Gene Regulation

Gene Regulation - BIO 181 Laboratory Exercise Molecular...

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BIO 181 Laboratory Exercise Name: Molecular Genetics 1 Enzyme Induction and Operon Theory In this exercise we will investigate an example of gene regulation in the bacteria Escherichia coli . These organisms are capable of producing an enzyme called β - galactosidase , which catalyzes catabolism of lactose , a disaccharide (carbohydrate), into its constituent monosac- charides, glucose and galactose . Although all E. coli have the gene coding for β -galactosidase, called the lac z gene, they will only transcribe it if lactose is available in their environment; otherwise, making β -galactosidase is a waste of energy and other resources. In mammals a different enzyme, lactase, performs the same function in the small intestines. Individuals who are “lactose-intolerant” suffer bloating and diarrhea when they ingest lactose because they fail to produce enough lactase to digest the lactose. So the sugar passes to the large intestine where bacteria resident there metabolize it and produce methane gas as a byproduct. Lactose intoler- ance afflicts about 70% of humans world-wide. The question we address in the first part of the lab is this: how do E. coli control transcription of the β -galactosidase gene? In the second part of the lab, you will design an experiment to address how other environmental factors influence the transcription of the β -galactosidase gene. The gene for β -galactosidase is found in an operon called the lac operon . An operon is a set of genes in prokaryotes (not eukaryotes) that are always transcribed together into a single mRNA molecule. The gene products usually participate in the same biochemical pathway. For example, the three genes in the lac operon are involved in transporting lactose into the cell and subsequently breaking it down. Imagine an E. coli bacterium sitting in an environment that does not contain any lactose. The bacteria will not transcribe the lac operon because it does not need it. The question is: what represses transcription? How does the bacteria, a creature with no brain, “know” not to make the products encoded by this operon’s genes, including β -galactosidase? The textbook suggests the following hypotheses: Hypothesis 1 In the absence of lactose, the lac operon is inactivated by a protein called the lac repressor protein , coded for by the lac i gene , which binds to a region of DNA between the promoter and the genes to be transcribed. That region is called the operator. The lac repressor protein bonding to the operator prevents transcription because the RNA polymerase cannot then bind to the promoter to begin transcription (Fig. 1). When a molecule, such as the lac repressor protein, prevents the expression of a gene by binding to it, it is called negative regulation . Hypothesis 2
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Gene Regulation - BIO 181 Laboratory Exercise Molecular...

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