BIOL 271 | BIOL271 Module 3: Microbiology
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. Which enzyme is responsible for unwinding the DNA double helix at the replication fork?
A. DNA Polymerase III
B. Helicase
C. DNA Ligase
D. Primase
Correct Answer: B
Explanation: Helicase is the specific enzyme that breaks hydrogen bonds between
nitrogenous bases to separate DNA strands. This action creates the replication fork where
synthesis occurs on both the leading and lagging strands. By unzipping the helix, it allows
other enzymes like primase to access the template. Without helicase activity, the DNA
remains in a double-stranded state and cannot be replicated. It is a critical early step in the
microbial DNA replication process.
2. In DNA replication, which enzyme synthesizes the RNA primer needed to start the process?
A. DNA Polymerase I
B. Primase
C. Gyrase
D. Helicase
Correct Answer: B
Explanation: Primase is an RNA polymerase that generates a short RNA sequence called a
primer. DNA polymerases cannot initiate synthesis on a bare template and require a free 3-
prime hydroxyl group. This RNA primer provides the necessary starting point for DNA
Polymerase III to begin adding deoxynucleotides. Once the primer is in place, DNA
synthesis can proceed in the 5-prime to 3-prime direction. The primer is eventually
removed and replaced with DNA by DNA Polymerase I.
3. What is the primary function of DNA Polymerase III during bacterial DNA replication?
A. Removing RNA primers
B. Joining Okazaki fragments
C. Adding nucleotides to the new DNA strand
D. Unwinding the parent DNA
,Correct Answer: C
Explanation: DNA Polymerase III is the main enzyme responsible for the elongation of the
new DNA strand. It reads the template strand and matches complementary nucleotides to
the growing chain. This enzyme works with high speed and precision to ensure the entire
genome is copied accurately. It also has proofreading functions that correct errors
immediately after they occur during synthesis. Therefore, it is the primary workhorse for
replicating the bulk of the bacterial chromosome.
4. Why is DNA replication considered a ‘semiconservative’ process?
A. Each new DNA molecule consists of two entirely new strands.
B. The original parent DNA remains intact and a completely new copy is made.
C. Only half of the DNA molecule is replicated during each cell cycle.
D. Each new DNA molecule contains one original parent strand and one newly synthesized
strand.
Correct Answer: D
Explanation: Semiconservative replication describes how the parent DNA molecule acts as
a template for two new molecules. Each resulting double helix contains one strand from the
original parent and one freshly synthesized daughter strand. This ensures that the genetic
information is passed down with high fidelity to offspring. This mechanism was famously
proven by the Meselson-Stahl experiment using nitrogen isotopes. It is the fundamental
way all microorganisms maintain genetic continuity through generations.
5. On the lagging strand, DNA is synthesized in short segments known as:
A. Primers
B. Transcription units
C. Codons
D. Okazaki fragments
Correct Answer: D
Explanation: The lagging strand is synthesized discontinuously because it runs in the
opposite direction of the replication fork movement. Since DNA polymerase only works in
the 5-prime to 3-prime direction, it must produce DNA in small pieces. These small
segments are named Okazaki fragments after the scientists who discovered them. These
fragments are eventually joined together by the enzyme DNA ligase to form a continuous
strand. This complex process ensures that both strands are replicated simultaneously at
the fork.
6. Which enzyme is responsible for sealing the gaps between Okazaki fragments?
A. Topoisomerase
, B. DNA Ligase
C. DNA Polymerase I
D. Helicase
Correct Answer: B
Explanation: DNA ligase acts as the molecular ‘glue’ that joins DNA fragments together by
forming phosphodiester bonds. After DNA Polymerase I replaces RNA primers with DNA,
small nicks remain in the sugar-phosphate backbone. Ligase recognizes these nicks and
catalyzes the final chemical reaction to seal them. This process is essential for creating a
continuous and stable DNA molecule on the lagging strand. Without ligase, the DNA strand
would remain fragmented and functionally compromised.
7. A mutation that results in the substitution of one amino acid for another in a protein is
called a:
A. Silent mutation
B. Nonsense mutation
C. Frameshift mutation
D. Missense mutation
Correct Answer: D
Explanation: A missense mutation occurs when a single nucleotide change results in a
different codon that codes for a different amino acid. This can alter the structure and
function of the resulting protein depending on the properties of the new amino acid. If the
change occurs in a critical region like an active site, the protein may become nonfunctional.
However, some missense mutations have little to no effect on the organism’s fitness. They
are a significant source of genetic variation in microbial populations.
8. Which type of mutation produces a premature stop codon, leading to a shortened protein?
A. Nonsense mutation
B. Missense mutation
C. Silent mutation
D. Inversion
Correct Answer: A
Explanation: Nonsense mutations convert a codon that specifies an amino acid into a stop
codon (UAA, UAG, or UGA). This causes translation to terminate prematurely, resulting in a
truncated polypeptide chain. These shortened proteins are almost always nonfunctional
and can be detrimental to the cell. The closer the mutation is to the start of the gene, the
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. Which enzyme is responsible for unwinding the DNA double helix at the replication fork?
