BIO 250 Exam 4 V1 | BIO 250 Microbiology | Actual Q&A with
Rationale (BIO250 Exam 4) | StraighterLine
1. During DNA replication, which enzyme is responsible for synthesizing a short RNA
sequence to provide a starting point for DNA polymerase III?
A. DNA Ligase
B. DNA Polymerase I
C. Helicase
D. Primase
Correct Answer: D
Explanation: Primase is a type of RNA polymerase that creates an RNA primer necessary
for DNA polymerase III to begin synthesis. DNA polymerase III requires a free 3’-OH group,
which the RNA primer provides. Without primase, the replication process could not initiate
on either the leading or lagging strands.
2. Select all of the following that are true regarding the structure and characteristics of the
DNA molecule:
A. It consists of two strands arranged in an antiparallel fashion.
B. The backbone is composed of alternating deoxyribose sugar and phosphate groups.
C. It uses uracil to pair with adenine in the primary sequence.
D. Adenine pairs with thymine via two hydrogen bonds.
E. Guanine pairs with cytosine via three hydrogen bonds.
F. It is primarily found in the cytoplasm of eukaryotic cells.
Correct Answer: A, B, D, E
Explanation: DNA is characterized by a double helix structure where the two strands run
in opposite directions, known as antiparallel. The nitrogenous bases are linked by
hydrogen bonds, specifically two for A-T pairs and three for G-C pairs, while uracil is
unique to RNA. In eukaryotes, the majority of DNA is housed within the nucleus rather than
the cytoplasm.
3. Which of the following describes the function of DNA Ligase during the replication of the
lagging strand?
A. It unwinds the DNA double helix to create a replication fork.
B. It removes the RNA primers and replaces them with DNA nucleotides.
,C. It joins Okazaki fragments together by catalyzing the formation of phosphodiester bonds.
D. It prevents the DNA from supercoiling ahead of the replication fork.
Correct Answer: C
Explanation: DNA Ligase acts as a ‘glue’ that seals the nicks between DNA fragments,
specifically Okazaki fragments on the lagging strand. This enzyme facilitates the formation
of a phosphodiester bond between the 3’ hydroxyl group of one fragment and the 5’
phosphate group of another. This step is crucial for completing a continuous DNA strand
after primers have been replaced.
4. In the context of the bacterial lac operon, what happens when lactose is present in the
environment and glucose is absent?
A. The repressor remains bound to the operator, preventing transcription.
B. The cell converts the lactose into glucose via reverse transcription.
C. The promoter is degraded by enzymes to prevent energy waste.
D. RNA polymerase is inhibited from binding to the DNA sequence.
E. Allolactose binds to the repressor, causing it to release from the operator, allowing
transcription.
Correct Answer: E
Explanation: Lactose serves as an inducer by being converted into allolactose, which then
binds to the lac repressor protein. This binding changes the shape of the repressor,
preventing it from attaching to the operator site. Consequently, RNA polymerase can
proceed with the transcription of the structural genes needed for lactose metabolism.
5. A mutation that results in the premature insertion of a stop codon in the mRNA sequence
is known as a:
A. Silent mutation
B. Missense mutation
C. Frameshift mutation
D. Nonsense mutation
Correct Answer: D
Explanation: A nonsense mutation is a point mutation that changes a codon specifying an
amino acid into a stop codon. This leads to the early termination of protein synthesis,
usually resulting in a nonfunctional, truncated protein. These mutations are typically more
severe than missense or silent mutations due to the loss of significant portions of the
polypeptide chain.
, 6. Which horizontal gene transfer mechanism involves the transfer of DNA from a donor to a
recipient via a bacteriophage?
A. Conjugation
B. Transformation
C. Transposition
D. Transduction
Correct Answer: D
Explanation: Transduction occurs when a virus that infects bacteria, known as a
bacteriophage, carries DNA from one host cell to another. This can occur through
generalized transduction during the lytic cycle or specialized transduction during the
lysogenic cycle. It is a significant method for moving genes, including antibiotic resistance,
between bacterial populations.
7. Which enzyme is responsible for ‘unzipping’ the DNA double helix by breaking the
hydrogen bonds between bases?
A. Topoisomerase
B. DNA Polymerase III
C. Helicase
D. RNA Polymerase
Correct Answer: C
Explanation: Helicase is the motor protein that moves along the DNA phosphodiester
backbone, separating the two strands. It utilizes energy from ATP hydrolysis to break the
hydrogen bonds holding the base pairs together. This action creates the replication fork,
providing the single-stranded templates needed for replication.
8. Select all the methods that are considered chemical means of microbial control:
A. Ethylene Oxide sterilization
B. Application of 70% Isopropyl Alcohol
C. Autoclaving at 121 degrees Celsius
D. Use of Phenolic compounds like Lysol
E. Ionizing Radiation treatment
F. Chlorination of water supplies
Correct Answer: A, B, D, F
Rationale (BIO250 Exam 4) | StraighterLine
1. During DNA replication, which enzyme is responsible for synthesizing a short RNA
sequence to provide a starting point for DNA polymerase III?
