BIO 250 Exam 3 V1 | BIO 250 Microbiology
| Actual Q&A with Rationale (BIO250 Exam
3) | StraighterLine
1. During DNA replication, which enzyme is responsible for unwinding the DNA double helix
at the replication fork?
A. DNA Ligase
B. Primase
C. Helicase
D. DNA Polymerase III
Answer: C
Rationale: Helicase is the enzyme that breaks the hydrogen bonds between the
nitrogenous bases to separate the strands. This process creates the replication fork where
other enzymes can begin their work. Without helicase, the DNA molecule would remain in
its double-stranded state, preventing replication.
2. The lac operon is an inducible system. Which of the following conditions must be met for
the structural genes of the lac operon to be transcribed at high levels?
A. Lactose is absent and glucose is present.
B. Lactose is present and glucose is present.
C. Lactose is absent and glucose is absent.
,D. Lactose is present and glucose is absent.
Answer: D
Rationale: For high transcription levels of the lac operon, lactose must be present to
inactivate the repressor. Additionally, glucose must be absent to allow cAMP levels to rise
and bind to the Catabolite Activator Protein (CAP). This dual-control mechanism ensures
that the cell prioritizes the most efficient energy source while still being able to utilize
alternative sugars.
3. Select all of the following that are considered physical methods of microbial control (Select
all that apply):
A. Autoclaving
B. Filtration
C. Ionizing radiation
D. Osmotic pressure
E. Ethylene oxide gas
F. Incineration
Answer: A, B, C, D, F
Rationale: Physical methods of control include heat (autoclaving and incineration),
mechanical removal (filtration), and environmental changes (osmotic pressure and
radiation). Ethylene oxide is considered a chemical method because it involves a reactive
, gas. These physical processes work by denaturing proteins, disrupting membranes, or
damaging nucleic acids to eliminate microbial life.
4. Which of the following describes a frameshift mutation?
A. A single base substitution that results in a different amino acid.
B. A mutation that changes a codon for an amino acid into a stop codon.
C. The insertion or deletion of one or more nucleotides that is not a multiple of three.
D. A mutation that does not change the amino acid sequence of the protein.
Answer: C
Rationale: A frameshift mutation alters the reading frame of the genetic message by
adding or removing bases. Because the genetic code is read in triplets, any change not
divisible by three shifts every subsequent codon. This usually results in a completely
nonfunctional protein or a premature stop codon.
5. The process of ‘competence’ in bacteria is specifically associated with which type of
horizontal gene transfer?
A. Transformation
B. Transduction
C. Conjugation
D. Transposition
Answer: A
| Actual Q&A with Rationale (BIO250 Exam
3) | StraighterLine
1. During DNA replication, which enzyme is responsible for unwinding the DNA double helix
at the replication fork?
A. DNA Ligase
B. Primase
C. Helicase
D. DNA Polymerase III
Answer: C
Rationale: Helicase is the enzyme that breaks the hydrogen bonds between the
nitrogenous bases to separate the strands. This process creates the replication fork where
other enzymes can begin their work. Without helicase, the DNA molecule would remain in
its double-stranded state, preventing replication.
2. The lac operon is an inducible system. Which of the following conditions must be met for
the structural genes of the lac operon to be transcribed at high levels?
A. Lactose is absent and glucose is present.
B. Lactose is present and glucose is present.
C. Lactose is absent and glucose is absent.
,D. Lactose is present and glucose is absent.
Answer: D
Rationale: For high transcription levels of the lac operon, lactose must be present to
inactivate the repressor. Additionally, glucose must be absent to allow cAMP levels to rise
and bind to the Catabolite Activator Protein (CAP). This dual-control mechanism ensures
that the cell prioritizes the most efficient energy source while still being able to utilize
alternative sugars.
3. Select all of the following that are considered physical methods of microbial control (Select
all that apply):
A. Autoclaving
B. Filtration
C. Ionizing radiation
D. Osmotic pressure
E. Ethylene oxide gas
F. Incineration
Answer: A, B, C, D, F
Rationale: Physical methods of control include heat (autoclaving and incineration),
mechanical removal (filtration), and environmental changes (osmotic pressure and
radiation). Ethylene oxide is considered a chemical method because it involves a reactive
, gas. These physical processes work by denaturing proteins, disrupting membranes, or
damaging nucleic acids to eliminate microbial life.
4. Which of the following describes a frameshift mutation?
A. A single base substitution that results in a different amino acid.
B. A mutation that changes a codon for an amino acid into a stop codon.
C. The insertion or deletion of one or more nucleotides that is not a multiple of three.
D. A mutation that does not change the amino acid sequence of the protein.
Answer: C
Rationale: A frameshift mutation alters the reading frame of the genetic message by
adding or removing bases. Because the genetic code is read in triplets, any change not
divisible by three shifts every subsequent codon. This usually results in a completely
nonfunctional protein or a premature stop codon.
5. The process of ‘competence’ in bacteria is specifically associated with which type of
horizontal gene transfer?
A. Transformation
B. Transduction
C. Conjugation
D. Transposition
Answer: A
