CH 18

Which of the following statements best defines the term operon? An operon is a region of DNA that consists of a single gene regulated by more than one promoter. An operon is a region of RNA that consists of the coding regions of more than one gene. An operon is a region of DNA that codes for a series of functionally related genes under the control of the same promoter. An operon is a region of DNA that codes for sugar-metabolizing enzymes.

An operon is a region of DNA that codes for a series of functionally related genes under the control of the same promoter. This arrangement of genes is common in bacteria. For example, genes involved in lactose metabolism are clustered in the lac operon of E. coli, and genes involved in tryptophan metabolism are in the trp operon.

The ________ is/are arranged sequentially after the promoter.

Genes of an operon

Which of the following best describes the biological role of the lac operon? It prevents other sugars from being metabolized until all available lactose has been used. It ensures that a cell produces enzymes involved in lactose metabolism in a constitutive manner. It ensures that a cell dedicates resources to the production of enzymes involved in lactose metabolism only when lactose is available in the environment. It ensures that bacterial cells produce lactose only when no other food sources are available.

It ensures that a cell dedicates resources to the production of enzymes involved in lactose metabolism only when lactose is available in the environment. The cell expends energy to produce the proteins necessary for lactose metabolism only when lactose is present.

A(n) _______ is a specific nucleotide sequence in DNA that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place.

Promoter

What molecule binds to promoters in bacteria and transcribes the coding regions of the genes? DNA polymerase A nucleotide RNA polymerase DNA ligase

RNA Polymerase RNA polymerase is the enzyme that binds to promoters and transcribes the coding regions of genes into RNA.

A(n) _______ codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes

Regulatory Gene

A(n) __________ is a protein that inhibits gene transcription. In prokaryotes, this protein binds to the DNA in or near the promoter.

Repressor

Which of the following enzymes converts ATP to cAMP? ATP synthase b-galactosidase Galactoside permease Adenylyl cyclase

adenylyl cyclase Adenylyl cyclase converts ATP to cAMP, which helps CAP bind and facilitates binding of RNA polymerase to the lac promoter.

Which of the following mutations could lead to constitutive expression of the genes of the lac operon? A mutation in the lac-Z gene A mutation in the lac-Y gene A mutation in the operator sequence A super repressor mutation

A mutation in the operator sequence Such a mutation could prevent binding of the repressor, allowing expression under all conditions.

True or false? The mechanism by which glucose inhibits expression of the lac structural genes is known as catabolite stimulation, whereas the mechanism by which lactose stimulates expression of the lac structural genes is known as allosteric regulation. True False

False The process by which lactose binds to the lac repressor and inactivates it by causing it to change shape is known as allosteric regulation. However, the process by which glucose causes cAMP levels in the cell to drop, thereby preventing CAP from stimulating expression of the lac structural genes, is known as catabolite repression.

What is allosteric regulation? In allosteric regulation, genes are expressed constitutively. In allosteric regulation, a gene is turned off by a repressor protein. In allosteric regulation, a small molecule binds to a large protein and causes it to change its shape and activity. In allosteric regulation, a gene is turned on by an activator protein.

In allosteric regulation, a small molecule binds to a large protein and causes it to change its shape and activity. Allosteric regulation is an important mechanism for changing enzyme activity, as well as for changing the function of some gene repressors and activators

A(n) _____ is a specific small molecule that binds to a bacterial regulatory protein and changes its shape so that it cannot bind to an operator, thus switching an operon on

Inducer

Regulatory proteins bind to the _________ to control expression of the operon.

Operator

A(n) ______ is a stretch of DNA consisting of an operator, a promoter, and genes for a related set of proteins, usually making up an entire metabolic pathway

Operon

What happens to the expression of the lacI gene if lactose is not available in the cell? There is no change—the lacI gene is constitutively expressed. The lacI gene turns off. The lacI gene increases its rate of transcription. The lacI gene turns on.

There is no change—the lacI gene is constitutively expressed. The lacI gene is expressed regardless of the presence of lactose. Only the structural genes of the lac operon are affected by the presence or absence of lactose. The lacZ gene encodes b-galactosidase, a key enzyme in lactose metabolism. When lactose is present in the cell, the cell expresses lacZ and metabolizes lactose.

The placement of the operator sequence between the promotor and the structural genes is critical to the proper function of the lac operon. True False

True When the repressor binds to the operator, RNA polymerase cannot transcribe the structural genes.