GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Operon Regulation

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Help With Operon Regulation

In regards to the lac operon in the presence of lactose, will the genes be transcribed in large amounts?

Possible Answers:

No; the lac operon does not utilize lactose sugars in its regulatory mechanism

Yes; the lactose sugars bind transcription factors that turn on transcription

Yes; the lactose sugars remove the repressor and the genes will be transcribed rigorously 

Maybe; it depends on the concentration of glucose

Correct answer:

Maybe; it depends on the concentration of glucose

Explanation:

Activation of the lac operon is necessary for the transport and metabolism of lactose sugars by E. coli. Lactose sugars actively work to remove a repressor that statically inhibits transcription; however, high concentrations of glucose (and, thus, low concentrations of cAMP) will prevent these genes from being transcribed rigorously. In order for the lac operon to be active at high levels, lactose must be present and glucose must be absent.

Example Question #2 : Help With Operon Regulation

Which of the following conditions are crucial to maintain high activation of the lac operon?

Possible Answers:

Large concentrations of cAMP and large concentrations of lactose

Low concentrations of cAMP and large concentrations of lactose

Large concentrations of glucose and large concentrations of lactose

Large concentrations of cAMP and large concentrations of glucose

Correct answer:

Large concentrations of cAMP and large concentrations of lactose

Explanation:

The lac operon is a system designed to only express particular proteins when the concentration of glucose is low and the concentration of lactose is high. The common cellular response to a low concentration of glucose is to increase the concentration of cAMP in order to activate various alternative metabolic pathways. Both a high concentration of cAMP and a high concentration of lactose are necessary to get sustained expression of the lac operon. When glucose levels begin to rise, the cAMP concentration will begin to fall and the operon function will deteriorate.

Example Question #3 : Help With Operon Regulation

Which of the following choices best represents the phenotype of a cell containing a mutation in the lac I gene?

Possible Answers:

No expression of the operon; RNA polymerase cannot bind properly

Constitutive expression of the lac operon

Lactose cannot enter the cell

Lactose can enter the cell, but cannot be broken down

Correct answer:

Constitutive expression of the lac operon

Explanation:

lac I is the gene that encodes for the repressor of the lac operon. If there is no repressor, the cell will constantly express the genes present in the lac operon whether or not the typical conditions are present.

A mutation of the gene encoding -galactosidase permease (lac Y) would prevent lactose from entering the cell. A mutation in the gene encoding -galactosidase (lac Z) would prevent the breakdown of lactose. A mutation in the promoter region would prevent RNA polymerase from binding.

Example Question #4 : Help With Operon Regulation

In prokaryotes, functionally related genes are sometimes position adjacent to each other in the genome and can under the control of the same regulatory machinery. What are these called?

Possible Answers:

Operons

Repressors

Promoters

Activators

Operators

Correct answer:

Operons

Explanation:

Prokaryotic organisms often have functionally related genes joined together on the chromosome under the direction of a single promoter. These structures are called operons. Operons have additional sequences, called operators that can be bound by either repressor or activator proteins, which will repress or activate transcription of the operon. One commonly studied example is the lac operon, whose genes encodes products required for lactose metabolism.

Example Question #5 : Help With Operon Regulation

Inducible operons are bound by a repressor and turned off under normal conditions. How are these operons turned on? 

Possible Answers:

An activator protein displaces the repressor on the operator

An inducer molecule competes with the repressor for binding to the operator

A second repressor protein binds to and represses the repressor

An inducer molecule binds to and inactivates the repressor

The transcription of the repressor protein is inactivated

Correct answer:

An inducer molecule binds to and inactivates the repressor

Explanation:

Negatively regulated operons that are said to be inducible have their operator sequence bound by a repressor molecule normally. That leads to these operons being off normally. For these operons to be turned on and transcribed, a small molecule called an inducer has to bind to and inactivate the repressor protein.

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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