Microbiology D3111 – WGU
OA (2025/2026) | Complete
Questions and Verified
Answers Updated To-Date.
Domain I: Microbiology Fundamentals & Biochemistry (15 Questions)
1. A student is examining a slide under a light microscope. The image shows a single-celled
organism with a distinct nucleus, mitochondria, and flagella. What domain does this
organism belong to?
A) Bacteria
B) Archaea
C) Eukarya
D) Virus
Answer: C) Eukarya
Rationale: The presence of a membrane-bound nucleus and organelles (mitochondria)
definitively places the organism in the Eukarya domain. Bacteria and Archaea are
prokaryotes lacking these structures.
2. Which of the following statements best explains why archaea are able to survive in
extreme environments like hot springs?
A) They possess peptidoglycan in their cell walls for structural integrity.
B) They have unique ether-linked lipids in their plasma membranes that resist denaturation.
C) They form endospores that are resistant to boiling temperatures.
D) They utilize a nucleus to compartmentalize DNA repair enzymes.
,Answer: B) They have unique ether-linked lipids in their plasma membranes that resist
denaturation.
Rationale: Archaeal membranes contain ether linkages (instead of ester linkages) and
isoprene chains, which provide stability at high temperatures and extreme pH. Peptidoglycan
is found in bacteria, not archaea.
3. A biochemist is analyzing a molecule that contains a nitrogenous base (adenine), a 5-
carbon sugar (ribose), and three phosphate groups. What is this molecule?
A) DNA
B) ATP
C) RNA
D) cAMP
Answer: B) ATP
Rationale: Adenosine triphosphate (ATP) consists of adenine (nitrogenous base), ribose
(pentose sugar), and three phosphate groups. This structure is distinct from DNA (which uses
deoxyribose) and standard RNA nucleotides (which have one phosphate).
4. Which type of bond is primarily responsible for the complementary base pairing between
adenine and thymine in DNA?
A) Covalent bond
B) Peptide bond
C) Hydrogen bond
D) Disulfide bridge
Answer: C) Hydrogen bond
Rationale: Hydrogen bonds are weak, non-covalent bonds that hold the two strands of DNA
together. A-T pairs form two hydrogen bonds; G-C pairs form three. Covalent bonds form the
sugar-phosphate backbone.
5. What is the primary function of ribosomes in a bacterial cell?
A) DNA replication
B) Protein synthesis
C) Lipid synthesis
D) ATP production
,Answer: B) Protein synthesis
Rationale: Ribosomes are the site of translation, where mRNA is decoded to assemble amino
acids into polypeptide chains (proteins).
6. A microbiologist observes a cell that is 50 µm in diameter. Based on size alone, what type
of cell is this most likely to be?
A) A typical bacterium
B) A virus
C) A eukaryotic cell
D) A ribosome
Answer: C) A eukaryotic cell
*Rationale: Eukaryotic cells typically range from 10–100 µm in diameter. Prokaryotic cells
(bacteria) are much smaller (0.5–5 µm). Viruses are sub-microscopic (20–300 nm).*
7. Which cellular structure is responsible for regulating the passage of substances into and
out of the cell?
A) Cell wall
B) Plasma membrane
C) Capsule
D) Nucleoid
Answer: B) Plasma membrane
Rationale: The plasma membrane is selectively permeable, controlling the movement of ions,
nutrients, and waste. The cell wall provides structural support but is not the primary
regulator of transport.
8. A Gram-positive bacterium is treated with lysozyme. What is the most immediate effect?
A) The plasma membrane dissolves.
B) The peptidoglycan layer is degraded, causing osmotic lysis.
C) The bacterium becomes resistant to penicillin.
D) The flagella are sheared off.
Answer: B) The peptidoglycan layer is degraded, causing osmotic lysis.
Rationale: Lysozyme hydrolyzes the bond between N-acetylglucosamine (NAG) and N-
, acetylmuramic acid (NAM) in peptidoglycan. Gram-positive cells have thick peptidoglycan
layers and are highly susceptible to osmotic lysis when this layer is compromised.
9. Which of the following is a key difference between prokaryotic and eukaryotic DNA
replication?
A) Prokaryotic replication is bidirectional; eukaryotic is unidirectional.
B) Prokaryotes use DNA polymerase; eukaryotes use RNA polymerase.
C) Prokaryotes have a single origin of replication; eukaryotes have multiple origins.
D) Prokaryotes replicate in the nucleus; eukaryotes replicate in the cytoplasm.
Answer: C) Prokaryotes have a single origin of replication; eukaryotes have multiple
origins.
Rationale: Bacterial chromosomes have a single origin of replication (oriC). Eukaryotic
chromosomes are linear and larger, requiring multiple origins to replicate within a
reasonable timeframe.
10. A Gram stain reveals purple cocci in clusters. What is the correct interpretation?
A) Gram-negative rods
B) Gram-positive cocci
C) Gram-negative cocci
D) Gram-positive rods
Answer: B) Gram-positive cocci
Rationale: Purple (crystal violet retention) indicates Gram-positive. Spherical shape indicates
cocci. Clusters suggest Staphylococcus morphology.
11. Which microscope would be most appropriate for viewing the internal ultrastructure of a
virus?
A) Compound light microscope
B) Phase-contrast microscope
C) Scanning electron microscope (SEM)
D) Transmission electron microscope (TEM)
OA (2025/2026) | Complete
Questions and Verified
Answers Updated To-Date.
