Name_________________________________________ Section_________7.014 Problem Set 6**Please note that due to the Patriots Day holiday, the deadline for this problemset has been extended. Copy your problem set as a study aid before you turn itin as problem sets will not be returned before the quiz.Answers to this problem set are to be turned in at the box outside 68-120 by 4:00 pm, Friday,April 19. Anyone wishing to turn in the problem set early can do so. Problem sets will not beaccepted late. Solutions will be posted at (http://web.mit.edu/7.01x/www/).Question 1In E. coli, the fictitious AB operon is induced by the presence of Compound W. A diagram ofthe operon, its regulatory proteins and regulatory sites is shown below:gene Xgene AP PSXgene BPXpromoter for the regulatory proteinX gene for the regulatory protein of the AB operonP promoter for the AB genesS sequence shown to be important for regulation by WA structural gene for enzyme AB structural gene for enzyme BThe following table shows the genotypes of different E. coli strains with a wild-type AB operonand various mutant AB operons, and the number of molecules of proteins A and B per cell inthe absence or presence of Compound W (–W or +W, respectively). The symbol "+" indicatesthat the gene or control element is functional (wt) and "–" indicates that the gene or controlelement is non-functional. Assume the genes not listed are wild type.Strain X P S A –W +W Expressionwt + + + + 0 200m1 - + + + 200 200m2 + - + + 0 0m3 + + - + 200 200m4 + + + - 0 0a) For each strain on the table above, label the expression as either inducible, uninducible orconstitutive.b) Based on the data shown above, does the regulatory protein X act as a repressor or anactivator of the AB operon? Explain your reasoning.7.014 Problem Set 6Spring 2002 1Question 1, continuedc) You make partial diploids of various E. coli mutant strains using a single-copy plasmidintroduced into the E. coli cell. In cells of the following genotypes, predict the number ofmolecules of enzyme A per cell (0, 200, 400) produced in the absence or presence ofCompound W (–W or +W, respectively). Put the total number of molecules of enzyme A onthe lines provided. # of molecules of enzymes A per cell Genotype –W +W X+P+S+A+0 400X+P+S+A+X+P+S+A+0 200X+P+S+A–X+P+S+A+X–P+S+A+X+P+S–A+X+P+S+A+X–P+S+A+X+P+S+A–X+P+S–A+X+P+S+A–X+P+S–A–X+P+S+A+7.014 Problem Set 6Spring 2002 2Question 1, continuedd) You examine a different operon, the H operon, containing genes 1 and 2 that encodeenzymes 1 and 2. In this operon compound H acts as an inducer. You look at the levels ofDNA, mRNA and protein in a typical E. coli cell. The results you find are shown below:–H +HDNA # of copies of gene 1 per cell 1 1mRNA # of copies of gene 1 mRNA per cell 50 50protein # of molecules of enzyme 1 per cell 0 200At which step of gene expression could this regulation occur? Explain your reasoning.e) In the lac operon, which is regulated at the transcriptional level, repressor protein is presentin E. coli at approximately 4 molecules per cell. In the H operon, the repressor protein ispresent at approximately 200 molecules per cell. Based on your answer to part d, why wouldyou expect there to be more molecules of the repressor protein for the H operon per cell thanmolecules of the lac repressor per cell?7.014 Problem Set 6Spring 2002 3Question 2a) Shown below is a schematic of the production of a heavy chain polypeptide for an antibody.At the top is the chromosomal arrangement found in an immature B cell, at the bottom isshown the heavy chain polypeptide.i) Label the process indicated by each arrow. Choose the one best option for each from:protein processingtranscriptiontranslationtransductionDNA ligationDNA rearrangementRNA splicingRNA ligationii) Indicate on the diagram below where you would expect to find each of the followingcomponents:PromoterTranscription terminatorstart codonstop codoniii) Indicate on the diagram below the variable and the constant region of the heavychain polypeptide.V segments D segmentsJ segments constant segment= intron regions}}DNARNA7.014 Problem Set 6Spring 2002 4Question 3a) Draw a schematic of a secreted antibody molecule and label the following structures onyour diagram: i) the light chains, ii) the heavy chains, iii) the antigen binding sites, iv) thevariable regions, v) If this were a secreted antibody, indicate the region of the antibody thatwould interact with macrophages.b)i) List two ways that combinatorial joining generates antibody diversity.ii) What are the interactions between the light and heavy chains that make themassociate with each other? What kind of bonding is this?7.014 Problem Set 6Spring 2002 5Question 4a) Name the cell type of the immune system that would:i) bind an antigen floating around in the blood.ii) recognize an antigenic peptide on a MHC Class II protein displayed by a macrophage.iii) recognize an antigenic peptide on a MHC Class I protein displayed by an infected skincell.iv) nonspecifically engulf and digest a variety of pathogens (disease-causing organisms).v) secrete large amounts of antibody in response to an infection.vi) recognize a virally infected body cell and destroy it.vii) the cell type that provides long-lived immunity to measles in children immunizedagainst measles by injection of viral proteins.b) Will an individual who does not express MHC Class II proteins on the surfaces ofB cells and macrophages produce a humoral immune response? Explain your answer.c) Below is a drawing of a putative human virus pathogen.viral protein "Nas T"viral protein "Stk E"i) Which viral protein(s) is likely to raise an antibody response in a natural infection?Why?ii) Which viral protein(s) is likely to raise cell-mediate response (killer T cell lines) in anatural infection? Explain your answer.7.014 Problem Set 6Spring 2002 67.014 Problem Set 6Spring 2002
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