BIO 250 Exam 3 V2 | BIO 250 Microbiology | Actual Q&A with
Rationale (BIO250 Exam 3) | StraighterLine
1. Which of the following processes are considered mechanisms of horizontal gene transfer
among bacterial populations? Select all that apply.
A. Transformation
B. Transduction
C. Transformation, Transduction, and Conjugation
D. Binary Fission
E. Conjugation
F. Mitosis and Meiosis
Correct Answer: C
Explanation: Horizontal gene transfer involves the movement of genetic material between
unicellular and/or multicellular organisms other than by the transmission of DNA from
parent to offspring. Transformation involves the uptake of naked DNA, transduction is
mediated by bacteriophages, and conjugation requires direct cell-to-cell contact via a pilus.
Binary fission, mitosis, and meiosis are forms of vertical transfer or cell division and do not
constitute horizontal exchange.
2. In DNA replication, which enzyme is responsible for adding bases to the new DNA chain
and proofreading the chain for mistakes?
A. DNA Ligase
B. Helicase
C. DNA Polymerase III
D. Primase
Correct Answer: C
Explanation: DNA Polymerase III is the primary enzyme involved in the synthesis of new
DNA strands by adding nucleotides to the 3 prime end of a growing chain. It also possesses
an exonuclease activity that allows it to proofread and correct mismatched bases
immediately after they are added. Other enzymes like helicase unwind the DNA, and ligase
seals the nicks in the sugar-phosphate backbone.
3. The specific sequence of three bases on a tRNA molecule that is complementary to a codon
on an mRNA molecule is known as the:
A. Codon
,B. Termination sequence
C. Promoter
D. Anticodon
Correct Answer: D
Explanation: The anticodon is a three-nucleotide sequence on tRNA that corresponds to a
specific amino acid and binds to the complementary codon on mRNA. This interaction
ensures that the correct amino acid is incorporated into the growing polypeptide chain
during translation. This specific base-pairing is fundamental to the fidelity of the genetic
code’s expression.
4. Which of the following best describes a ‘silent mutation’?
A. A mutation that results in a stop codon and premature termination.
B. A mutation that changes the base sequence but does not change the resulting amino acid.
C. A mutation that changes a single amino acid in the protein.
D. A mutation caused by the insertion or deletion of nucleotides.
Correct Answer: B
Explanation: A silent mutation occurs when a change in the DNA nucleotide sequence does
not result in a change in the amino acid sequence of the protein produced. This is possible
due to the degeneracy of the genetic code, where multiple codons can code for the same
amino acid. While the genotype is altered, the phenotype typically remains unchanged
because the protein structure is preserved.
5. The process of using a cleansing technique to mechanically remove and reduce
microorganisms and debris to safe public health levels is:
A. Sanitization
B. Sterilization
C. Disinfection
D. Antisepsis
Correct Answer: A
Explanation: Sanitization is a decontamination process that reduces microbial populations
to levels deemed safe by public health standards. It is often used in the food industry and
for cleaning public surfaces, involving both mechanical cleaning and chemical agents.
Unlike sterilization, it does not guarantee the destruction of all microbial life, especially
endospores.
, 6. Which of the following factors affect the germicidal activity of chemical agents? Select all
that apply.
A. The nature of the microorganisms being treated
B. The degree of contamination (microbial load)
C. The time of exposure to the agent
D. All of the above (A, B, C, and D) are correct.
E. The concentration of the chemical agent
Correct Answer: D
Explanation: The effectiveness of a chemical germicide is influenced by several critical
factors including the resistance level of the microbes present and the total number of cells
in the population. Environmental conditions like temperature and pH, as well as the
concentration of the chemical and the duration of contact, also play vital roles. High
concentrations and longer exposure times generally increase the efficacy of the treatment.
7. What is the primary action of the autoclave as a physical method of microbial control?
A. Dry heat at high atmospheric pressure
B. Incineration of microbial cells
C. Ionizing radiation to disrupt DNA
D. Steam under pressure to achieve sterilization
Correct Answer: D
Explanation: An autoclave uses pressurized steam to reach temperatures above the
boiling point of water (typically 121 degrees Celsius at 15 psi). This intense moist heat is
highly effective at denaturing proteins and destroying all forms of microbial life, including
highly resistant bacterial endospores. It is considered the gold standard for sterilization in
clinical and laboratory settings.
8. An antimicrobial drug that inhibits the growth of bacteria but does not necessarily kill them
is described as:
A. Bacteriostatic
B. Bactericidal
C. Narrow-spectrum
D. Broad-spectrum
Correct Answer: A
Explanation: Bacteriostatic agents prevent the growth and reproduction of bacteria
without killing them directly, relying on the host’s immune system to eliminate the
Rationale (BIO250 Exam 3) | StraighterLine
1. Which of the following processes are considered mechanisms of horizontal gene transfer
among bacterial populations? Select all that apply.