Domain I: Microbiology Fundamentals & Biochemistry (15 Questions)
1. A student is examining a slide under a light microscope. The image shows a single-celled
organism with a distinct nucleus, mitochondria, and flagella. What domain does this
organism belong to?
A) Bacteria
B) Archaea
C) Eukarya
D) Virus
Answer: C) Eukarya
Rationale: The presence of a membrane-bound nucleus and organelles (mitochondria)
definitively places the organism in the Eukarya domain. Bacteria and Archaea are
prokaryotes lacking these structures.
2. Which of the following statements best explains why archaea are able to survive in
extreme environments like hot springs?
A) They possess peptidoglycan in their cell walls for structural integrity.
B) They have unique ether-linked lipids in their plasma membranes that resist denaturation.
C) They form endospores that are resistant to boiling temperatures.
D) They utilize a nucleus to compartmentalize DNA repair enzymes.
,Answer: B) They have unique ether-linked lipids in their plasma membranes that resist
denaturation.
Rationale: Archaeal membranes contain ether linkages (instead of ester linkages) and
isoprene chains, which provide stability at high temperatures and extreme pH. Peptidoglycan
is found in bacteria, not archaea.
3. A biochemist is analyzing a molecule that contains a nitrogenous base (adenine), a 5-
carbon sugar (ribose), and three phosphate groups. What is this molecule?
A) DNA
B) ATP
C) RNA
D) cAMP
Answer: B) ATP
Rationale: Adenosine triphosphate (ATP) consists of adenine (nitrogenous base), ribose
(pentose sugar), and three phosphate groups. This structure is distinct from DNA (which uses
deoxyribose) and standard RNA nucleotides (which have one phosphate).
4. Which type of bond is primarily responsible for the complementary base pairing between
adenine and thymine in DNA?
A) Covalent bond
B) Peptide bond
C) Hydrogen bond
D) Disulfide bridge
Answer: C) Hydrogen bond
Rationale: Hydrogen bonds are weak, non-covalent bonds that hold the two strands of DNA
together. A-T pairs form two hydrogen bonds; G-C pairs form three. Covalent bonds form the
sugar-phosphate backbone.
5. What is the primary function of ribosomes in a bacterial cell?
A) DNA replication
B) Protein synthesis
C) Lipid synthesis
D) ATP production
,Answer: B) Protein synthesis
Rationale: Ribosomes are the site of translation, where mRNA is decoded to assemble amino
acids into polypeptide chains (proteins).
6. A microbiologist observes a cell that is 50 µm in diameter. Based on size alone, what type
of cell is this most likely to be?
A) A typical bacterium
B) A virus
C) A eukaryotic cell
D) A ribosome
Answer: C) A eukaryotic cell
*Rationale: Eukaryotic cells typically range from 10–100 µm in diameter. Prokaryotic cells
(bacteria) are much smaller (0.5–5 µm). Viruses are sub-microscopic (20–300 nm).*
7. Which cellular structure is responsible for regulating the passage of substances into and
out of the cell?
A) Cell wall
B) Plasma membrane
C) Capsule
D) Nucleoid
Answer: B) Plasma membrane
Rationale: The plasma membrane is selectively permeable, controlling the movement of ions,
nutrients, and waste. The cell wall provides structural support but is not the primary
regulator of transport.
8. A Gram-positive bacterium is treated with lysozyme. What is the most immediate effect?
A) The plasma membrane dissolves.
B) The peptidoglycan layer is degraded, causing osmotic lysis.
C) The bacterium becomes resistant to penicillin.
D) The flagella are sheared off.
Answer: B) The peptidoglycan layer is degraded, causing osmotic lysis.
Rationale: Lysozyme hydrolyzes the bond between N-acetylglucosamine (NAG) and N-
, acetylmuramic acid (NAM) in peptidoglycan. Gram-positive cells have thick peptidoglycan
layers and are highly susceptible to osmotic lysis when this layer is compromised.
9. Which of the following is a key difference between prokaryotic and eukaryotic DNA
replication?
A) Prokaryotic replication is bidirectional; eukaryotic is unidirectional.
B) Prokaryotes use DNA polymerase; eukaryotes use RNA polymerase.
C) Prokaryotes have a single origin of replication; eukaryotes have multiple origins.
D) Prokaryotes replicate in the nucleus; eukaryotes replicate in the cytoplasm.
Answer: C) Prokaryotes have a single origin of replication; eukaryotes have multiple
origins.
Rationale: Bacterial chromosomes have a single origin of replication (oriC). Eukaryotic
chromosomes are linear and larger, requiring multiple origins to replicate within a
reasonable timeframe.
10. A Gram stain reveals purple cocci in clusters. What is the correct interpretation?
A) Gram-negative rods
B) Gram-positive cocci
C) Gram-negative cocci
D) Gram-positive rods
Answer: B) Gram-positive cocci
Rationale: Purple (crystal violet retention) indicates Gram-positive. Spherical shape indicates
cocci. Clusters suggest Staphylococcus morphology.
11. Which microscope would be most appropriate for viewing the internal ultrastructure of a
virus?
A) Compound light microscope
B) Phase-contrast microscope
C) Scanning electron microscope (SEM)
D) Transmission electron microscope (TEM)
