BIOL 3200 Exam 3 Actual Exam V3 | BIOL 3200 General Microbiology
(BIOL 3200 Exam 3) | Auburn University
1. Which component of the RNA polymerase holoenzyme is responsible for recognizing the
promoter sequence in prokaryotes?
A. Sigma factor
B. Beta subunit
C. Alpha subunit
D. Rho protein
Answer: A
Rationale: The sigma factor is a protein needed for initiation of transcription in bacteria. It
enables specific binding of RNA polymerase to gene promoters. Once transcription begins,
the sigma factor is typically released from the core enzyme.
2. In the lac operon, what is the specific role of allolactose?
A. It acts as a corepressor by binding to the operator.
B. It facilitates the binding of RNA polymerase to the promoter.
C. It acts as an inducer by binding to the repressor protein.
D. It increases the levels of cAMP in the cell.
Answer: C
Rationale: Allolactose is an isomer of lactose that functions as the inducer molecule for the
lac operon. When allolactose binds to the LacI repressor, it causes a conformational change
that prevents the repressor from binding to the operator. This relief of repression allows
RNA polymerase to transcribe the structural genes.
3. Which enzyme is primarily responsible for synthesizing the new DNA strand during
replication in E. coli?
A. DNA Polymerase I
B. DNA Polymerase II
C. DNA Polymerase III
D. RNA Polymerase
Answer: C
,Rationale: DNA Polymerase III is the main replicative enzyme in bacteria that adds
nucleotides to the 3’ end of the growing DNA strand. It possesses high processivity and
proofreading capabilities to ensure accuracy. DNA Polymerase I is mainly involved in
removing RNA primers and filling gaps.
4. What occurs during the process of specialized transduction?
A. Specific host genes adjacent to the phage integration site are transferred.
B. Random fragments of host DNA are packaged into phage heads.
C. Naked DNA is taken up by a competent cell from the environment.
D. DNA is transferred through a sex pilus via cell-to-cell contact.
Answer: A
Rationale: Specialized transduction occurs when a temperate bacteriophage incorrectly
excises from the host genome. Because the phage integrates at a specific site (like ‘att’ in
lambda), it only carries nearby bacterial genes. This differs from generalized transduction
where any part of the host genome can be packaged.
5. Which type of mutation results in a premature stop codon and a truncated protein?
A. Missense mutation
B. Silent mutation
C. Nonsense mutation
D. Frameshift mutation
Answer: C
Rationale: A nonsense mutation changes a codon that specifies an amino acid into a
termination (stop) codon. This causes translation to end early, resulting in a shorter, often
non-functional protein. Missense mutations, by contrast, merely change one amino acid to
another.
6. An Hfr (High Frequency of Recombination) strain is defined by which characteristic?
A. It has the F factor integrated into the bacterial chromosome.
B. It contains an F plasmid existing independently in the cytoplasm.
C. It lacks the F factor and acts as a recipient cell.
D. It contains a mutated F factor that cannot form a pilus.
Answer: A
Rationale: In Hfr strains, the F plasmid has integrated into the host cell’s chromosome via
homologous recombination. During conjugation, the Hfr cell attempts to transfer its entire
, chromosome to the recipient, starting from the integrated F factor. This leads to a high
frequency of transfer of chromosomal genes compared to standard F+ cells.
7. Which molecular feature is common to the recognition sites of most Type II restriction
endonucleases?
A. High AT content
B. Methylated cytosines
C. Poly-A tails
D. Palindromic sequences
Answer: D
Rationale: Most Type II restriction enzymes recognize specific DNA sequences that are
palindromic, meaning they read the same 5’ to 3’ on both strands. These enzymes cut the
DNA at or near these specific sites to produce either blunt or ‘sticky’ ends. This specificity is
crucial for recombinant DNA technology and molecular cloning.
8. What is the primary function of the Shine-Dalgarno sequence in prokaryotic translation?
A. It serves as the binding site for the 16S rRNA in the small ribosomal subunit.
B. It signals the termination of transcription.
C. It is the site where the repressor binds to block the operon.
D. It acts as a start codon for the methionine amino acid.
Answer: A
Rationale: The Shine-Dalgarno sequence is a ribosomal binding site in bacterial messenger
RNA, generally located 8 bases upstream of the start codon AUG. It is complementary to a
sequence at the 3’ end of the 16S rRNA. This base-pairing helps align the ribosome
correctly on the mRNA to initiate protein synthesis.
9. In the context of the tryptophan (trp) operon, what occurs when tryptophan levels are
high?
A. Tryptophan acts as a corepressor, activating the repressor to block transcription.
B. The repressor is inactive and transcription proceeds.
C. The ribosome stalls at the leader sequence, allowing transcription.
D. The cAMP levels rise to activate the operon.
Answer: A
Rationale: The trp operon is a repressible system where the presence of tryptophan
inhibits gene expression. Tryptophan binds to the Trp repressor protein, enabling it to bind
(BIOL 3200 Exam 3) | Auburn University
1. Which component of the RNA polymerase holoenzyme is responsible for recognizing the
promoter sequence in prokaryotes?
