BIOL 3200 Exam 3 Actual Exam V1 | BIOL 3200 General Microbiology
(BIOL 3200 Exam 3) | Auburn University
1. Which enzyme is primarily responsible for synthesizing the leading strand during DNA
replication in Escherichia coli?
A. DNA Polymerase I
B. Primase
C. DNA Polymerase III
D. Helicase
Answer: C
Rationale: DNA Polymerase III is the main replicative enzyme in prokaryotes that
catalyzes the synthesis of both the leading and lagging strands. It possesses high
processivity due to the beta-clamp subunit which keeps it attached to the DNA template.
This enzyme adds nucleotides in a 5’ to 3’ direction, requiring a free 3’ OH group provided
by an RNA primer.
2. In prokaryotic transcription, what is the specific role of the sigma factor?
A. Catalyzing the formation of phosphodiester bonds
B. Recognition of the promoter sequence
C. Unwinding the DNA double helix at the replication fork
D. Termination of the RNA transcript
Answer: B
Rationale: The sigma factor is a protein needed for initiation of transcription in bacteria,
enabling specific binding of RNA polymerase to gene promoters. Once the RNA polymerase
core enzyme has successfully initiated transcription, the sigma factor is usually released.
This mechanism allows the cell to regulate which genes are transcribed in response to
environmental changes by using different sigma factors.
3. Which horizontal gene transfer mechanism involves the uptake of ‘naked’ DNA from the
surrounding environment?
A. Conjugation
B. Transduction
C. Transformation
D. Transposition
,Answer: C
Rationale: Transformation is the process by which a competent bacterial cell takes up free
DNA fragments from its environment that were released by dead cells. For this to occur, the
recipient cell must be in a state of competence, which can be natural or induced in a
laboratory setting. Once inside, the DNA can be integrated into the host genome through
homologous recombination.
4. What occurs when the lac repressor binds to the operator region of the lac operon?
A. Transcription of the structural genes is enhanced
B. The CAP protein is activated by cAMP
C. Lactose is converted into allolactose
D. RNA polymerase is physically blocked from transcribing the genes
Answer: D
Rationale: The lac repressor is a protein that negatively regulates the lac operon by
binding to the operator site in the absence of lactose. This binding prevents RNA
polymerase from moving forward and transcribing the Z, Y, and A genes. When allolactose
is present, it binds to the repressor, causing a conformational change that releases it from
the DNA.
5. A mutation that results in a premature stop codon is known as a:
A. Silent mutation
B. Nonsense mutation
C. Frameshift mutation
D. Missense mutation
Answer: B
Rationale: A nonsense mutation occurs when a point mutation changes a codon that
specifies an amino acid into a termination codon. This leads to the production of a
truncated, and usually non-functional, protein product. Such mutations are often
deleterious to the organism because they disrupt the primary structure of essential
enzymes.
6. The process of generalized transduction is mediated by which of the following?
A. Bacteriophages
B. F-plasmids
C. Competence factors
D. Insertion sequences
, Answer: A
Rationale: Generalized transduction occurs during the lytic cycle of a bacteriophage when
random fragments of bacterial DNA are accidentally packaged into a phage head. These
‘transducing particles’ then deliver the host DNA to a new recipient cell upon infection. This
mechanism is a significant driver of genetic diversity and the spread of antibiotic resistance
in bacterial populations.
7. During DNA replication, Okazaki fragments are joined together by which enzyme?
A. DNA Polymerase II
B. DNA Gyrase
C. DNA Ligase
D. Topoisomerase
Answer: C
Rationale: DNA Ligase is essential for sealing nicks in the sugar-phosphate backbone of
DNA during replication and repair. It catalyzes the formation of a phosphodiester bond
between the 3’-OH and 5’-phosphate ends of adjacent nucleotides. This action is
particularly crucial on the lagging strand to connect the discontinuous Okazaki fragments
into a continuous strand.
8. What is the function of the Shine-Dalgarno sequence in bacteria?
A. It acts as a binding site for the 16S rRNA of the ribosome
B. It signals the termination of transcription
C. It serves as the origin of replication
D. It codes for the first methionine in a protein
Answer: A
Rationale: The Shine-Dalgarno sequence is a ribosomal binding site in bacterial messenger
RNA, generally located around 8 bases upstream of the start codon AUG. It helps recruit the
30S ribosomal subunit to the mRNA to initiate protein synthesis by aligning the ribosome
with the start codon. This sequence is complementary to a region at the 3’ end of the 16S
rRNA molecule.
