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Protein synthesis O O O O O O O O O O O O O O O Gene DNA Triplet code Degenerate Amino acid Polypeptide Transcription mRNA Template strand Hydrogen bonds Complementary Phosphoryl groups (energy) Coding strand Nuclear pore Ribosome O Translation O Codon O Anticodon O tRNA O AUG=start mRNA O Amino acid O Peptide bond O Stop codon: UAA,UAG,UGA How is protein synthesis controlled? O So you should already know the mechanism of protein synthesis…… O How is the process controlled?? O Learning outcome: O Explain genetic control of protein production in a prokaryote using the lac operon O Pages 112-113 Genetic Control of Enzyme Production in Prokaryotes O Genes control protein synthesis. Some proteins are required all the time by cells whilst others are only needed in specific circumstances and so are not made all the time. O Such genes must be capable of being switched on or off – it would be wasteful if they were made and not required! Switching on is known as induction, whilst switching off is known as repression. Gene control in Prokaryotes O Research was carried out using bacteria, when it was noticed that bacteria could vary the synthesis of enzymes in response to environmental change such as food O This operon mechanism was proposed by Jacob and Monod to account for the regulation of gene activity in response to the needs of a cell. O (Genes are not found in operons in Eukaryotes, so does not apply) O Operons consist of a group of closely linked genes that act together and code for enzymes that control a particular metabolic pathway. An operon consists of at least one structural gene coding for the primary structure of an enzyme and 2 regulatory elements, the operator and promotor AS revision……… first! Example of the control of gene expression: switching genes on! O A bacterium called Escherichia coli has the O O O O O gene to make the enzyme β-galactocidase Breaks down lactose to glucose and galactose Only produced when lactose is present! In it’s normal environment lactose is not present to be used as a food source So, since enzymes are ‘expensive’ to make (need energy, amino acids etc) the enzyme is not required if no lactose is present SO: When there is NO LACTOSE promoter In normal circumstances when lactose is not present a repressor protein binds to the DNA close to the promoter region (where RNA polymerase has to bind to start transcribing the DNA strand) so RNA polymerase cannot bind so the gene is not transcribed into mRNA so no enzyme produced When there is LACTOSE PRESENT.......... mRNA bound to promoter •If lactose is present it gets into the cell and binds with the repressor protein •this now cannot bind to the DNA •so RNA polymerase can bind to the promoter •so the gene is transcribed into mRNA •and the enzyme is produced to break down the lactose as a food source i.e. lactose is the signal to switch on the gene Lac system genes form an operon O An operon: is a section of DNA, made up of structural O O O O O O genes and control sites The structural genes code for proteins, such as enzymes The control sites are the operator region and a promotor region. The operator and promotor are both genes, as they are lengths of DNA but they do not code for polypeptides The operator region: Non-coding sequence of DNA that is the binding site for the repressor molecule A promoter region where the RNA polymerase attaches to, so allowing transcription There is also regulator gene (not actually part of the operon) on another part of DNA molecule, that codes for a repressor protein. Behaviour of the repressor protein is determined by whether the gene is induced or repressed. Whiteboards 1. Draw and label an operon 2. Define: an operon, structural gene, operator, promotor, regulatory gene. O Exam question: O All human babies produce the enzyme lactase. The genetic change that allows adults to produce this enzyme is thought to involve a mutation in a regulatory gene. This mutation causes the structural gene to be expressed in adults. O Distinguish between the terms ‘regulatory gene’ and ‘structural gene’ 2 marks 2 marks O Regulatory: O Idea that makes regulatory/repressor protein O OR O Idea that it switches another/structural gene on/off O Structural: O Idea that it makes enzyme/polypeptide/protein O Relationship between the 2: O Idea that the regulatory gene controls/affects the expression of the structural gene Your specification requires only that you know about the lac operon of Escherichia coli.