ch19 - Chapter 19 19.1 How can inducible and repressible...

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Chapter 19 19.1 How can inducible and repressible enzymes of microorganisms be distinguished? ANS: By studying the synthesis or lack of synthesis of the enzyme in cells grown on chemically defined media. If the enzyme is synthesized only in the presence of a certain metabolite or a particular set of metabolites, it is probably inducible. If it is synthesized in the absence but not in the presence of a particular metabolite or group of metabolites, it is probably repressible. FEEDBACK: 19.1 DIFFICULTY: medium 19.2 Distinguish between (a) repression and (b) feedback inhibition caused by the end product of a biosynthetic pathway. How do these two regulatory phenomena complement each other to provide for the efficient regulation of metabolism? ANS: Repression occurs at the level of transcription during enzyme synthesis. The end-product, or a derivative of the end-product, of a repressible system acts as an effector molecule that usually, if not always, combines with the product of one or more regulator genes to turn off the synthesis of the enzymes in the biosynthetic pathway. Feedback inhibition occurs at the level of enzyme activity; it usually involved the first enzyme of the biosynthetic pathway. Feedback inhibition thus brings about an immediate arrest of the biosynthesis of the end-product. Together, feedback inhibition and repssion rapidly and efficiently turn off the synthesis of both the enzymes and the end-products that no longer need to be synthesized by the cell. FEEDBACK: 19.1 DIFFICULTY: easy 19.3 In the lactose operon of E. coli , what is the function of each of the following genes or sites: (a) regulator, (b) operator, (c) promoter, (d) structural gene Z, and (e) structural gene Y ? ANS: Gene or Regulatory Element Function (a) Regulator gene Codes for repressor (b) Operator Binding site of repressor (c) Promoter Binding site of RNA polymerase and CAP-cAMP complex (d) Structural gene Z Encodes β -galactosidase (e) Structural gene Y Encodes β -galactoside permease
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FEEDBACK: 19.4 DIFFICULTY: easy 19.4. What would be the result of inactivation by mutation of the following genes or sites in the E. coli lactose operon: (a) regulator, (b) operator, (c) promoter, (d) structural gene Z , and (e) structural gene Y ? ANS: (a) Constitutive synthesis of the lac enzymes. (b) Constitutive synthesis of the lac enzymes. (c) Uninducibility of the lac enzymes. (d) No β -galactosidase activity. (e) No β -galactosidase permease activity. FEEDBACK: 19.4 DIFFICULTY: medium 19.5 Groups of alleles associated with the lactose operon are as follows (in order of dominance for each allelic series): repressor, I s (superrepressor), I + (inducible), and I - (constitutive); operator, O c (constitutive, cis - dominant) and O + (inducible, cis -dominant); structural, Z + and Y + . ( a ) Which of the following genotypes will produce β -galactosidase and β -galactoside permease if lactose is present: (1) I + O + Z + Y + , (2) I - O c Z + Y + , (3) I s O c Z + Y + , (4) I s O
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This note was uploaded on 11/26/2011 for the course BIOLOGY 2c03 taught by Professor Dej during the Spring '11 term at McMaster University.

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ch19 - Chapter 19 19.1 How can inducible and repressible...

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