PCB 3063 Exam 4 V2 | PCB 3063 Genetics
| Actual Q&A with Rationale (PCB3063
Exam 4) | University of Central Florida
1. In the context of the lac operon in E. coli, what occurs when both glucose and lactose are
present in the medium?
A. Transcription occurs at a high rate because lactose is present.
B. Transcription occurs at a very low level because glucose inhibits cAMP production,
preventing CAP binding.
C. The repressor remains bound to the operator because glucose is the preferred energy
source.
D. The lac operon is completely shut down and no mRNA is produced at all.
Answer: B
Rationale: When glucose is present, adenylate cyclase activity is low, resulting in low
levels of cAMP. Without cAMP, the Catabolite Activator Protein (CAP) cannot bind to the
promoter to recruit RNA polymerase effectively. Therefore, even if lactose is present to
remove the repressor, transcription remains at a basal, low level due to the lack of positive
activation.
2. Which mechanism of gene regulation is unique to prokaryotes because transcription and
translation are coupled?
A. Alternative Splicing
,B. Chromatin Remodeling
C. Attenuation
D. RNA Interference
Answer: C
Rationale: Attenuation is a regulatory mechanism found in operons like the trp operon
where the speed of translation affects the folding of the mRNA leader sequence. Because
prokaryotes do not have a nuclear envelope, ribosomes begin translating mRNA while it is
still being transcribed. This allows the formation of specific secondary structures that can
prematurely terminate transcription.
3. What is the primary function of the TATA box in eukaryotic promoters?
A. It acts as an enhancer to increase the rate of transcription.
B. It serves as a recognition site for the binding of TFIID and the assembly of the pre-
initiation complex.
C. It serves as the binding site for the rho factor.
D. It is the site where the poly-A tail is added to the mRNA.
Answer: B
Rationale: The TATA box is a conserved DNA sequence located approximately 25-30 base
pairs upstream of the transcription start site. It is recognized by the TATA-binding protein
(TBP), which is a component of the general transcription factor TFIID. The binding of TFIID
, facilitates the recruitment of other transcription factors and RNA Polymerase II to initiate
transcription.
4. A mutation in a tumor-suppressor gene typically follows which pattern of inheritance at
the cellular level to contribute to cancer?
A. Haploinsufficiency where one mutant allele is enough to cause cancer.
B. Dominant gain-of-function mutation.
C. Epigenetic silencing of only the paternal allele.
D. Recessive loss-of-function, requiring ‘two hits’ to both alleles.
Answer: D
Rationale: Tumor-suppressor genes generally act in a recessive manner at the cellular
level, meaning both copies must be inactivated to lose growth control. This is often referred
to as Knudson’s ‘two-hit’ hypothesis. A single functional allele is usually sufficient to
maintain normal cell cycle regulation.
5. Which enzyme is responsible for synthesizing DNA from an RNA template during the
creation of a cDNA library?
A. DNA Polymerase III
B. Restriction Endonuclease
C. RNA Polymerase II
D. Reverse Transcriptase
| Actual Q&A with Rationale (PCB3063
Exam 4) | University of Central Florida
1. In the context of the lac operon in E. coli, what occurs when both glucose and lactose are
present in the medium?
A. Transcription occurs at a high rate because lactose is present.
B. Transcription occurs at a very low level because glucose inhibits cAMP production,
preventing CAP binding.
C. The repressor remains bound to the operator because glucose is the preferred energy
source.
D. The lac operon is completely shut down and no mRNA is produced at all.
Answer: B
Rationale: When glucose is present, adenylate cyclase activity is low, resulting in low
levels of cAMP. Without cAMP, the Catabolite Activator Protein (CAP) cannot bind to the
promoter to recruit RNA polymerase effectively. Therefore, even if lactose is present to
remove the repressor, transcription remains at a basal, low level due to the lack of positive
activation.
2. Which mechanism of gene regulation is unique to prokaryotes because transcription and
translation are coupled?
A. Alternative Splicing
,B. Chromatin Remodeling
C. Attenuation
D. RNA Interference
Answer: C
Rationale: Attenuation is a regulatory mechanism found in operons like the trp operon
where the speed of translation affects the folding of the mRNA leader sequence. Because
prokaryotes do not have a nuclear envelope, ribosomes begin translating mRNA while it is
still being transcribed. This allows the formation of specific secondary structures that can
prematurely terminate transcription.
3. What is the primary function of the TATA box in eukaryotic promoters?
A. It acts as an enhancer to increase the rate of transcription.
B. It serves as a recognition site for the binding of TFIID and the assembly of the pre-
initiation complex.
C. It serves as the binding site for the rho factor.
D. It is the site where the poly-A tail is added to the mRNA.
Answer: B
Rationale: The TATA box is a conserved DNA sequence located approximately 25-30 base
pairs upstream of the transcription start site. It is recognized by the TATA-binding protein
(TBP), which is a component of the general transcription factor TFIID. The binding of TFIID
, facilitates the recruitment of other transcription factors and RNA Polymerase II to initiate
transcription.
4. A mutation in a tumor-suppressor gene typically follows which pattern of inheritance at
the cellular level to contribute to cancer?
A. Haploinsufficiency where one mutant allele is enough to cause cancer.
B. Dominant gain-of-function mutation.
C. Epigenetic silencing of only the paternal allele.
D. Recessive loss-of-function, requiring ‘two hits’ to both alleles.
Answer: D
Rationale: Tumor-suppressor genes generally act in a recessive manner at the cellular
level, meaning both copies must be inactivated to lose growth control. This is often referred
to as Knudson’s ‘two-hit’ hypothesis. A single functional allele is usually sufficient to
maintain normal cell cycle regulation.
5. Which enzyme is responsible for synthesizing DNA from an RNA template during the
creation of a cDNA library?
A. DNA Polymerase III
B. Restriction Endonuclease
C. RNA Polymerase II
D. Reverse Transcriptase