A. Transformation
B. Transduction
C. Transformation, Transduction, and Conjugation
D. Binary Fission
E. Conjugation
F. Mitosis and Meiosis
Correct Answer: C
Explanation: Horizontal gene transfer involves the movement of genetic material between
unicellular and/or multicellular organisms other than by the transmission of DNA from
parent to offspring. Transformation involves the uptake of naked DNA, transduction is
mediated by bacteriophages, and conjugation requires direct cell-to-cell contact via a pilus.
Binary fission, mitosis, and meiosis are forms of vertical transfer or cell division and do not
constitute horizontal exchange.
2. In DNA replication, which enzyme is responsible for adding bases to the new DNA chain
and proofreading the chain for mistakes?
A. DNA Ligase
B. Helicase
C. DNA Polymerase III
D. Primase
Correct Answer: C
Explanation: DNA Polymerase III is the primary enzyme involved in the synthesis of new
DNA strands by adding nucleotides to the 3 prime end of a growing chain. It also possesses
an exonuclease activity that allows it to proofread and correct mismatched bases
immediately after they are added. Other enzymes like helicase unwind the DNA, and ligase
seals the nicks in the sugar-phosphate backbone.
3. The specific sequence of three bases on a tRNA molecule that is complementary to a codon
on an mRNA molecule is known as the:
A. Codon
,B. Termination sequence
C. Promoter
D. Anticodon
Correct Answer: D
Explanation: The anticodon is a three-nucleotide sequence on tRNA that corresponds to a
specific amino acid and binds to the complementary codon on mRNA. This interaction
ensures that the correct amino acid is incorporated into the growing polypeptide chain
during translation. This specific base-pairing is fundamental to the fidelity of the genetic
code’s expression.
4. Which of the following best describes a ‘silent mutation’?
A. A mutation that results in a stop codon and premature termination.
B. A mutation that changes the base sequence but does not change the resulting amino acid.
C. A mutation that changes a single amino acid in the protein.
D. A mutation caused by the insertion or deletion of nucleotides.
Correct Answer: B
Explanation: A silent mutation occurs when a change in the DNA nucleotide sequence does
not result in a change in the amino acid sequence of the protein produced. This is possible
due to the degeneracy of the genetic code, where multiple codons can code for the same
amino acid. While the genotype is altered, the phenotype typically remains unchanged
because the protein structure is preserved.
5. The process of using a cleansing technique to mechanically remove and reduce
microorganisms and debris to safe public health levels is:
A. Sanitization
B. Sterilization
C. Disinfection
D. Antisepsis
Correct Answer: A
Explanation: Sanitization is a decontamination process that reduces microbial populations
to levels deemed safe by public health standards. It is often used in the food industry and
for cleaning public surfaces, involving both mechanical cleaning and chemical agents.
Unlike sterilization, it does not guarantee the destruction of all microbial life, especially
endospores.
, 6. Which of the following factors affect the germicidal activity of chemical agents? Select all
that apply.
A. The nature of the microorganisms being treated
B. The degree of contamination (microbial load)
C. The time of exposure to the agent
D. All of the above (A, B, C, and D) are correct.
E. The concentration of the chemical agent
Correct Answer: D
Explanation: The effectiveness of a chemical germicide is influenced by several critical
factors including the resistance level of the microbes present and the total number of cells
in the population. Environmental conditions like temperature and pH, as well as the
concentration of the chemical and the duration of contact, also play vital roles. High
concentrations and longer exposure times generally increase the efficacy of the treatment.
7. What is the primary action of the autoclave as a physical method of microbial control?
A. Dry heat at high atmospheric pressure
B. Incineration of microbial cells
C. Ionizing radiation to disrupt DNA
D. Steam under pressure to achieve sterilization
Correct Answer: D
Explanation: An autoclave uses pressurized steam to reach temperatures above the
boiling point of water (typically 121 degrees Celsius at 15 psi). This intense moist heat is
highly effective at denaturing proteins and destroying all forms of microbial life, including
highly resistant bacterial endospores. It is considered the gold standard for sterilization in
clinical and laboratory settings.
8. An antimicrobial drug that inhibits the growth of bacteria but does not necessarily kill them
is described as:
A. Bacteriostatic
B. Bactericidal
C. Narrow-spectrum
D. Broad-spectrum
Correct Answer: A
Explanation: Bacteriostatic agents prevent the growth and reproduction of bacteria
without killing them directly, relying on the host’s immune system to eliminate the
