Answers to review questions – chapter 12

  1. What is the function of a promoter? (p. 250, Figure 12.2)

The promoter provides the site to which the RNA polymerase binds to initiate transcription.

  1. What is the role of the lacI gene in lac operon regulation? (p. 251)

In the absence of lactose, the lacI gene produces repressor protein that diffuses to the lac operon to block transcription.

  1. Is the location of the lacO operator between the promoter and lacZ gene important for this role? (p. 251)

Yes, lacO is the specific binding site for lac repressor proteins. When bound it prevents transcription of the lac operon.

  1. In the lac operon, when cells growing in a non-lactose-containing medium are switched to a medium containing only lactose, what is the effect on gene expression? (pp. 252–253)

In the absence of lactose, transcription of the lac operon is repressed. When cells are switched to a medium containing only lactose, de-repression occurs and gene expression is the result.

  1. Does the Trp repressor bind to DNA in the presence or absence of tryptophan? (p. 254)

Absence. When tryptophan levels are low the trp repressor cannot bind to the trp operator, therefore tryptophan is synthesised.

  1. Compare and contrast the regulation of lac and trp operons. (pp. 251–254)

Regulation of lac operon / Regulation of trp operon
controlled by two regulating proteins / controlled by one regulating protein
lac repressor active in the absence of lactose and in the presence of glucose / trp repressor active in the presence of tryptophan
negative regulation: bound repressor complex prevents transcription / negative regulation: bound repressor complex prevents transcription
positive regulation: bound CAP-cAMP complex promotes transcription / no positive regulation
low levels of glucose in the presence of lactose induce lac operon / low levels of tryptophan induce trp operon
  1. What is the relationship between an enhancer and a transcription factor? (pp. 257–258)

Transcription factors are proteins that regulate the initiation of transcription. Transcription is stimulated when transcription factors bind to specific DNA sequences called enhancers.

  1. How do nucleosomes influence eukaryotic transcriptional regulation? (pp. 257–258)

Nucleosomes are composed of tightly wrapped DNA, which hinders transcription by RNA polymerase II. The more tightly the DNA is bound within the nucleosome, the greater the repression. Acetylation of histones within nucleosomes decreases the strength of the histone-DNA bonds, thereby favouring transcription.

  1. In eukaryotes, what sequence elements other than the promoter are important in regulating transcription? (pp. 257–258)

Two sequence elements other than the promoter that are important in regulating transcription are enhancers and silencers.

  1. What is meant by the modular nature of eukaryotic gene control? (p. 255)

The regulation of transcription of a single gene through independent action of an enhancer or silencer element.