9. In the context of the tryptophan (trp) operon, what is ‘attenuation’?
A. The degradation of mRNA by RNases
B. A form of transcriptional regulation based on the rate of translation
C. The binding of a repressor to the operator
D. A post-translational modification of the TrpE enzyme
(BIOL 3200 Exam 3) | Auburn University
1. Which enzyme is primarily responsible for synthesizing the leading strand during DNA
replication in Escherichia coli?
A. DNA Polymerase I
B. Primase
C. DNA Polymerase III
D. Helicase
Answer: C
Rationale: DNA Polymerase III is the main replicative enzyme in prokaryotes that
catalyzes the synthesis of both the leading and lagging strands. It possesses high
processivity due to the beta-clamp subunit which keeps it attached to the DNA template.
This enzyme adds nucleotides in a 5’ to 3’ direction, requiring a free 3’ OH group provided
by an RNA primer.
2. In prokaryotic transcription, what is the specific role of the sigma factor?
A. Catalyzing the formation of phosphodiester bonds
B. Recognition of the promoter sequence
C. Unwinding the DNA double helix at the replication fork
D. Termination of the RNA transcript
Answer: B
Rationale: The sigma factor is a protein needed for initiation of transcription in bacteria,
enabling specific binding of RNA polymerase to gene promoters. Once the RNA polymerase
core enzyme has successfully initiated transcription, the sigma factor is usually released.
This mechanism allows the cell to regulate which genes are transcribed in response to
environmental changes by using different sigma factors.
3. Which horizontal gene transfer mechanism involves the uptake of ‘naked’ DNA from the
surrounding environment?
A. Conjugation
B. Transduction
C. Transformation
D. Transposition
,Answer: C
Rationale: Transformation is the process by which a competent bacterial cell takes up free
DNA fragments from its environment that were released by dead cells. For this to occur, the
recipient cell must be in a state of competence, which can be natural or induced in a
laboratory setting. Once inside, the DNA can be integrated into the host genome through
homologous recombination.
4. What occurs when the lac repressor binds to the operator region of the lac operon?
A. Transcription of the structural genes is enhanced
B. The CAP protein is activated by cAMP
C. Lactose is converted into allolactose
D. RNA polymerase is physically blocked from transcribing the genes
Answer: D
Rationale: The lac repressor is a protein that negatively regulates the lac operon by
binding to the operator site in the absence of lactose. This binding prevents RNA
polymerase from moving forward and transcribing the Z, Y, and A genes. When allolactose
is present, it binds to the repressor, causing a conformational change that releases it from
the DNA.
5. A mutation that results in a premature stop codon is known as a:
A. Silent mutation
B. Nonsense mutation
C. Frameshift mutation
D. Missense mutation
Answer: B
Rationale: A nonsense mutation occurs when a point mutation changes a codon that
specifies an amino acid into a termination codon. This leads to the production of a
truncated, and usually non-functional, protein product. Such mutations are often
deleterious to the organism because they disrupt the primary structure of essential
enzymes.
6. The process of generalized transduction is mediated by which of the following?
A. Bacteriophages
B. F-plasmids
C. Competence factors
D. Insertion sequences
, Answer: A
Rationale: Generalized transduction occurs during the lytic cycle of a bacteriophage when
random fragments of bacterial DNA are accidentally packaged into a phage head. These
‘transducing particles’ then deliver the host DNA to a new recipient cell upon infection. This
mechanism is a significant driver of genetic diversity and the spread of antibiotic resistance
in bacterial populations.
7. During DNA replication, Okazaki fragments are joined together by which enzyme?
A. DNA Polymerase II
B. DNA Gyrase
C. DNA Ligase
D. Topoisomerase
Answer: C
Rationale: DNA Ligase is essential for sealing nicks in the sugar-phosphate backbone of
DNA during replication and repair. It catalyzes the formation of a phosphodiester bond
between the 3’-OH and 5’-phosphate ends of adjacent nucleotides. This action is
particularly crucial on the lagging strand to connect the discontinuous Okazaki fragments
into a continuous strand.
8. What is the function of the Shine-Dalgarno sequence in bacteria?
A. It acts as a binding site for the 16S rRNA of the ribosome
B. It signals the termination of transcription
C. It serves as the origin of replication
D. It codes for the first methionine in a protein
Answer: A
Rationale: The Shine-Dalgarno sequence is a ribosomal binding site in bacterial messenger
RNA, generally located around 8 bases upstream of the start codon AUG. It helps recruit the
30S ribosomal subunit to the mRNA to initiate protein synthesis by aligning the ribosome
with the start codon. This sequence is complementary to a region at the 3’ end of the 16S
rRNA molecule.
9. In the context of the tryptophan (trp) operon, what is ‘attenuation’?
A. The degradation of mRNA by RNases
B. A form of transcriptional regulation based on the rate of translation
C. The binding of a repressor to the operator
D. A post-translational modification of the TrpE enzyme
