BIO 250 Exam 3 V1 | BIO 250 Microbiology | Actual Q&A with
Rationale (BIO250 Exam 3) | StraighterLine
1. Which enzyme is responsible for unwinding the DNA double helix by breaking hydrogen
bonds during the initiation of replication?
A. DNA Polymerase III
B. Helicase
C. DNA Ligase
D. Primase
Correct Answer: B
Explanation: Helicase is the specific enzyme that utilizes ATP to break the hydrogen bonds
between complementary base pairs. This action creates the replication fork, allowing other
enzymes to access the single-stranded DNA templates. Without helicase activity, the
double-stranded DNA would remain coiled and inaccessible for replication.
2. Select all that apply: Which of the following are characteristics of the DNA molecule in
prokaryotes?
A. Antiparallel strands
B. Composed of deoxyribonucleotides
C. Contains uracil instead of thymine
D. Double-stranded helix
E. Hydrogen bonding between A-T and G-C
F. Enclosed within a nuclear membrane
Correct Answer: A, B, D, E
Explanation: Prokaryotic DNA is a double-stranded, antiparallel helix composed of
deoxyribonucleotides containing thymine, not uracil. The strands are held together by
hydrogen bonds between specific nitrogenous bases, namely adenine with thymine and
guanine with cytosine. Unlike eukaryotes, prokaryotic DNA is not enclosed in a nuclear
membrane but resides in the nucleoid region.
3. In the process of transcription, what is the role of the sigma factor in bacteria?
A. It helps RNA polymerase recognize and bind to the promoter region.
B. It catalyzes the addition of ribonucleotides to the growing RNA chain.
C. It terminates the transcription process at the hairpin loop.
,D. It acts as a primer for DNA synthesis.
Correct Answer: A
Explanation: The sigma factor is a subunit of the bacterial RNA polymerase holoenzyme
that ensures site-specific initiation. It identifies the -10 and -35 promoter sequences to
position the enzyme correctly for transcription start. Once transcription begins and the
elongation phase is reached, the sigma factor typically dissociates from the core enzyme.
4. During translation, which site in the ribosome holds the tRNA that is carrying the next
amino acid to be added to the polypeptide chain?
A. E site
B. P site
C. A site
D. Promoter site
Correct Answer: C
Explanation: The A (aminoacyl) site is the entry point for the charged tRNA molecule
during the elongation phase of translation. It matches the tRNA anticodon with the mRNA
codon to ensure the correct amino acid is incorporated. The P (peptidyl) site holds the
tRNA with the growing chain, while the E (exit) site releases uncharged tRNAs.
5. A mutation that results in the replacement of a single amino acid with a different amino
acid in the resulting protein is known as a:
A. Missense mutation
B. Silent mutation
C. Nonsense mutation
D. Frameshift mutation
Correct Answer: A
Explanation: A missense mutation occurs when a point mutation changes a codon so that
it codes for a different amino acid. This can alter the protein’s function or stability
depending on the chemical properties of the substituted amino acid. In contrast, nonsense
mutations create premature stop codons, and silent mutations do not change the amino
acid sequence.
6. Which mechanism of horizontal gene transfer involves the uptake of ‘naked’ DNA from the
surrounding environment by a competent cell?
A. Conjugation
B. Transduction
, C. Transformation
D. Transposition
Correct Answer: C
Explanation: Transformation is the process where bacteria take up free DNA fragments
released by dead or lysed cells in their vicinity. For this to occur, the recipient cell must be
in a state of ‘competence,’ allowing the DNA to pass through the cell wall and membrane.
Once inside, the DNA may be integrated into the host genome through homologous
recombination.
7. In the lac operon, what happens when lactose is present in the environment but glucose is
absent?
A. The repressor remains bound to the operator, preventing transcription.
B. The promoter is degraded by nucleases.
C. Lactose binds to the repressor, inactivating it and allowing transcription.
D. RNA polymerase is inhibited by high levels of cAMP.
Correct Answer: C
Explanation: Lactose acts as an inducer by binding to the lac repressor protein, causing a
conformational change that prevents it from binding to the operator. This removes the
physical block, allowing RNA polymerase to transcribe the genes needed for lactose
metabolism. Low glucose levels also lead to high cAMP, which further activates the operon
via the Catabolite Activator Protein (CAP).
8. Which of the following describes specialized transduction?
A. Random fragments of host DNA are packaged into a phage head.
B. Plasmids are transferred through a sex pilus.
C. DNA is transferred between cells in direct contact.
D. Specific host genes near the phage integration site are transferred.
Correct Answer: D
Explanation: Specialized transduction occurs during the lysogenic cycle when a prophage
is excised incorrectly from the bacterial chromosome. Because the phage integrates at a
specific site, only the adjacent bacterial genes are captured and transferred to the next host.
This differs from generalized transduction, where any part of the bacterial genome can be
packaged into the phage head during the lytic cycle.