A. Sigma factor
B. Beta subunit
C. Alpha subunit
D. Rho protein
Answer: A
Rationale: The sigma factor is a protein needed for initiation of transcription in bacteria. It
enables specific binding of RNA polymerase to gene promoters. Once transcription begins,
the sigma factor is typically released from the core enzyme.
2. In the lac operon, what is the specific role of allolactose?
A. It acts as a corepressor by binding to the operator.
B. It facilitates the binding of RNA polymerase to the promoter.
C. It acts as an inducer by binding to the repressor protein.
D. It increases the levels of cAMP in the cell.
Answer: C
Rationale: Allolactose is an isomer of lactose that functions as the inducer molecule for the
lac operon. When allolactose binds to the LacI repressor, it causes a conformational change
that prevents the repressor from binding to the operator. This relief of repression allows
RNA polymerase to transcribe the structural genes.
3. Which enzyme is primarily responsible for synthesizing the new DNA strand during
replication in E. coli?
A. DNA Polymerase I
B. DNA Polymerase II
C. DNA Polymerase III
D. RNA Polymerase
Answer: C
,Rationale: DNA Polymerase III is the main replicative enzyme in bacteria that adds
nucleotides to the 3’ end of the growing DNA strand. It possesses high processivity and
proofreading capabilities to ensure accuracy. DNA Polymerase I is mainly involved in
removing RNA primers and filling gaps.
4. What occurs during the process of specialized transduction?
A. Specific host genes adjacent to the phage integration site are transferred.
B. Random fragments of host DNA are packaged into phage heads.
C. Naked DNA is taken up by a competent cell from the environment.
D. DNA is transferred through a sex pilus via cell-to-cell contact.
Answer: A
Rationale: Specialized transduction occurs when a temperate bacteriophage incorrectly
excises from the host genome. Because the phage integrates at a specific site (like ‘att’ in
lambda), it only carries nearby bacterial genes. This differs from generalized transduction
where any part of the host genome can be packaged.
5. Which type of mutation results in a premature stop codon and a truncated protein?
A. Missense mutation
B. Silent mutation
C. Nonsense mutation
D. Frameshift mutation
Answer: C
Rationale: A nonsense mutation changes a codon that specifies an amino acid into a
termination (stop) codon. This causes translation to end early, resulting in a shorter, often
non-functional protein. Missense mutations, by contrast, merely change one amino acid to
another.
6. An Hfr (High Frequency of Recombination) strain is defined by which characteristic?
A. It has the F factor integrated into the bacterial chromosome.
B. It contains an F plasmid existing independently in the cytoplasm.
C. It lacks the F factor and acts as a recipient cell.
D. It contains a mutated F factor that cannot form a pilus.
Answer: A
Rationale: In Hfr strains, the F plasmid has integrated into the host cell’s chromosome via
homologous recombination. During conjugation, the Hfr cell attempts to transfer its entire
, chromosome to the recipient, starting from the integrated F factor. This leads to a high
frequency of transfer of chromosomal genes compared to standard F+ cells.
7. Which molecular feature is common to the recognition sites of most Type II restriction
endonucleases?
A. High AT content
B. Methylated cytosines
C. Poly-A tails
D. Palindromic sequences
Answer: D
Rationale: Most Type II restriction enzymes recognize specific DNA sequences that are
palindromic, meaning they read the same 5’ to 3’ on both strands. These enzymes cut the
DNA at or near these specific sites to produce either blunt or ‘sticky’ ends. This specificity is
crucial for recombinant DNA technology and molecular cloning.
8. What is the primary function of the Shine-Dalgarno sequence in prokaryotic translation?
A. It serves as the binding site for the 16S rRNA in the small ribosomal subunit.
B. It signals the termination of transcription.
C. It is the site where the repressor binds to block the operon.
D. It acts as a start codon for the methionine amino acid.
Answer: A
Rationale: The Shine-Dalgarno sequence is a ribosomal binding site in bacterial messenger
RNA, generally located 8 bases upstream of the start codon AUG. It is complementary to a
sequence at the 3’ end of the 16S rRNA. This base-pairing helps align the ribosome
correctly on the mRNA to initiate protein synthesis.
9. In the context of the tryptophan (trp) operon, what occurs when tryptophan levels are
high?
A. Tryptophan acts as a corepressor, activating the repressor to block transcription.
B. The repressor is inactive and transcription proceeds.
C. The ribosome stalls at the leader sequence, allowing transcription.
D. The cAMP levels rise to activate the operon.
Answer: A
Rationale: The trp operon is a repressible system where the presence of tryptophan
inhibits gene expression. Tryptophan binds to the Trp repressor protein, enabling it to bind
