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 #393 : Gre Subject Test: Biochemistry, Cell, And Molecular Biology
In regards to the lac operon in the presence of lactose, will the genes be transcribed in large amounts?
Maybe; it depends on the concentration of glucose
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
No; the lac operon does not utilize lactose sugars in its regulatory mechanism
Maybe; it depends on the concentration of glucose
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 #1 : Gene Regulation
Which of the following conditions are crucial to maintain high activation of the lac operon?
Large concentrations of glucose and large concentrations of lactose
Large concentrations of cAMP and large concentrations of lactose
Low concentrations of cAMP and large concentrations of lactose
Large concentrations of cAMP and large concentrations of glucose
Large concentrations of cAMP and large concentrations of lactose
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 #1 : Gene Regulation
Which of the following choices best represents the phenotype of a cell containing a mutation in the lac I gene?
No expression of the operon; RNA polymerase cannot bind properly
Lactose cannot enter the cell
Constitutive expression of the lac operon
Lactose can enter the cell, but cannot be broken down
Constitutive expression of the lac operon
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 
Example Question #396 : Gre Subject Test: Biochemistry, Cell, And Molecular Biology
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?
Activators
Operators
Promoters
Repressors
Operons
Operons
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 #395 : Gre Subject Test: Biochemistry, Cell, And Molecular Biology
Inducible operons are bound by a repressor and turned off under normal conditions. How are these operons turned on?
An activator protein displaces the repressor on the operator
An inducer molecule competes with the repressor for binding to the operator
The transcription of the repressor protein is inactivated
An inducer molecule binds to and inactivates the repressor
A second repressor protein binds to and represses the repressor
An inducer molecule binds to and inactivates the repressor
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.
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