Ch. 18 Study Guide
A hypothetical bacterium isolated from a Martian sea uses silicone as its main
energy source. This silica-based sugar is one of many sugars presents on Mars, and
tests have shown that silicone is present everywhere on the planet. Which of the
following would be the most efficient type of control fro the production of silicase, the
enzyme this bacterium uses to metabolize silicose?
A. constitute transcription of silicase gene
B. negative control of transcription of silicase gene
✅✅
C. catabolite repression transcription of silicase gene
D. inducible operon in control of transcription of silicase gene - -A
A regulon is a set of genes controlled by:
A. one type of regulator of transcription
B. two or more different alternative sigma proteins
✅✅
C. many different types of promoters
D. glucose - -A
A type of mutation in the lac operator known as lacO^c prevents repressor benign to
DNA and uses constitutive transcription of the lac operon. Which of the following
secondary mutations might restore normal regulation to the lac operon in a lacO^c
mutant?
A. a lacI mutation that decreases the ability of the repressor to bind the inducer
B. a lacI mutation that produces a repressor than can recognize the mutated lacO^c
DNA sequence
C. a promoter mutation that prevents it from being recognized by sigma
✅✅
D. an RNA polymerase mutation that allows it to bind to the promoter without using
sigma - -B
✅✅
Activators bind to regulatory sequences in _____ and to ______ polymerase. -
-DNA, RNA
An example of negative control of a promoter element would be:
A. protein R binds to the promoter to induce transcription
B. protein R binds to RNA polymerase
✅✅
C. protein R is allosterically regulated by sucrose binding
D. Protein R binds to the promoter to inhibit transcription - -D
Constitutive lactose operon mutants would be at a metabolic disadvantage
compared to wild-type cells because:
A. they would metabolize lactose more slowly than wild-type cells.
B. they would exert energy converting a disaccharide (lactose) to monosaccharides
when this process is unnecessary
, C. constitutive mutations inhibit cell division processes.
✅✅
D. lactose metabolites would accumulate to toxic levels and harm the bacteria. -
-B
E. coli expresses genes for glycolytic enzyme constitutively. Why? - ✅✅-The
glycolytic enzymes are always needed in the cell because they are required to
produce ATP, and ATP is always needed.
✅✅
Explain the difference between positive and negative control over transcription -
-Positive control means that a regulatory protein, when present, causes
transcription to increase. Negative control means that a regulatory protein, when
present, prevents transcription
✅✅
How does inducer exclusion control gene expression in the lac operon? -
-When glucose and another sugar are present in the environment, inducer
exclusion prevents the use of the other sugar and allows only use of glucose.
If any of the following hypothetical drugs could be developed, which would be most
effective in preventing cholera?
A. a drug that increased the amount of the ToxR activator
B. a drug that blocked the DNA-binding activity of the activator
✅✅
C. a drug that increased rates of transcriptions in V. cholerae
D. a drug that increased rates of translation in V. cholerae - -B, since the
activator needs to bind to regulatory sequences to activate gene expression,
preventing DNA binding would cripple the region and prevent cholera.
Imagine that you are repeating the replica-plating procedure of Jacob and Monod to
find mutants that can't grow using lactose. After treating cells with a mutagen, you
anticipate a mutation rate of 1×10⁻⁴ lactose-nonutilizing mutants per mutagen-treated
✅✅
cell. Based on this estimate, how many cells should you replica-plate to have a good
chance of finding one mutant? - -At a rate of 1×10⁻⁴ mutants per cell, you would
on average find one mutant in every 10,000 (1×10⁴) cells. Therefor, you should
screen a bit more (~2-3 times more) than 10,000 cells to be reasonably sure of
finding at least one mutant.
In the lac operon, the repressor inhibits transcription when:
A. the repressor is bound to the inducer
B. the repressor is not bound to the inducer
✅✅
C. the repressor is bound to glucose
D. the repressor is not bound to the operator - -B
IPTG is a molecule with a structure very similar to lactose. IPTG can be transported
into cells by galactoside permeate and can bind to the lac repressor protein.
However, unlike lactose, IPTG is not broken down by β-galactosidase. Predict what
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