A. DNA Ligase
B. DNA Polymerase I
C. Helicase
D. Primase
Correct Answer: D
Explanation: Primase is a type of RNA polymerase that creates an RNA primer necessary
for DNA polymerase III to begin synthesis. DNA polymerase III requires a free 3’-OH group,
which the RNA primer provides. Without primase, the replication process could not initiate
on either the leading or lagging strands.
2. Select all of the following that are true regarding the structure and characteristics of the
DNA molecule:
A. It consists of two strands arranged in an antiparallel fashion.
B. The backbone is composed of alternating deoxyribose sugar and phosphate groups.
C. It uses uracil to pair with adenine in the primary sequence.
D. Adenine pairs with thymine via two hydrogen bonds.
E. Guanine pairs with cytosine via three hydrogen bonds.
F. It is primarily found in the cytoplasm of eukaryotic cells.
Correct Answer: A, B, D, E
Explanation: DNA is characterized by a double helix structure where the two strands run
in opposite directions, known as antiparallel. The nitrogenous bases are linked by
hydrogen bonds, specifically two for A-T pairs and three for G-C pairs, while uracil is
unique to RNA. In eukaryotes, the majority of DNA is housed within the nucleus rather than
the cytoplasm.
3. Which of the following describes the function of DNA Ligase during the replication of the
lagging strand?
A. It unwinds the DNA double helix to create a replication fork.
B. It removes the RNA primers and replaces them with DNA nucleotides.
,C. It joins Okazaki fragments together by catalyzing the formation of phosphodiester bonds.
D. It prevents the DNA from supercoiling ahead of the replication fork.
Correct Answer: C
Explanation: DNA Ligase acts as a ‘glue’ that seals the nicks between DNA fragments,
specifically Okazaki fragments on the lagging strand. This enzyme facilitates the formation
of a phosphodiester bond between the 3’ hydroxyl group of one fragment and the 5’
phosphate group of another. This step is crucial for completing a continuous DNA strand
after primers have been replaced.
4. In the context of the bacterial lac operon, what happens when lactose is present in the
environment and glucose is absent?
A. The repressor remains bound to the operator, preventing transcription.
B. The cell converts the lactose into glucose via reverse transcription.
C. The promoter is degraded by enzymes to prevent energy waste.
D. RNA polymerase is inhibited from binding to the DNA sequence.
E. Allolactose binds to the repressor, causing it to release from the operator, allowing
transcription.
Correct Answer: E
Explanation: Lactose serves as an inducer by being converted into allolactose, which then
binds to the lac repressor protein. This binding changes the shape of the repressor,
preventing it from attaching to the operator site. Consequently, RNA polymerase can
proceed with the transcription of the structural genes needed for lactose metabolism.
5. A mutation that results in the premature insertion of a stop codon in the mRNA sequence
is known as a:
A. Silent mutation
B. Missense mutation
C. Frameshift mutation
D. Nonsense mutation
Correct Answer: D
Explanation: A nonsense mutation is a point mutation that changes a codon specifying an
amino acid into a stop codon. This leads to the early termination of protein synthesis,
usually resulting in a nonfunctional, truncated protein. These mutations are typically more
severe than missense or silent mutations due to the loss of significant portions of the
polypeptide chain.
, 6. Which horizontal gene transfer mechanism involves the transfer of DNA from a donor to a
recipient via a bacteriophage?
A. Conjugation
B. Transformation
C. Transposition
D. Transduction
Correct Answer: D
Explanation: Transduction occurs when a virus that infects bacteria, known as a
bacteriophage, carries DNA from one host cell to another. This can occur through
generalized transduction during the lytic cycle or specialized transduction during the
lysogenic cycle. It is a significant method for moving genes, including antibiotic resistance,
between bacterial populations.
7. Which enzyme is responsible for ‘unzipping’ the DNA double helix by breaking the
hydrogen bonds between bases?
A. Topoisomerase
B. DNA Polymerase III
C. Helicase
D. RNA Polymerase
Correct Answer: C
Explanation: Helicase is the motor protein that moves along the DNA phosphodiester
backbone, separating the two strands. It utilizes energy from ATP hydrolysis to break the
hydrogen bonds holding the base pairs together. This action creates the replication fork,
providing the single-stranded templates needed for replication.
8. Select all the methods that are considered chemical means of microbial control:
A. Ethylene Oxide sterilization
B. Application of 70% Isopropyl Alcohol
C. Autoclaving at 121 degrees Celsius
D. Use of Phenolic compounds like Lysol
E. Ionizing Radiation treatment
F. Chlorination of water supplies
Correct Answer: A, B, D, F
