosidase converts it into allolactose
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In the presence of lactose When E. coli has lactose available, -galactosidase converts it into allolactose Allolactose binds the repressor protein and inactivates it
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In the presence of lactose With the repressor out of the way, RNA polymerase is free to do it’s job and transcribe the genes for lactose metabolising enzymes
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In the presence of lactose With the repressor out of the way, RNA polymerase is free to do it’s job and transcribe the genes for lactose metabolising enzymes
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In the presence of lactose With the repressor out of the way, RNA polymerase is free to do it’s job and transcribe the genes for lactose metabolising enzymes http://highered.mheducation.com/sites/0072995246/student_view0/chapt er7/the_lac_operon__induction.html
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Eukaryotic gene expression Eukaryotic gene regulation is similar to that in prokaryotes in that… …protein-DNA interactions occur But there are complicating factors such as… …far more proteins are involved …DNA is densely packed as chromatin
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Transcription factors Protein that bind to specific DNA sequences andcontrol their rate of transcription are called transcription factors eg. trp repressor and lac repressor In eukaryotes transcription is naturally repressed by DNA packaging Transcription factors bind to promoter sites to keep DNA available to RNA polymerase for transcription Often numerous transcription factors are involved Read pg. 312 – 313 to learn about the complexity of eukaryotic gene regulation
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