BIOTECHNOLOGY Lecture Notes - Lecture 3: Lac Operon, Lac Repressor, Operon

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learnthebasics07

20 Dec 2022

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Guru Nanak Dev University

Department

Biology

Course

BIOTECHNOLOGY

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Lac operon: why lac operon is called as inducible operon, some confusing terms in lac operon, negative regulation, positive regulation, repressors, operators, inducers, activators/enhancers. Lac operon in the presence of glucose: lac operon in the absence of glucose, catabolic repression. Trp operon: in the presence of tryptophan, in the absence of tryptophan. This operon remains switched on till lactose is there in the environment. As lactose/allolactose induces this operon, hence the name inducible operon: some confusing terms in lac operon. Negative regulation: rna polymerase binding or its movement along the dna after binding, is blocked when the repressor is present. Regulation by means of a repressor protein that blocks transcription is referred to as negative regulation. Positive regulation: activators provide a molecular counterpoint to repressors, they bind to dna and enhance the activity of rna polymerase at a promoter, this is positive regulation.

Related Questions

The following test deals with the regulation of the trp operon of Escherichia coli. Two mutants of this operon are described: in mutant A, the operator region of the operon carries a point mutation so that it is unable to carry out its function; mutant B expresses a trp repressor protein unable to bind tryptophan. Review the principle of the regulation of this operon and then solve the following questions related to the behavior of the mutant cells using the options below.

A: Mutant A B: Mutant B C: Both of them D: Neither of them

a) _____ In the presence of tryptophan these cells behave like wild-type cells. b) _____ In the absence of tryptophan these cells behave like wild-type cells. c) _____ These cells are able to synthesize tryptophan.
d) _____ These cells are able to break down tryptophan.

e) _____ Tryptophan is able to act as an inducer in these cells.
f) _____ Tryptophan is able to act as a corepressor in these cells.
g) _____ The structural genes of the trp operon can be transcribed in these cells.
h) _____ In the presence of tryptophan RNA polymerase binds to the promoter of the trp operon in these cells.
i) _____ In the absence of tryptophan RNA polymerase binds to the promoter of the trp operon in these cells.
j) _____ These cells die if tryptophan is omitted from their medium.
k) _____ These mutants can be ââcuredââ by expressing wild-type trp repressor in them.
l) _____ These mutants can be ââcuredââ by transforming them with a wild-type trp operator.

1.) What type of control best describes the lac operon when it is regulated by allolactose? Explain the features of this type of regulation using the lac operon as an example.

2.) What is positive gene regulation?

a) when an activator protein binds to the operator of a gene to turn expression off

b) when a repressor protein binds to the operator of a gene to turn expression off

c) when an activator protein binds to the operator of a gene to turn expression on

d) when a repressor protein binds to the operator of a gene to turn expression on

e) when a protein binds to DNA to change gene expression

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?

a) regulator gene

b) operator

c) promoter

d) structural gene Z

e) structural gene Y

4.) In a negative, repressible regulatory mechanism, transcription of the structural gene(s) occurs in

a) the presence of the co-repressor, but not in its absence.

b) the absence of the co-repressor, but not in its presence.

c) the presence of the inducer, but not in its absence.

d) the absence of the inducer but not in its presence.

e) the absence or presence of the co-repressor.

What are answers and can you explain them to me?

Given the following mutants and conditions, predict the expression level of the of the lac Z gene. Please indicate your answer with one of followings: no expression, basal level of expression, or activated level of expression. Explain why, in the context of activities by activator and repressor:

I. A mutant of E.coli that has a mutation in the repressor gene (Lac I) for regulating lac operon

a. In the presence of glucose, presence of lactose

b. In the presence of glucose, absence of lactose-

c. In the absence of glucose, absence of lactose-

d. In absence of glucose, presence of lactose-

II. A mutant of E.coli that has a mutation in the cAMP binding site of CAP

a. In the presence of glucose, presence of lactose â

b. In the presence of glucose, absence of lactose â

c. In the absence of glucose, absence of lactose â

d. In absence of glucose, presence of lactose -

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BIOTECHNOLOGY Lecture Notes - Lecture 3: Lac Operon, Lac Repressor, Operon

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