Genetics Exam 4 Study Guide Chapter 15 Regulation of Gene Expression in Prokaryotes This section describes how the cell makes the choice of what protein to make when and in what amount gene expression The most important step in the choice of making a protein is the initiation of transcription by RNA polymerase Know what regulation of gene expression means Prokaryotes respond rapidly to changes in their environment The goal of regulating gene expression is to maximize growth and ensure that cells can live efficiently Understand that gene expression in bacteria is primarily regulated to maximize growth rate and so bacterial gene expression is mainly controlled by small molecules nutrients from the environment monocistronic genes Understand what an operon is polycistronic gene and how they differ from typical Some enzymes are inducible meaning that they induce a reaction Some enzymes are repressible meaning that the enzymes are regulated by the intracellular concentration of certain metabolites Lactose is present so the cells produce enzymes to metabolize lactose inducible 3 genes and an adjacent regulatory region constitute the Lactose Operon The cluster functions in an integrated fashion to provide a rapid response to the presence or absence of lactose 3 structural genes in the lac operon that encode for B galactosidase permease transacetylase These 3 units are transcribed as a single unit resulting in a polycistronic gene An operon is a genetic unit consisting of one or more structural genes encoding polypeptides and operator region that regulates the transcriptional activity of the adjacent structural genes Be able to draw a diagram showing the positions of the major elements of the lac Operon and the I gene I gene CAP site lac promoter operator Z gene Y gene A gene and understand the function of each of these in transcription of the lac operon Know and be able to use appropriately the terms inducible gene expression constitutive and uninducible Inducible gene expression is under the control of an inducer a regulatory molecule that acts to block a repressor and allow transcription ex in the presence of lactose B Galactosidase is made Constitutive are genes that are always on ex lacI gene mutation causes cell to make B galactosidase all the time with or without lactose Uninducible genes are never on ex Is cannot make B galactosidase in presence of lactose or not Be able to describe the regulation of the lac Operon by lactose and glucose and be able to predict the effect of mutations in the lac Operon the I gene or the CAP gene on expression of the lac Operon in the presence of lactose and or glucose The I gene is a repressor gene because it codes for a protein that blocks transcription mutations can cause constitutive behavior Lactose is an inducer that causes the cell to produce lactase Operator region which consists of Z Y A structural genes and other regional sequences People argue that the LacI gene regulates the transcription of structural genes by producing a repressor molecule and that the repressor is allosteric it reversibly reacts with another molecule causing both a conformational change in the repressor s 3D shape and a change in its chemical activity Repressor binds to the DNA sequence of operator region to inhibit action of RNA polymerase Lactose binds to the repressor and inhibits the molecule from interacting with the operator This allows RNA polymerase to transcribe the DNA sequence NEGATIVE CONTROL Enzymes are produced to degrade the lactose and then repressor binds back onto the operator to stop all transcription Glucose inhibits transcription acts through a catabolite activation protein CAP CAP requires cAMP to bind to the lac promoter which helps RNA polymerase bind When glucose is present cAMP levels decrease and the CAP is repressed and cannot bind well Without glucose sufficient cAMP is present allowing transcriptase Understand why binding of the repressor to the operator blocks transcription of the lac Operon and how CAP activates transcription Be able to state the active sites on the lac Repressor and the CAP protein Be able to define the term transcription factor Transcription factor is a protein that binds to a specific DNA sequence in order to facilitate inhit the binding of RNA polymerase CAP the repressor Sequence is recognized on where to bind Understand the difference between positive and negative control of gene expression Negative control gene expression occurs unless it is shut off by some form of a regulator Positive control Transcription only occurs if a regulator molecule directly simulates molecule RNA production Regulation of Gene Expression in Eukaryotes For eukaryotes understand that gene expression is regulated primarily over the course of development and in response to internal signals like hormones and not to external signals from the environment Understand that in eukaryotes the initiation of transcription is regulated both through the activity of transcription factors and through the packing of chromatin Know eukaryotic promoters are much larger than prokaryotic promoters and contain many more sequences that must be bound by specific transcription factors for RNA polymerase to initiate transcription This complexity allows for the increased regulation of gene expression that is necessary in complex multicellular organisms Be able to explain the difference between promoters and enhancers Understand that transcription factors contain at least two functional sites one that binds to a specific DNA sequence in the promoter or enhancer of a gene and a second site that activates gene expression Know what heterochromatin and euchromatin are and that histone deacetylation and DNA methylation are involved in the inactivation of gene expression during differentiation Actively expressed genes have acetylated histones and have little cytosine methylation Deacetylation of histones represses transcription and in heterochromatin transcription is permanently inactivated and has acetylated histones and highly methylated DNA The condensation of chromatin for regulation of transcription involves activator or repressor proteins that bind specific DNA sequences and attract enzymes that acetylate or deacetylate histone proteins or that methylate cytosine bases Be able to explain how steroid hormones activate the expression of specific genes in specific cells only certain cells make the hormone receptor and this receptor is a transcription factor which
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