BIOS 242 / BIOS242 Fundamentals of Microbiology
Exam 3 Practice Test Actual 2026/2027 with Detailed
Rationales | 100% Verified | Pass Guaranteed – A+
Graded
SECTION 1: MICROBIAL GENETICS & GENE REGULATION (12
Questions)
Q1: A bacterium acquires antibiotic resistance after exposure to DNA from a lysed
neighboring cell. Which mechanism of gene transfer is occurring?
A. Conjugation via sex pili
B. Transduction by bacteriophage
C. Transformation through uptake of naked DNA
D. Transposition by jumping genes
Correct Answer: C
Rationale: Correct because transformation involves the uptake of free DNA from the
environment; when a cell lyses, its DNA becomes available for uptake by competent
recipient bacteria.
Q2: In the lac operon, the repressor protein binds to which DNA sequence to prevent
transcription?
A. The promoter region where RNA polymerase attaches
B. The operator sequence adjacent to the promoter
C. The structural genes lacZ, lacY, and lacA
D. The CAP binding site upstream of the promoter
Correct Answer: B
,Rationale: Correct because the lac repressor binds specifically to the operator
sequence, blocking RNA polymerase access to the promoter and preventing
transcription of the structural genes.
Q3: A bacteriophage transfers bacterial DNA from one host cell to another during the
lytic cycle. Which gene transfer mechanism is described?
A. Generalized transduction
B. Specialized transduction
C. Conjugation
D. Transformation
Correct Answer: A
Rationale: Correct because generalized transduction occurs when bacteriophage
packaging mistakenly incorporates random host bacterial DNA instead of viral DNA,
transferring it to a new host during infection.
Q4: Which enzyme is responsible for synthesizing a complementary DNA strand
during bacterial replication?
A. DNA ligase
B. DNA polymerase III
C. RNA polymerase
D. Helicase
Correct Answer: B
Rationale: Correct because DNA polymerase III is the primary replicative polymerase
in bacteria that synthesizes new DNA strands by adding nucleotides in the 5' to 3'
direction using the parental strand as template.
Q5: A mutation changes a single nucleotide in a codon but does not alter the amino
acid sequence of the resulting protein. What type of mutation is this?
A. Nonsense mutation
B. Missense mutation
C. Silent mutation
D. Frameshift mutation
Correct Answer: C
, Rationale: Correct because silent mutations occur when a nucleotide substitution
results in a different codon that codes for the same amino acid due to the
degeneracy of the genetic code.
Q6: During translation, which RNA molecule carries amino acids to the ribosome?
A. Messenger RNA (mRNA)
B. Transfer RNA (tRNA)
C. Ribosomal RNA (rRNA)
D. Small nuclear RNA (snRNA)
Correct Answer: B
Rationale: Correct because tRNA molecules have anticodons that pair with mRNA
codons and carry the corresponding amino acid to the ribosome for protein
synthesis.
Q7: In bacterial conjugation, which plasmid carries the genes for sex pilus
formation?
A. R plasmid
B. F plasmid
C. Col plasmid
D. Virulence plasmid
Correct Answer: B
Rationale: Correct because the F (fertility) plasmid contains the tra genes that
encode the sex pilus and other proteins necessary for DNA transfer during
conjugation.
Q8: Which regulatory mechanism allows bacteria to respond to environmental
changes by altering gene expression without changing the DNA sequence?
A. Point mutation
B. Epigenetic regulation through DNA methylation
C. Chromosomal rearrangement
D. Gene duplication
Correct Answer: B
Exam 3 Practice Test Actual 2026/2027 with Detailed
Rationales | 100% Verified | Pass Guaranteed – A+
Graded
SECTION 1: MICROBIAL GENETICS & GENE REGULATION (12
Questions)
Q1: A bacterium acquires antibiotic resistance after exposure to DNA from a lysed
neighboring cell. Which mechanism of gene transfer is occurring?
A. Conjugation via sex pili
B. Transduction by bacteriophage
C. Transformation through uptake of naked DNA
D. Transposition by jumping genes
Correct Answer: C
Rationale: Correct because transformation involves the uptake of free DNA from the
environment; when a cell lyses, its DNA becomes available for uptake by competent
recipient bacteria.
Q2: In the lac operon, the repressor protein binds to which DNA sequence to prevent
transcription?
A. The promoter region where RNA polymerase attaches
B. The operator sequence adjacent to the promoter
C. The structural genes lacZ, lacY, and lacA
D. The CAP binding site upstream of the promoter
Correct Answer: B
,Rationale: Correct because the lac repressor binds specifically to the operator
sequence, blocking RNA polymerase access to the promoter and preventing
transcription of the structural genes.
Q3: A bacteriophage transfers bacterial DNA from one host cell to another during the
lytic cycle. Which gene transfer mechanism is described?
A. Generalized transduction
B. Specialized transduction
C. Conjugation
D. Transformation
Correct Answer: A
Rationale: Correct because generalized transduction occurs when bacteriophage
packaging mistakenly incorporates random host bacterial DNA instead of viral DNA,
transferring it to a new host during infection.
Q4: Which enzyme is responsible for synthesizing a complementary DNA strand
during bacterial replication?
A. DNA ligase
B. DNA polymerase III
C. RNA polymerase
D. Helicase
Correct Answer: B
Rationale: Correct because DNA polymerase III is the primary replicative polymerase
in bacteria that synthesizes new DNA strands by adding nucleotides in the 5' to 3'
direction using the parental strand as template.
Q5: A mutation changes a single nucleotide in a codon but does not alter the amino
acid sequence of the resulting protein. What type of mutation is this?
A. Nonsense mutation
B. Missense mutation
C. Silent mutation
D. Frameshift mutation
Correct Answer: C
, Rationale: Correct because silent mutations occur when a nucleotide substitution
results in a different codon that codes for the same amino acid due to the
degeneracy of the genetic code.
Q6: During translation, which RNA molecule carries amino acids to the ribosome?
A. Messenger RNA (mRNA)
B. Transfer RNA (tRNA)
C. Ribosomal RNA (rRNA)
D. Small nuclear RNA (snRNA)
Correct Answer: B
Rationale: Correct because tRNA molecules have anticodons that pair with mRNA
codons and carry the corresponding amino acid to the ribosome for protein
synthesis.
Q7: In bacterial conjugation, which plasmid carries the genes for sex pilus
formation?
A. R plasmid
B. F plasmid
C. Col plasmid
D. Virulence plasmid
Correct Answer: B
Rationale: Correct because the F (fertility) plasmid contains the tra genes that
encode the sex pilus and other proteins necessary for DNA transfer during
conjugation.
Q8: Which regulatory mechanism allows bacteria to respond to environmental
changes by altering gene expression without changing the DNA sequence?
A. Point mutation
B. Epigenetic regulation through DNA methylation
C. Chromosomal rearrangement
D. Gene duplication
Correct Answer: B
