PCB 3063 Exam 4 V3 | PCB 3063 Genetics
| Actual Q&A with Rationale (PCB3063
Exam 4) | University of Central Florida
1. In the presence of both glucose and lactose, which of the following describes the state of
the lac operon?
A. High levels of transcription occur because lactose is present.
B. Basal levels of transcription occur as CAP is inactive due to low cAMP.
C. The repressor is bound to the operator, preventing transcription.
D. Transcription is completely inhibited by the presence of glucose regardless of lactose.
Answer: B
Rationale: When glucose is present, adenylate cyclase is inhibited, leading to low levels of
cAMP. Consequently, the CAP-cAMP complex does not form to enhance RNA polymerase
recruitment. This results in only a low, basal level of transcription even if the repressor is
removed by lactose.
2. Which mechanism is responsible for the attenuation of the trp operon in E. coli?
A. The binding of the Trp repressor to the operator sequence.
B. The formation of a 3-4 hairpin loop in the leader mRNA when tryptophan is abundant.
C. The degradation of the mRNA by specialized nucleases in the cytosol.
D. The interaction between glucose levels and the CAP-cAMP activator complex.
,Answer: B
Rationale: Attenuation relies on the coupled nature of transcription and translation in
prokaryotes. When tryptophan levels are high, the ribosome moves quickly through the
leader sequence, allowing the 3-4 termination hairpin to form. This structure causes RNA
polymerase to detach before reaching the structural genes.
3. What is the primary effect of histone acetylation on eukaryotic gene expression?
A. It neutralizes positive charges on histone tails, loosening chromatin structure.
B. It facilitates the recruitment of DNA methyltransferases to promoters.
C. It causes DNA to wrap more tightly around the histone octamer.
D. It directly inhibits the binding of the TATA-binding protein to the DNA.
Answer: A
Rationale: Histone acetyltransferases (HATs) add acetyl groups to lysine residues on the
N-terminal tails of histones. This modification reduces the affinity between histones and
the negatively charged DNA backbone. The resulting open chromatin, or euchromatin, is
more accessible to the transcriptional machinery.
4. In eukaryotic cells, where are enhancers typically located relative to the gene they
regulate?
A. They can be located upstream, downstream, or within introns of the gene.
B. Strictly within the first 100 base pairs upstream of the TATA box.
, C. Only on the sister chromatid that is not being actively transcribed.
D. At the 3’ end of the mRNA molecule after the poly-A tail.
Answer: A
Rationale: Enhancers are cis-acting elements that function independently of their
orientation and distance from the promoter. They can be found thousands of base pairs
away from the transcription start site. DNA looping allows proteins bound at enhancers to
interact with the basal transcription apparatus.
5. Which of the following describes the function of the Bicoid protein in Drosophila
development?
A. It functions as a transcription factor that establishes the anterior-posterior axis.
B. It acts as a signaling molecule for the posterior end of the embryo.
C. It is a structural protein that forms the cellular membrane of the blastoderm.
D. It inhibits the translation of all maternal mRNAs in the zygote.
Answer: A
Rationale: Bicoid is a maternal effect gene that forms a concentration gradient from the
anterior pole. It acts as both a transcription factor and a translation repressor to regulate
downstream gap genes. High levels of Bicoid are essential for the development of head and
thorax structures.
| Actual Q&A with Rationale (PCB3063
Exam 4) | University of Central Florida
1. In the presence of both glucose and lactose, which of the following describes the state of
the lac operon?
A. High levels of transcription occur because lactose is present.
B. Basal levels of transcription occur as CAP is inactive due to low cAMP.
C. The repressor is bound to the operator, preventing transcription.
D. Transcription is completely inhibited by the presence of glucose regardless of lactose.
Answer: B
Rationale: When glucose is present, adenylate cyclase is inhibited, leading to low levels of
cAMP. Consequently, the CAP-cAMP complex does not form to enhance RNA polymerase
recruitment. This results in only a low, basal level of transcription even if the repressor is
removed by lactose.
2. Which mechanism is responsible for the attenuation of the trp operon in E. coli?
A. The binding of the Trp repressor to the operator sequence.
B. The formation of a 3-4 hairpin loop in the leader mRNA when tryptophan is abundant.
C. The degradation of the mRNA by specialized nucleases in the cytosol.
D. The interaction between glucose levels and the CAP-cAMP activator complex.
,Answer: B
Rationale: Attenuation relies on the coupled nature of transcription and translation in
prokaryotes. When tryptophan levels are high, the ribosome moves quickly through the
leader sequence, allowing the 3-4 termination hairpin to form. This structure causes RNA
polymerase to detach before reaching the structural genes.
3. What is the primary effect of histone acetylation on eukaryotic gene expression?
A. It neutralizes positive charges on histone tails, loosening chromatin structure.
B. It facilitates the recruitment of DNA methyltransferases to promoters.
C. It causes DNA to wrap more tightly around the histone octamer.
D. It directly inhibits the binding of the TATA-binding protein to the DNA.
Answer: A
Rationale: Histone acetyltransferases (HATs) add acetyl groups to lysine residues on the
N-terminal tails of histones. This modification reduces the affinity between histones and
the negatively charged DNA backbone. The resulting open chromatin, or euchromatin, is
more accessible to the transcriptional machinery.
4. In eukaryotic cells, where are enhancers typically located relative to the gene they
regulate?
A. They can be located upstream, downstream, or within introns of the gene.
B. Strictly within the first 100 base pairs upstream of the TATA box.
, C. Only on the sister chromatid that is not being actively transcribed.
D. At the 3’ end of the mRNA molecule after the poly-A tail.
Answer: A
Rationale: Enhancers are cis-acting elements that function independently of their
orientation and distance from the promoter. They can be found thousands of base pairs
away from the transcription start site. DNA looping allows proteins bound at enhancers to
interact with the basal transcription apparatus.
5. Which of the following describes the function of the Bicoid protein in Drosophila
development?
A. It functions as a transcription factor that establishes the anterior-posterior axis.
B. It acts as a signaling molecule for the posterior end of the embryo.
C. It is a structural protein that forms the cellular membrane of the blastoderm.
D. It inhibits the translation of all maternal mRNAs in the zygote.
Answer: A
Rationale: Bicoid is a maternal effect gene that forms a concentration gradient from the
anterior pole. It acts as both a transcription factor and a translation repressor to regulate
downstream gap genes. High levels of Bicoid are essential for the development of head and
thorax structures.