A. DNA Polymerase III
B. Helicase
C. DNA Ligase
D. Primase
Correct Answer: B
Explanation: Helicase is the specific enzyme that breaks hydrogen bonds between
nitrogenous bases to separate DNA strands. This action creates the replication fork where
synthesis occurs on both the leading and lagging strands. By unzipping the helix, it allows
other enzymes like primase to access the template. Without helicase activity, the DNA
remains in a double-stranded state and cannot be replicated. It is a critical early step in the
microbial DNA replication process.
2. In DNA replication, which enzyme synthesizes the RNA primer needed to start the process?
A. DNA Polymerase I
B. Primase
C. Gyrase
D. Helicase
Correct Answer: B
Explanation: Primase is an RNA polymerase that generates a short RNA sequence called a
primer. DNA polymerases cannot initiate synthesis on a bare template and require a free 3-
prime hydroxyl group. This RNA primer provides the necessary starting point for DNA
Polymerase III to begin adding deoxynucleotides. Once the primer is in place, DNA
synthesis can proceed in the 5-prime to 3-prime direction. The primer is eventually
removed and replaced with DNA by DNA Polymerase I.
3. What is the primary function of DNA Polymerase III during bacterial DNA replication?
A. Removing RNA primers
B. Joining Okazaki fragments
C. Adding nucleotides to the new DNA strand
D. Unwinding the parent DNA
,Correct Answer: C
Explanation: DNA Polymerase III is the main enzyme responsible for the elongation of the
new DNA strand. It reads the template strand and matches complementary nucleotides to
the growing chain. This enzyme works with high speed and precision to ensure the entire
genome is copied accurately. It also has proofreading functions that correct errors
immediately after they occur during synthesis. Therefore, it is the primary workhorse for
replicating the bulk of the bacterial chromosome.
4. Why is DNA replication considered a ‘semiconservative’ process?
A. Each new DNA molecule consists of two entirely new strands.
B. The original parent DNA remains intact and a completely new copy is made.
C. Only half of the DNA molecule is replicated during each cell cycle.
D. Each new DNA molecule contains one original parent strand and one newly synthesized
strand.
Correct Answer: D
Explanation: Semiconservative replication describes how the parent DNA molecule acts as
a template for two new molecules. Each resulting double helix contains one strand from the
original parent and one freshly synthesized daughter strand. This ensures that the genetic
information is passed down with high fidelity to offspring. This mechanism was famously
proven by the Meselson-Stahl experiment using nitrogen isotopes. It is the fundamental
way all microorganisms maintain genetic continuity through generations.
5. On the lagging strand, DNA is synthesized in short segments known as:
A. Primers
B. Transcription units
C. Codons
D. Okazaki fragments
Correct Answer: D
Explanation: The lagging strand is synthesized discontinuously because it runs in the
opposite direction of the replication fork movement. Since DNA polymerase only works in
the 5-prime to 3-prime direction, it must produce DNA in small pieces. These small
segments are named Okazaki fragments after the scientists who discovered them. These
fragments are eventually joined together by the enzyme DNA ligase to form a continuous
strand. This complex process ensures that both strands are replicated simultaneously at
the fork.
6. Which enzyme is responsible for sealing the gaps between Okazaki fragments?
A. Topoisomerase
, B. DNA Ligase
C. DNA Polymerase I
D. Helicase
Correct Answer: B
Explanation: DNA ligase acts as the molecular ‘glue’ that joins DNA fragments together by
forming phosphodiester bonds. After DNA Polymerase I replaces RNA primers with DNA,
small nicks remain in the sugar-phosphate backbone. Ligase recognizes these nicks and
catalyzes the final chemical reaction to seal them. This process is essential for creating a
continuous and stable DNA molecule on the lagging strand. Without ligase, the DNA strand
would remain fragmented and functionally compromised.
7. A mutation that results in the substitution of one amino acid for another in a protein is
called a:
A. Silent mutation
B. Nonsense mutation
C. Frameshift mutation
D. Missense mutation
Correct Answer: D
Explanation: A missense mutation occurs when a single nucleotide change results in a
different codon that codes for a different amino acid. This can alter the structure and
function of the resulting protein depending on the properties of the new amino acid. If the
change occurs in a critical region like an active site, the protein may become nonfunctional.
However, some missense mutations have little to no effect on the organism’s fitness. They
are a significant source of genetic variation in microbial populations.
8. Which type of mutation produces a premature stop codon, leading to a shortened protein?
A. Nonsense mutation
B. Missense mutation
C. Silent mutation
D. Inversion
Correct Answer: A
Explanation: Nonsense mutations convert a codon that specifies an amino acid into a stop
codon (UAA, UAG, or UGA). This causes translation to terminate prematurely, resulting in a
truncated polypeptide chain. These shortened proteins are almost always nonfunctional
and can be detrimental to the cell. The closer the mutation is to the start of the gene, the
