BIO 420 MICROBIOLOGY FINAL EXAM
QUESTIONS COMPLETE WITH 100% VERIFIED
ANSWERS AND DETAILED EXPLANATIONS
1. Which of the following metabolic processes is unique to prokaryotes?
A. Glycolysis
B. Chemolithotrophy
C. Photosynthesis
D. Aerobic respiration
Correct answer: B
Explanation: Chemolithotrophy (using inorganic compounds like H₂S, NH₃, or Fe²⁺
as electron donors) is found only in certain bacteria and archaea. Glycolysis,
photosynthesis (though not oxygenic in all), and aerobic respiration occur in some
eukaryotes as well.
2. A clinical isolate shows growth on MacConkey agar as colorless colonies. This
indicates the bacterium is most likely:
A. Lactose-fermenting coliform
B. Gram-positive coccus
C. Non-lactose-fermenting Gram-negative rod
D. Obligate anaerobe
Correct answer: C
Explanation: MacConkey agar selects for Gram-negatives and differentiates
lactose fermenters (pink). Colorless colonies = no lactose fermentation, typical of
pathogens like Salmonella, Shigella, or Proteus.
,3. Which structure is directly responsible for transfer of plasmid DNA during
conjugation?
A. Flagellum
B. Pilus (sex pilus)
C. Capsule
D. Ribosome
Correct answer: B
Explanation: The sex pilus, encoded by F-plasmid genes, forms a bridge between
donor and recipient cells for single-stranded plasmid DNA transfer.
4. A bacterium that grows best at 45°C but can survive at 37°C is termed:
A. Psychrophile
B. Mesophile
C. Thermophile
D. Hyperthermophile
Correct answer: C
Explanation: Thermophiles have an optimum above 45°C. Hyperthermophiles grow
above 80°C. Mesophiles prefer ~20–40°C.
5. The endosymbiotic theory for the origin of mitochondria is supported by
which evidence?
A. Mitochondria have 70S ribosomes
B. Mitochondria reproduce by binary fission
C. Mitochondria have circular DNA
D. All of the above
Correct answer: D
Explanation: All three are true: 70S ribosomes (prokaryotic size), binary fission (not
,mitosis), and circular DNA (like bacteria) strongly support endosymbiosis from an
alphaproteobacterium.
6. Which enzyme is used by retroviruses to convert RNA into DNA?
A. RNA polymerase
B. DNA polymerase III
C. Reverse transcriptase
D. Helicase
Correct answer: C
Explanation: Reverse transcriptase is a retroviral enzyme (e.g., HIV) that
synthesizes DNA from an RNA template, allowing integration into the host
genome.
7. The Ames test detects:
A. Antibiotic resistance
B. Mutagenicity of chemicals
C. Viral titer
D. Bacterial motility
Correct answer: B
Explanation: The Ames test uses histidine-auxotrophic Salmonella; if a chemical
causes back-mutations to prototrophy (growth on His-free medium), it’s a
potential mutagen/carcinogen.
8. Which bacterial genus produces endospores?
A. Escherichia
B. Staphylococcus
C. Clostridium
D. Pseudomonas
, Correct answer: C
Explanation: Clostridium (anaerobic) and Bacillus (aerobic) are the main
endospore-formers. Endospores resist heat, chemicals, and radiation.
9. A culture tube shows growth only at the very top (surface pellicle). The
organism is:
A. Obligate aerobe
B. Obligate anaerobe
C. Facultative anaerobe
D. Aerotolerant anaerobe
Correct answer: A
Explanation: Obligate aerobes require O₂ and grow only at the air-liquid interface.
Facultative anaerobes grow throughout but best aerobically; aerotolerant
anaerobes grow uniformly.
10. Which of the following is a mechanism of horizontal gene transfer that
requires cell-to-cell contact?
A. Transformation
B. Transduction
C. Conjugation
D. Transposition
Correct answer: C
Explanation: Conjugation requires direct contact via pili. Transformation = uptake
free DNA; transduction = phage-mediated; transposition = within same genome.
