Question
A repressible operon in bacteria codes for the genes required to produce the amino acid serine. Serine functions as a corepressor for the operon. If serine is present in the bacteria’s environment, which of the following is most likely?
(A) increased digestion of serine
(B) increase in the levels of serine
(C) increased production of the repressor protein
(D) increased binding of the repressor protein to the operator
▶️Answer/Explanation
Ans:
(D) Corepressors help the repressor protein bind to the operator. Since
serine is a corepressor, its presence would result in increased binding of
the repressor protein to the operator. Choice (A) is incorrect because
repressible operons are usually anabolic, not catabolic, in function and
thus they would not be involved in the digestion of serine. Since serine
functions as a corepressor, the presence of serine would reduce the
expression of the operon and would not result in increased levels of
serine. Thus, choice (B) is incorrect. The presence of the corepressor
does not affect the production of the repressor protein, so choice (C) is
incorrect.
Question
The arabinose operon is an inducible operon that codes for the genes required to digest the sugar arabinose. Arabinose functions as an
inducer molecule for the operon. If arabinose is present in the bacteria’s environment, which of the following is most likely?
(A) increased digestion of arabinose
(B) increase in the levels of arabinose
(C) increased production of the repressor protein
(D) increased binding of the repressor protein to the operator
▶️Answer/Explanation
Ans:
(A) In inducible operons, the inducer molecule binds to the repressor
protein, which prevents the repressor protein from binding to the
operator, increasing the level of expression of the operon. Choice (B) is
incorrect because inducible operons are usually catabolic, not anabolic,
and would not be involved in the production of arabinose. The presence
of the inducer does not affect the production of the repressor protein, so
choice (C) is incorrect. The presence of the inducer results in decreased
binding of the repressor protein to the operator, so choice (D) is also
incorrect.
Question
Which sequences in bacterial operons are noncoding sequences?
(A) operators and promoters only
(B) operators, promoters, and genes for regulatory proteins
(C) operators, promoters, and genes for structural proteins
(D) promoters, genes for regulatory proteins, and genes for structural
proteins
▶️Answer/Explanation
Ans:
(A) The operators and promoters of operons both serve as noncoding
sequences to which regulatory proteins bind. Repressor proteins bind to
the operator, and RNA polymerase binds to the promoter. Genes for
both regulatory proteins and structural proteins contain sequences that
code for proteins, so choices (B), (C), and (D) are incorrect.
Question
Which of the following is a key difference between inducible operons and repressible operons?
(A) Inducible operons have promoter sequences, and repressible
operons do not have promoter sequences.
(B) Inducible operons have operator sequences, and repressible
operons do not have operator sequences.
(C) The repressor in an inducible operon can bind to the operator
sequence on its own, but the repressor in a repressible operon
requires a corepressor in order to bind to the operator.
(D) In inducible operons, RNA polymerase requires the presence of a
corepressor in order to bind to the promoter; in repressible
operons, RNA polymerase does not require a corepressor.
▶️Answer/Explanation
Ans:
(C) In inducible operons, repressor proteins can bind to the operator
without the assistance of a corepressor. In repressible operons, the
presence of a corepressor is required in order for the repressor protein
to bind to the operator sequence. Choice (A) is incorrect because both
inducible and repressible operons have promoter sequences. Operator sequences are present in both inducible and repressible operons, so
choice (B) is incorrect. RNA polymerase does not require a corepressor
in order to bind to the promoter in both inducible and repressible
operons, so choice (D) is incorrect.