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Control, Genomes and Environment Cellular Control – The lac operon Starter • Cystic fibrosis (CF) in humans is caused by mutations of a gene coding for transmembrane protein (CFTR) which acts as an ion pump. A large number of different mutations of the gene have been found. Explain what is meant by a gene mutation. (2 marks) • a change in the genetic material; unpredictable / AW; extra detail; e.g. addition / substitution / deletion / frame shift / small part of chromosome / may code for different protein /may code for no protein Learning Outcomes • Explain genetic control of protein production in a prokaryote using the lac operon Control of protein synthesis and activation • Protein synthesis and activation is tightly controlled - ensuring body development, cellular processes and reactions all take place at appropriate times and rates. • In eukaryotes: • Protein synthesis can be controlled at the transcriptional and translational levels. • Genes that aren’t transcribed are ‘switched off’ • Genes that are transcribed, but not translated are ‘silenced’ Gene regulation in Prokaryotes • Lactose: can be an excellent meal for E. coli bacteria. However, they'll only gobble up lactose when other, better sugars – like glucose – are unavailable. • With that for context, what exactly is the lac operon? The lac operon is an operon, or group of genes with a single promoter (transcribed as a single mRNA). The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source. What is the lac operon? • The lac operon of E. coli contains genes involved in lactose metabolism. It's expressed only when lactose is present and glucose is absent. • Two regulators turn the operon "on" and "off" in response to lactose and glucose levels: the lac repressor and catabolite activator protein (CAP). • The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon, but stops acting as a repressor when lactose is present. Genetic control in prokaryotes • Differential gene expression due to environment, eg. nutrient availability. • Involves ‘operons’ - a section of DNA containing several genes: – Eg Structural genes (protein products), control elements (P,O) – regulatory gene is not strictly part of the operon, but its product has an important role. Regulatory gene Control sites Structural genes A Complete the table Part of operon Letter I P O Z Function name Regulatory Gene Structural gene Product/detail of function Repressor protein bgalactosidase Y Lactose • Bacteria produce enzymes to metabolise certain nutrients only when they are present in their environment. • E.coli normally respires glucose, but can also use lactose as its source of carbon. • The essential enzyme to allow the metabolism of lactose is b-galactosidase. • If lactose is not present, there will be very few bgalactosidase molecules in the cell. • The presence of lactose in the environment causes a large increase in the production of b-galactosidase Lactose not present A I- Regulator gene – produces a repressor protein which binds to operator (O) P- Promoter region – binding site for RNA polymerase. Repressor at O prevents it binding. Z is b-galactosidase, Y is lactase permease. A is transacetylase Z, Y, A are not transcribed when R is at O Lactose present A I- Regulator gene – produces a repressor protein which is prevented from binding to O by lactose binding and changing the shape of the repressor. P- Promoter region – binding site for RNA polymerase. Z, Y, A are transcribed when RNA polymerase binds at P Plenary Component Function A. structural genes 1. produces repressor protein B. regulator gene 2. binds to repressor C. promoter 3. codes for lac enzymes D. operator 4. binds to RNA polymerase A–3 B–1 C–4 D-2 Activators • Proteins which activate genes. • Activators and repressors are transcription factors. • Eg: cAMP can bind to protein kinase A and cause the protein to change shape, thus activating the protein. cAMP activates PKA which phosphorylates other proteins and enzymes Plenary • The bacterium Escherichia coli (E. coli) uses glucose as a respiratory substrate. In the absence of glucose, E. coli can use lactose. The use of a different substrate is determined by the interaction between genes and the environment. Describe how this happens in this example. • (5 marks) Answer • 1. ref to operon; 2 normally repressor substance bound to operator; 3 prevents RNA polymerase binding (at promoter) / prevents transcription; 4 lactose binds to repressor; 5 changes shape of protein molecule; 6 unable to bind (to operator); 7 RNA polymerase binds (at promoter) / transcription occurs / genes switched on; 8 AVP; e.g. production of lactose permease / production of beta-galactosidase; Homework • Gene control in prokaryotes sheet • Q p103 green book • Q4 p149 purple book Learning Outcomes • Explain genetic control of protein production in a prokaryote using the lac operon