MCB 3020C Exam 2 V1 | MCB 3020C General Microbiology | Actual
Q&A with Rationale (MCB3020C Exam 2) | University of Central
Florida
1. During which phase of the bacterial growth curve is the population of cells increasing at its
maximum rate?
A. Log phase
B. Lag phase
C. Stationary phase
D. Death phase
Answer: A
Explanation: The log phase, also known as the exponential growth phase, is the period
where cells divide at their maximum rate under the given conditions. During this time, the
generation time is constant and the population increases geometrically. This phase is also
when bacteria are most sensitive to metabolic disruptions such as antibiotic treatment.
2. Which of the following conditions is standard for effective sterilization in an autoclave?
A. 100°C at 10 psi for 10 minutes
B. 121°C at 15 psi for 15 minutes
C. 100°C at 15 psi for 60 minutes
D. 115°C at 20 psi for 30 minutes
Answer: B
Explanation: Autoclaving utilizes pressurized steam to achieve temperatures above the
boiling point of water. The standard parameters of 121 degrees Celsius at 15 pounds per
square inch for 15 minutes are designed to kill all vegetative cells and, critically, bacterial
endospores. Failure to reach these specific conditions may result in incomplete sterilization
and the persistence of thermoduric organisms.
3. What is the primary function of DNA Polymerase III in prokaryotic DNA replication?
A. To unzip the DNA double helix
B. To remove RNA primers and replace them with DNA
C. To add nucleotides in the 5’ to 3’ direction on the new strand
D. To join Okazaki fragments together
Answer: C
,Explanation: DNA Polymerase III is the primary enzyme responsible for synthesizing the
new DNA strand during replication in bacteria. It catalyzes the addition of
deoxyribonucleotides to the 3’ hydroxyl end of a growing chain, moving in a 5’ to 3’
direction. This enzyme is highly processive and includes a proofreading function to ensure
high fidelity in the genetic code.
4. In the Lac Operon, what role does allolactose play when lactose is present in the
environment?
A. It acts as an inducer by binding to the repressor
B. It binds to the promoter to recruit RNA polymerase
C. It acts as a corepressor to shut down transcription
D. It degrades the mRNA produced by the operon
Answer: A
Explanation: Allolactose is an isomer of lactose that functions as the inducer molecule for
the lac operon. When lactose is present, it is converted to allolactose, which then binds to
the lac repressor protein, changing its shape. This conformational change prevents the
repressor from binding to the operator, allowing RNA polymerase to transcribe the genes
needed for lactose metabolism.
5. Microorganisms that grow optimally at temperatures between 20°C and 45°C are classified
as:
A. Psychrophiles
B. Thermophiles
C. Hyperthermophiles
D. Mesophiles
Answer: D
Explanation: Mesophiles are organisms that prefer moderate temperature environments,
typically matching the internal temperature of humans and many animals. Most human
pathogens fall into this category because the human body provides an ideal incubator at 37
degrees Celsius. Psychrophiles prefer colder temperatures, while thermophiles thrive in
heat exceeding 45 degrees Celsius.
6. Which of the following describes a ‘missense’ mutation?
A. A mutation that results in a premature stop codon
B. A mutation that results in a different amino acid in the protein
C. A mutation that does not change the amino acid sequence
D. A mutation caused by the insertion of a single nucleotide
, Answer: B
Explanation: A missense mutation occurs when a single nucleotide change in the DNA
sequence results in a codon that codes for a different amino acid. This can lead to a protein
with altered function or stability, depending on the properties of the new amino acid. It
contrasts with silent mutations, which have no effect on the protein, and nonsense
mutations, which terminate the protein early.
7. What is the net yield of ATP produced per molecule of glucose during glycolysis?
A. 1 ATP
B. 38 ATP
C. 4 ATP
D. 2 ATP
Answer: D
Explanation: Glycolysis involves an initial energy investment phase where two ATP
molecules are consumed to activate glucose. Subsequently, four ATP molecules are
generated through substrate-level phosphorylation, resulting in a net gain of two ATP.
While the total produced is four, the ‘net’ refers to the profit available to the cell for other
metabolic processes.
8. In aerobic respiration, which molecule serves as the final electron acceptor?
A. Pyruvate
B. Oxygen
C. Nitrate
D. NAD+
Answer: B
Explanation: Aerobic respiration is defined by the use of molecular oxygen as the ultimate
recipient of electrons at the end of the electron transport chain. The reduction of oxygen by
these electrons and protons results in the formation of water. This process allows for the
maximum extraction of energy from glucose compared to anaerobic pathways.
9. Which enzyme is responsible for relieving torsional strain (supercoiling) ahead of the
replication fork?
