Chapter 17 Gene Expression Bacteria How do bacteria regulate which genes are expressed in what amounts and when Review All cells in an organism contain the same Regulation of gene is one of the ways that different cells function differently according to their location in the organism The cell regulates gene expression to avoid waste Gene Expression Regulation DNA X mRNA X Protein X Activated protein Regulation can occur at any of these stages Mechanisms of Regulation Different Levels of Control Transcriptional control is efficient and saves the cell the most Translational control allows a cell to make changes in the relative of different proteins Post translational control provides the most response of the the three mechanisms Understanding Gene Expression Regulation Regulation of gene expression is often complex with many different involved When studying regulation scientist often try and identify the proteins that are involved in regulation To do this scientists create mutations in the genes that code for those proteins Mutations Any change in the hereditary material DNA organism Mutagen is any physical or agent that causes a change in the hereditary material DNA UV radiation different chemicals tobacco radiation DNA Damage UV light can also damage DNA This causes mRNA to not be transcribed and can result in a change in the base during replication DNA strand with adjacent thymine bases UV light Kin k Thymine dimer DNA Damage X ray radiation and chemicals can mutate the DNA changing the base pair Mutagen Identifying Regulators of Gene Expression To study the regulation of gene expression you must identify the components that regulate gene expression Sometimes a mutation in a results in its product protein not being made We can use a to create a mutant and screen look for those mutants that fail to regulate a specific process DNA mRNA Protein Lactose metabolism Lactose Metabolism Model System Glucose is E coli s preferred carbon source but when glucose levels are low it will use lactose This requires the galactosidase to break lactose down into glucose and galactose Recognizing a potential site of regulation scientists tested 1 E coli does not produce galactosidase when glucose is present even when in the presence of lactose Lactose Metabolism Model System Conclusion prevents the expression of galactosidase galactosidase is only expressed in the presence of and absence of glucose Identify Genes Under Control How do we identify genes whose product function in regulation 1 Generate a large number of mutants using 2 Screen the mutants to find individuals with Lactose Metabolism Mutants Constitutive mutant Expression is on all the time Lactose Metabolism Mutants Mechanism of Control Negative control occurs when a regulatory protein binds to DNA and shuts down transcription Positive control occurs when a regulatory protein binds to DNA and triggers transcription The Hypothesis of Negative Control The lac Operon 1 The lacZ lacA genes are adjacent and transcribed into one mRNA 2 The repressor is a protein encoded by lacI that binds to DNA operator and prevents transcription lacZ Y and A 3 The inducer lactose binds to the repressor changing its shape so that it drops off the DNA
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