9. What is the primary function of the Polymerase Chain Reaction (PCR)?
A. To sequence an entire bacterial genome in one step.
B. To visualize proteins on a polyacrylamide gel.
Rationale (BIO250 Exam 3) | StraighterLine
1. Which enzyme is responsible for unwinding the DNA double helix by breaking hydrogen
bonds during the initiation of replication?
A. DNA Polymerase III
B. Helicase
C. DNA Ligase
D. Primase
Correct Answer: B
Explanation: Helicase is the specific enzyme that utilizes ATP to break the hydrogen bonds
between complementary base pairs. This action creates the replication fork, allowing other
enzymes to access the single-stranded DNA templates. Without helicase activity, the
double-stranded DNA would remain coiled and inaccessible for replication.
2. Select all that apply: Which of the following are characteristics of the DNA molecule in
prokaryotes?
A. Antiparallel strands
B. Composed of deoxyribonucleotides
C. Contains uracil instead of thymine
D. Double-stranded helix
E. Hydrogen bonding between A-T and G-C
F. Enclosed within a nuclear membrane
Correct Answer: A, B, D, E
Explanation: Prokaryotic DNA is a double-stranded, antiparallel helix composed of
deoxyribonucleotides containing thymine, not uracil. The strands are held together by
hydrogen bonds between specific nitrogenous bases, namely adenine with thymine and
guanine with cytosine. Unlike eukaryotes, prokaryotic DNA is not enclosed in a nuclear
membrane but resides in the nucleoid region.
3. In the process of transcription, what is the role of the sigma factor in bacteria?
A. It helps RNA polymerase recognize and bind to the promoter region.
B. It catalyzes the addition of ribonucleotides to the growing RNA chain.
C. It terminates the transcription process at the hairpin loop.
,D. It acts as a primer for DNA synthesis.
Correct Answer: A
Explanation: The sigma factor is a subunit of the bacterial RNA polymerase holoenzyme
that ensures site-specific initiation. It identifies the -10 and -35 promoter sequences to
position the enzyme correctly for transcription start. Once transcription begins and the
elongation phase is reached, the sigma factor typically dissociates from the core enzyme.
4. During translation, which site in the ribosome holds the tRNA that is carrying the next
amino acid to be added to the polypeptide chain?
A. E site
B. P site
C. A site
D. Promoter site
Correct Answer: C
Explanation: The A (aminoacyl) site is the entry point for the charged tRNA molecule
during the elongation phase of translation. It matches the tRNA anticodon with the mRNA
codon to ensure the correct amino acid is incorporated. The P (peptidyl) site holds the
tRNA with the growing chain, while the E (exit) site releases uncharged tRNAs.
5. A mutation that results in the replacement of a single amino acid with a different amino
acid in the resulting protein is known as a:
A. Missense mutation
B. Silent mutation
C. Nonsense mutation
D. Frameshift mutation
Correct Answer: A
Explanation: A missense mutation occurs when a point mutation changes a codon so that
it codes for a different amino acid. This can alter the protein’s function or stability
depending on the chemical properties of the substituted amino acid. In contrast, nonsense
mutations create premature stop codons, and silent mutations do not change the amino
acid sequence.
6. Which mechanism of horizontal gene transfer involves the uptake of ‘naked’ DNA from the
surrounding environment by a competent cell?
A. Conjugation
B. Transduction
, C. Transformation
D. Transposition
Correct Answer: C
Explanation: Transformation is the process where bacteria take up free DNA fragments
released by dead or lysed cells in their vicinity. For this to occur, the recipient cell must be
in a state of ‘competence,’ allowing the DNA to pass through the cell wall and membrane.
Once inside, the DNA may be integrated into the host genome through homologous
recombination.
7. In the lac operon, what happens when lactose is present in the environment but glucose is
absent?
A. The repressor remains bound to the operator, preventing transcription.
B. The promoter is degraded by nucleases.
C. Lactose binds to the repressor, inactivating it and allowing transcription.
D. RNA polymerase is inhibited by high levels of cAMP.
Correct Answer: C
Explanation: Lactose acts as an inducer by binding to the lac repressor protein, causing a
conformational change that prevents it from binding to the operator. This removes the
physical block, allowing RNA polymerase to transcribe the genes needed for lactose
metabolism. Low glucose levels also lead to high cAMP, which further activates the operon
via the Catabolite Activator Protein (CAP).
8. Which of the following describes specialized transduction?
A. Random fragments of host DNA are packaged into a phage head.
B. Plasmids are transferred through a sex pilus.
C. DNA is transferred between cells in direct contact.
D. Specific host genes near the phage integration site are transferred.
Correct Answer: D
Explanation: Specialized transduction occurs during the lysogenic cycle when a prophage
is excised incorrectly from the bacterial chromosome. Because the phage integrates at a
specific site, only the adjacent bacterial genes are captured and transferred to the next host.
This differs from generalized transduction, where any part of the bacterial genome can be
packaged into the phage head during the lytic cycle.
9. What is the primary function of the Polymerase Chain Reaction (PCR)?
A. To sequence an entire bacterial genome in one step.
B. To visualize proteins on a polyacrylamide gel.