A. Helicase
B. Primase
C. DNA Gyrase (Topoisomerase II)
D. Ligase
Q&A with Rationale (MCB3020C Exam 2) | University of Central
Florida
1. During which phase of the bacterial growth curve is the population of cells increasing at its
maximum rate?
A. Log phase
B. Lag phase
C. Stationary phase
D. Death phase
Answer: A
Explanation: The log phase, also known as the exponential growth phase, is the period
where cells divide at their maximum rate under the given conditions. During this time, the
generation time is constant and the population increases geometrically. This phase is also
when bacteria are most sensitive to metabolic disruptions such as antibiotic treatment.
2. Which of the following conditions is standard for effective sterilization in an autoclave?
A. 100°C at 10 psi for 10 minutes
B. 121°C at 15 psi for 15 minutes
C. 100°C at 15 psi for 60 minutes
D. 115°C at 20 psi for 30 minutes
Answer: B
Explanation: Autoclaving utilizes pressurized steam to achieve temperatures above the
boiling point of water. The standard parameters of 121 degrees Celsius at 15 pounds per
square inch for 15 minutes are designed to kill all vegetative cells and, critically, bacterial
endospores. Failure to reach these specific conditions may result in incomplete sterilization
and the persistence of thermoduric organisms.
3. What is the primary function of DNA Polymerase III in prokaryotic DNA replication?
A. To unzip the DNA double helix
B. To remove RNA primers and replace them with DNA
C. To add nucleotides in the 5’ to 3’ direction on the new strand
D. To join Okazaki fragments together
Answer: C
,Explanation: DNA Polymerase III is the primary enzyme responsible for synthesizing the
new DNA strand during replication in bacteria. It catalyzes the addition of
deoxyribonucleotides to the 3’ hydroxyl end of a growing chain, moving in a 5’ to 3’
direction. This enzyme is highly processive and includes a proofreading function to ensure
high fidelity in the genetic code.
4. In the Lac Operon, what role does allolactose play when lactose is present in the
environment?
A. It acts as an inducer by binding to the repressor
B. It binds to the promoter to recruit RNA polymerase
C. It acts as a corepressor to shut down transcription
D. It degrades the mRNA produced by the operon
Answer: A
Explanation: Allolactose is an isomer of lactose that functions as the inducer molecule for
the lac operon. When lactose is present, it is converted to allolactose, which then binds to
the lac repressor protein, changing its shape. This conformational change prevents the
repressor from binding to the operator, allowing RNA polymerase to transcribe the genes
needed for lactose metabolism.
5. Microorganisms that grow optimally at temperatures between 20°C and 45°C are classified
as:
A. Psychrophiles
B. Thermophiles
C. Hyperthermophiles
D. Mesophiles
Answer: D
Explanation: Mesophiles are organisms that prefer moderate temperature environments,
typically matching the internal temperature of humans and many animals. Most human
pathogens fall into this category because the human body provides an ideal incubator at 37
degrees Celsius. Psychrophiles prefer colder temperatures, while thermophiles thrive in
heat exceeding 45 degrees Celsius.
6. Which of the following describes a ‘missense’ mutation?
A. A mutation that results in a premature stop codon
B. A mutation that results in a different amino acid in the protein
C. A mutation that does not change the amino acid sequence
D. A mutation caused by the insertion of a single nucleotide
, Answer: B
Explanation: A missense mutation occurs when a single nucleotide change in the DNA
sequence results in a codon that codes for a different amino acid. This can lead to a protein
with altered function or stability, depending on the properties of the new amino acid. It
contrasts with silent mutations, which have no effect on the protein, and nonsense
mutations, which terminate the protein early.
7. What is the net yield of ATP produced per molecule of glucose during glycolysis?
A. 1 ATP
B. 38 ATP
C. 4 ATP
D. 2 ATP
Answer: D
Explanation: Glycolysis involves an initial energy investment phase where two ATP
molecules are consumed to activate glucose. Subsequently, four ATP molecules are
generated through substrate-level phosphorylation, resulting in a net gain of two ATP.
While the total produced is four, the ‘net’ refers to the profit available to the cell for other
metabolic processes.
8. In aerobic respiration, which molecule serves as the final electron acceptor?
A. Pyruvate
B. Oxygen
C. Nitrate
D. NAD+
Answer: B
Explanation: Aerobic respiration is defined by the use of molecular oxygen as the ultimate
recipient of electrons at the end of the electron transport chain. The reduction of oxygen by
these electrons and protons results in the formation of water. This process allows for the
maximum extraction of energy from glucose compared to anaerobic pathways.
9. Which enzyme is responsible for relieving torsional strain (supercoiling) ahead of the
replication fork?
A. Helicase
B. Primase
C. DNA Gyrase (Topoisomerase II)
D. Ligase