11. The Gram stain differentiates bacteria based on:
A. Cell shape
B. Cell wall structure
QUESTIONS COMPLETE WITH 100% VERIFIED
ANSWERS AND DETAILED EXPLANATIONS
1. Which of the following metabolic processes is unique to prokaryotes?
A. Glycolysis
B. Chemolithotrophy
C. Photosynthesis
D. Aerobic respiration
Correct answer: B
Explanation: Chemolithotrophy (using inorganic compounds like H₂S, NH₃, or Fe²⁺
as electron donors) is found only in certain bacteria and archaea. Glycolysis,
photosynthesis (though not oxygenic in all), and aerobic respiration occur in some
eukaryotes as well.
2. A clinical isolate shows growth on MacConkey agar as colorless colonies. This
indicates the bacterium is most likely:
A. Lactose-fermenting coliform
B. Gram-positive coccus
C. Non-lactose-fermenting Gram-negative rod
D. Obligate anaerobe
Correct answer: C
Explanation: MacConkey agar selects for Gram-negatives and differentiates
lactose fermenters (pink). Colorless colonies = no lactose fermentation, typical of
pathogens like Salmonella, Shigella, or Proteus.
,3. Which structure is directly responsible for transfer of plasmid DNA during
conjugation?
A. Flagellum
B. Pilus (sex pilus)
C. Capsule
D. Ribosome
Correct answer: B
Explanation: The sex pilus, encoded by F-plasmid genes, forms a bridge between
donor and recipient cells for single-stranded plasmid DNA transfer.
4. A bacterium that grows best at 45°C but can survive at 37°C is termed:
A. Psychrophile
B. Mesophile
C. Thermophile
D. Hyperthermophile
Correct answer: C
Explanation: Thermophiles have an optimum above 45°C. Hyperthermophiles grow
above 80°C. Mesophiles prefer ~20–40°C.
5. The endosymbiotic theory for the origin of mitochondria is supported by
which evidence?
A. Mitochondria have 70S ribosomes
B. Mitochondria reproduce by binary fission
C. Mitochondria have circular DNA
D. All of the above
Correct answer: D
Explanation: All three are true: 70S ribosomes (prokaryotic size), binary fission (not
,mitosis), and circular DNA (like bacteria) strongly support endosymbiosis from an
alphaproteobacterium.
6. Which enzyme is used by retroviruses to convert RNA into DNA?
A. RNA polymerase
B. DNA polymerase III
C. Reverse transcriptase
D. Helicase
Correct answer: C
Explanation: Reverse transcriptase is a retroviral enzyme (e.g., HIV) that
synthesizes DNA from an RNA template, allowing integration into the host
genome.
7. The Ames test detects:
A. Antibiotic resistance
B. Mutagenicity of chemicals
C. Viral titer
D. Bacterial motility
Correct answer: B
Explanation: The Ames test uses histidine-auxotrophic Salmonella; if a chemical
causes back-mutations to prototrophy (growth on His-free medium), it’s a
potential mutagen/carcinogen.
8. Which bacterial genus produces endospores?
A. Escherichia
B. Staphylococcus
C. Clostridium
D. Pseudomonas
, Correct answer: C
Explanation: Clostridium (anaerobic) and Bacillus (aerobic) are the main
endospore-formers. Endospores resist heat, chemicals, and radiation.
9. A culture tube shows growth only at the very top (surface pellicle). The
organism is:
A. Obligate aerobe
B. Obligate anaerobe
C. Facultative anaerobe
D. Aerotolerant anaerobe
Correct answer: A
Explanation: Obligate aerobes require O₂ and grow only at the air-liquid interface.
Facultative anaerobes grow throughout but best aerobically; aerotolerant
anaerobes grow uniformly.
10. Which of the following is a mechanism of horizontal gene transfer that
requires cell-to-cell contact?
A. Transformation
B. Transduction
C. Conjugation
D. Transposition
Correct answer: C
Explanation: Conjugation requires direct contact via pili. Transformation = uptake
free DNA; transduction = phage-mediated; transposition = within same genome.
11. The Gram stain differentiates bacteria based on:
A. Cell shape
B. Cell wall structure