(E-coli) It is however possible that you may be provided with detail of another system and asked to compare it with the lac operon. E-coli lac operon and its regulator gene The lac operon is a section of DNA within the bacterium's DNA Structural genes: Z=enzyme beta-galactosidase Y= lactose permease Operator region=O, next to structural genes, can switch genes on/off The promotor region=P, length of DNA to which the enzyme RNA polymerase binds to begin the transcription of Z and Y structural genes Regulatory gene is not part of the operon SO: When there is NO LACTOSE Regulatory gene is expressed A repressor protein synthesised, with 2 binding sites (lactose+operator) a) Binds to operator region b) Covers the promotor region where RNA polymerase normally attaches So structural genes cannot be transcribed into mRNA So no enzymes produced Whiteboards! When there is LACTOSE PRESENT.......... Lactose added Lactose inducer molecules attach to other site on repressor protein Repressor protein changes shape so its other binding site cannot bind to operator region Inducer-repressor complex The repressor breaks away from operator region So RNA polymerase can bind to unblocked promotor region and initiate transcription of genes for Z and Y So E coli can use lactose permease to take up lactose from the cells The enzyme beta galactosidase can convert lactose to glucose and galactose…respiration!!! O Explain the advantage of having an inducible O O O O enzyme system that is regulated by the presence of a substrate 2 marks Extension: Suggest how gene control in a noninducible system may be achieved? 2 marks In an inducible system the enzymes required for the metabolism of a particular substrate are only produced when the substrate is present.(1) This saves the cell valuable energy in not producing enzymes that have no immediate use.(1) Gene repression in Prokaryotes, structural genes are transcribed all the time,(1) and when the end product is present in excess to requirements the gene is switched off.(1) Watch O http://www.sumanasinc.com/webcontent/anim O O O O O ations/content/lacoperon.html Summarise the structure of a Lac system in E coli How can this system control protein production in a Prokaryote! Use diagrams to explain to each other O PLENARY: Exam question: ‘The control of the expression.’ (3 marks) http://www.sumanasinc.com/webcontent/anim ations/content/lacoperon.html Whiteboards quiz! Learning outcome O State that cyclic AMP activates proteins by altering their 3 dimensional structure O Importance of referring to the spec!! O NB How do you make your notes?? O You probably covered this in Homeostasis/ endocrine system?? O We have looked at the genetic involvement in development..other molecules involved.. O As we go through jot key words onto your whiteboard Enzymes activating proteins O Enzymes are responsible for the metabolism O O O O of cells. Some function continuously; some are present all the time, but are ‘idle’, only working when given the appropriate signal; Some are only needed if particular substrates are present Some may be needed for particular aspects of development. Clearly their action is regulated. Such regulation can be via chemical signals such as hormones or under genetic control. Protein activation by cyclic AMP O Cyclic adenosine monophosphate (cAMP) is a second messenger which targets specific intracellular enzymes to induce a cellular response. O Its actions allows hormones that cannot pass across the plasma membrane to still bring about an effect inside the target cell. O cAMP acts via other enzymes to initiate a response O What do you think cAMP is a derivative of? Remember? Mechanism of action of peptide hormones: peptide hormone: first messenger receptor cell surface membrane Inactive secondary messenger Secondary messenger activated: activates enzymes or transcription factors Can act as a transcription factor which switches on gene for a specific protein O Cyclic AMP is a second messenger, allowing for transmission O O O O O O of messages across the cell membrane. Hormones such as insulin and glucagon cannot pass through cell membranes. Instead, there are receptors on the outer surface of the cell to which the hormone binds. This activates a system that produces a second messenger – in the case of glucagon, adenylate cyclase brings about the production of cyclic AMP from ATP. The cyclic AMP then activates intracellular enzymes like protein kinase A (PKA) by altering their three dimensional structure. The activated enzyme then catalyses the required reaction in muscle cells which then switches on glycogen phosphorylase and switches off glycogen synthase. In short, the required enzyme action is switched on. Protein kinases O Are an important family of target enzymes for cAMP. O Kinase activation modifies the function of other proteins by adding phosphate groups to them O About 30% of human proteins can be modified by protein kinases. O Protein kinase A is involved in controlling the transcription of many genes. O It has an important role in regulating glycogen metabolism O It adds a phosphate group to enzyme phosphorylase kinase to activate it, this then phosphorylates and activates glycogen phosphorylase which catalyses the breakdown of glycogen to glucose 1 phosphate. Review: Learning outcome O State that cyclic AMP activates proteins by altering their 3 dimensional structure O WATCH?? https://www.youtube.com/watch?v=Y2er_Df gg44 https://www.youtube.com/watch?v=NaOBRv AFiJQ Make a simple flow diagram to explain the LO O State that cyclic AMP activates proteins by altering their 3 dimensional structure O Glucagon binds to receptor on cell surface membrane O Stimulates production of adenylate cyclase O Stimulates production of second messenger cAMP O This activates intracellular enzymes: protein kinase A by altering 3D structure O It adds a phosphate group to enzyme phosphorylase kinase to activate it O This then phosphorylates and activates glycogen phosphorylase O Which catalyses the breakdown of glycogen to glucose 1 phosphate.