BIO 151 EXAM PREP QUESTIONS AND
ANSWERS GRADED A PLUS STUDY
RESOURCE
●● Which of the following statements best summarizes how the lac
operon is regulated?
Answer: Positive and negative control elements regulate the lac operon.
●● What happens when the interaction of the repressor protein with the
DNA of the lac operator is disrupted?
Answer: Lactose binds the repressor and causes the protein to change
shape and fall off the DNA.
●● What is the mutant phenotype of bacteria that lack a functional lacY
gene?
Answer: They do not accumulate lactose inside the cell.
●● What effects will growth in glucose and lactose have on E. coli sugar
metabolism and gene expression?
Answer: The bacteria will use glucose as their primary sugar source until
it is used up and then switch to lactose.
●● The lactose operon is likely to be transcribed when __________.
,Answer: lactose levels are high within the cell
(The lactose operon is likely to be transcribed when lactose levels are
high within the cell.
The presence of lactose in the cell increases as the lac operon is turned
on due to the increased production of the lactose permease. The presence
of high levels of lactose would result in release of the repressor protein
(the product of the lacI gene) from the operator DNA sequence and
transcription of the lacZ, lacY, and lacA genes.
When lactose levels are low, the lac operon is not likely to be transcribed
due to binding of the repressor protein to the operator sequence.
The presence of glucose and no lactose in the cell would result in tight
binding of the repressor to the operator sequence and inhibition of
transcription of the lac operon.
Glucose inhibits the lactose transport activity of galactoside permease.
When both glucose and lactose are present in the environment, the
transport of lactose into the cell is inhibited. Because lactose does not
accumulate in the cytoplasm, the repressor remains bound to the
operator.)
●● Which of the following experiments would help you to determine
whether the β-galactosidase gene is induced by lactose or glucose?
, Answer: Measure the amount of β-galactosidase produced by E. coli
grown on a glucose plate, a lactose plate, and a glucose + lactose plate.
●● Which of the following choices (A-D) best represent(s) the
mechanisms for control of gene expression that can occur in bacteria?
A. reduced production of mRNA
B. increased life span of mRNA molecules
C. decreased efficiency of mRNA translation
D. protein inactivation
Answer: All of them (A, B, C, and D) can occur in bacteria.
●● In a mouse experiment, researchers used the lac operon to control the
expression of the tyrosinase gene. If the tyrosinase gene contained the
protein-coding sequence and the normal mouse regulatory sequences,
these regulatory sequences would provide positive control of tyrosinase
transcription. If nothing else was done to this DNA and it was introduced
into a mouse, tyrosinase would always be expressed. Because the goal
was to regulate expression of tyrosinase, what portion of the lac operon
should be added to this mouse gene?
Answer: The lac operator
(Addition of the lac operator regulatory DNA sequence could allow for
additional regulation of the tyrosinase gene to be imposed. However,
additional sequences would also need to be added so that the operator
sequence could be used as a regulatory element in a eukaryote like a
mouse.
ANSWERS GRADED A PLUS STUDY
RESOURCE
●● Which of the following statements best summarizes how the lac
operon is regulated?
Answer: Positive and negative control elements regulate the lac operon.
●● What happens when the interaction of the repressor protein with the
DNA of the lac operator is disrupted?
Answer: Lactose binds the repressor and causes the protein to change
shape and fall off the DNA.
●● What is the mutant phenotype of bacteria that lack a functional lacY
gene?
Answer: They do not accumulate lactose inside the cell.
●● What effects will growth in glucose and lactose have on E. coli sugar
metabolism and gene expression?
Answer: The bacteria will use glucose as their primary sugar source until
it is used up and then switch to lactose.
●● The lactose operon is likely to be transcribed when __________.
,Answer: lactose levels are high within the cell
(The lactose operon is likely to be transcribed when lactose levels are
high within the cell.
The presence of lactose in the cell increases as the lac operon is turned
on due to the increased production of the lactose permease. The presence
of high levels of lactose would result in release of the repressor protein
(the product of the lacI gene) from the operator DNA sequence and
transcription of the lacZ, lacY, and lacA genes.
When lactose levels are low, the lac operon is not likely to be transcribed
due to binding of the repressor protein to the operator sequence.
The presence of glucose and no lactose in the cell would result in tight
binding of the repressor to the operator sequence and inhibition of
transcription of the lac operon.
Glucose inhibits the lactose transport activity of galactoside permease.
When both glucose and lactose are present in the environment, the
transport of lactose into the cell is inhibited. Because lactose does not
accumulate in the cytoplasm, the repressor remains bound to the
operator.)
●● Which of the following experiments would help you to determine
whether the β-galactosidase gene is induced by lactose or glucose?
, Answer: Measure the amount of β-galactosidase produced by E. coli
grown on a glucose plate, a lactose plate, and a glucose + lactose plate.
●● Which of the following choices (A-D) best represent(s) the
mechanisms for control of gene expression that can occur in bacteria?
A. reduced production of mRNA
B. increased life span of mRNA molecules
C. decreased efficiency of mRNA translation
D. protein inactivation
Answer: All of them (A, B, C, and D) can occur in bacteria.
●● In a mouse experiment, researchers used the lac operon to control the
expression of the tyrosinase gene. If the tyrosinase gene contained the
protein-coding sequence and the normal mouse regulatory sequences,
these regulatory sequences would provide positive control of tyrosinase
transcription. If nothing else was done to this DNA and it was introduced
into a mouse, tyrosinase would always be expressed. Because the goal
was to regulate expression of tyrosinase, what portion of the lac operon
should be added to this mouse gene?
Answer: The lac operator
(Addition of the lac operator regulatory DNA sequence could allow for
additional regulation of the tyrosinase gene to be imposed. However,
additional sequences would also need to be added so that the operator
sequence could be used as a regulatory element in a eukaryote like a
mouse.
