BIO 171 BASIC MICROBIOLOGY FINAL
PRACTICE EXAM 2026/2027 QUESTIONS
AND ANSWERS
1. Which of the following describes the mechanism of action for the diphtheria toxin?
A. Inhibition of the 30S ribosomal subunit
B. ADP-ribosylation of elongation factor 2 (EF-2)
C. Cleavage of the host cell plasma membrane phospholipids
D. Degradation of the extracellular matrix using hyaluronidase
Answer: B
Conceptual Explanation: Diphtheria toxin acts by ADP-ribosylating EF-2, which halts
protein synthesis in eukaryotic cells, leading to cell death.
2. In the context of bacterial genetics, what is the role of the DnaA protein during DNA
replication?
A. Binding to the oriC region to initiate strand separation
B. Synthesizing RNA primers
C. Relieving torsional strain ahead of the replication fork
D. Connecting Okazaki fragments on the lagging strand
,Answer: A
Conceptual Explanation: DnaA is the initiator protein that binds to specific 9-mer repeats
in the origin of replication (oriC), causing the DNA to wrap around the DnaA complex and
promoting the melting of the 13-mer AT-rich region.
3. Which enzyme is responsible for the detoxification of superoxide radicals in obligate
aerobes?
A. Catalase
B. Superoxide dismutase
C. Peroxidase
D. Oxidase
Answer: B
Conceptual Explanation: Superoxide dismutase (SOD) converts superoxide radicals into
hydrogen peroxide and oxygen. Catalase then typically breaks down the hydrogen
peroxide.
4. The bacterial cell wall component ‘Lipid A’ is primarily responsible for which biological
effect?
A. Facilitating adherence to mucosal surfaces
B. Acting as a channel for the transport of small hydrophilic molecules
C. Providing structural rigidity to the peptidoglycan layer
, D. Inducing a potent inflammatory response and toxic shock
Answer: D
Conceptual Explanation: Lipid A is the toxic moiety of Lipopolysaccharide (LPS) in Gram-
negative bacteria and acts as an endotoxin by stimulating TLR4.
5. During the process of specialized transduction, which of the following is true?
A. Any random piece of host DNA can be packaged into the phage head
B. Only specific genes adjacent to the phage integration site are transferred
C. The phage must be a virulent (lytic) phage
D. The process requires direct cell-to-cell contact through a pilus
Answer: B
Conceptual Explanation: Specialized transduction occurs when a temperate phage excises
incorrectly from the host genome, taking specific flanking bacterial genes with it.
6. What is the primary function of the SecA protein in the Sec-dependent secretion pathway?
A. Acting as an ATPase to drive the polypeptide through the SecYEG translocon
B. Recognizing the signal sequence in the cytoplasm
C. Cleaving the signal peptide once the protein reaches the periplasm
D. Folding the protein into its final tertiary structure
Answer: A
PRACTICE EXAM 2026/2027 QUESTIONS
AND ANSWERS
1. Which of the following describes the mechanism of action for the diphtheria toxin?
A. Inhibition of the 30S ribosomal subunit
B. ADP-ribosylation of elongation factor 2 (EF-2)
C. Cleavage of the host cell plasma membrane phospholipids
D. Degradation of the extracellular matrix using hyaluronidase
Answer: B
Conceptual Explanation: Diphtheria toxin acts by ADP-ribosylating EF-2, which halts
protein synthesis in eukaryotic cells, leading to cell death.
2. In the context of bacterial genetics, what is the role of the DnaA protein during DNA
replication?
A. Binding to the oriC region to initiate strand separation
B. Synthesizing RNA primers
C. Relieving torsional strain ahead of the replication fork
D. Connecting Okazaki fragments on the lagging strand
,Answer: A
Conceptual Explanation: DnaA is the initiator protein that binds to specific 9-mer repeats
in the origin of replication (oriC), causing the DNA to wrap around the DnaA complex and
promoting the melting of the 13-mer AT-rich region.
3. Which enzyme is responsible for the detoxification of superoxide radicals in obligate
aerobes?
A. Catalase
B. Superoxide dismutase
C. Peroxidase
D. Oxidase
Answer: B
Conceptual Explanation: Superoxide dismutase (SOD) converts superoxide radicals into
hydrogen peroxide and oxygen. Catalase then typically breaks down the hydrogen
peroxide.
4. The bacterial cell wall component ‘Lipid A’ is primarily responsible for which biological
effect?
A. Facilitating adherence to mucosal surfaces
B. Acting as a channel for the transport of small hydrophilic molecules
C. Providing structural rigidity to the peptidoglycan layer
, D. Inducing a potent inflammatory response and toxic shock
Answer: D
Conceptual Explanation: Lipid A is the toxic moiety of Lipopolysaccharide (LPS) in Gram-
negative bacteria and acts as an endotoxin by stimulating TLR4.
5. During the process of specialized transduction, which of the following is true?
A. Any random piece of host DNA can be packaged into the phage head
B. Only specific genes adjacent to the phage integration site are transferred
C. The phage must be a virulent (lytic) phage
D. The process requires direct cell-to-cell contact through a pilus
Answer: B
Conceptual Explanation: Specialized transduction occurs when a temperate phage excises
incorrectly from the host genome, taking specific flanking bacterial genes with it.
6. What is the primary function of the SecA protein in the Sec-dependent secretion pathway?
A. Acting as an ATPase to drive the polypeptide through the SecYEG translocon
B. Recognizing the signal sequence in the cytoplasm
C. Cleaving the signal peptide once the protein reaches the periplasm
D. Folding the protein into its final tertiary structure
Answer: A
