MCB 3020C Exam 3 V2 | MCB 3020C General Microbiology | Actual
Q&A with Rationale (MCB3020C Exam 3) | University of Central
Florida
1. Which enzyme is primarily responsible for synthesizing the new DNA strand during
replication in E. coli?
A. DNA Polymerase I
B. DNA Gyrase
C. DNA Ligase
D. DNA Polymerase III
Answer: D
Explanation: DNA Polymerase III is the main enzyme responsible for the elongation of the
DNA strand. It adds nucleotides to the 3’ end of the growing chain with high processivity.
This enzyme also possesses 3’ to 5’ exonuclease activity for proofreading.
2. The role of the sigma factor in bacterial transcription is to:
A. Recognize and bind to the promoter sequence
B. Terminate transcription at the rho site
C. Catalyze the formation of phosphodiester bonds
D. Unwind the DNA double helix
Answer: A
Explanation: The sigma factor is a subunit of the RNA polymerase holoenzyme that directs
the core enzyme to specific promoter regions. It ensures that transcription initiates at the
correct start site. Once transcription begins, the sigma factor is typically released.
3. In the lac operon, which molecule acts as the inducer by binding to the repressor protein?
A. Glucose
B. cAMP
C. Allolactose
D. Galactose
Answer: C
,Explanation: Allolactose is an isomer of lactose that binds to the lac repressor, causing a
conformational change. This change prevents the repressor from binding to the operator
site. Consequently, RNA polymerase can proceed with transcription of the structural genes.
4. What is the function of the enzyme aminoacyl-tRNA synthetase?
A. Synthesizing mRNA from a DNA template
B. Attaching the correct amino acid to its corresponding tRNA
C. Forming peptide bonds between amino acids
D. Binding the mRNA to the 30S ribosomal subunit
Answer: B
Explanation: Aminoacyl-tRNA synthetase is responsible for the ‘charging’ of tRNA
molecules. Each enzyme is specific for one amino acid and its corresponding tRNA. This
process is crucial for ensuring the accuracy of the genetic code during translation.
5. Which type of horizontal gene transfer involves the uptake of ‘naked’ DNA from the
environment?
A. Conjugation
B. Transduction
C. Transposition
D. Transformation
Answer: D
Explanation: Transformation is the process by which a competent bacterial cell takes up
free DNA fragments from its surroundings. This DNA can then be integrated into the host
genome through recombination. This phenomenon was first demonstrated by Frederick
Griffith in his experiments with Streptococcus pneumoniae.
6. During DNA replication, which enzyme relieves the torsional strain (supercoiling) ahead of
the replication fork?
A. Helicase
B. Single-strand binding proteins
C. Primase
D. DNA Gyrase (Topoisomerase II)
Answer: D
Explanation: DNA Gyrase introduces negative supercoils to counteract the positive
supercoiling caused by helicase unwinding the DNA. By breaking and rejoining DNA
, strands, it prevents the DNA from becoming overwound. This is an essential step to allow
the replication fork to continue moving.
7. A mutation that results in the replacement of a single amino acid with a different one is
known as a:
A. Silent mutation
B. Nonsense mutation
C. Missense mutation
D. Frameshift mutation
Answer: C
Explanation: A missense mutation occurs when a single nucleotide change results in a
codon that codes for a different amino acid. Depending on the properties of the new amino
acid, this may or may not affect protein function. This is a common type of point mutation
observed in bacterial genomes.
8. Which ribosomal site does the initiator tRNA-fMet occupy at the beginning of translation?
A. P site
B. A site
C. E site
D. Binding site
Answer: A
Explanation: In bacteria, the initiator tRNA carrying formylmethionine (fMet) binds
directly to the P (peptidyl) site of the ribosome. All subsequent tRNAs enter through the A
(aminoacyl) site. This positioning allows the formation of the first peptide bond during the
elongation phase.
9. The transfer of DNA from a donor to a recipient cell via a bacteriophage is called:
A. Transfection
D. Transformation
C. Conjugation
D. Transduction
Answer: D
Explanation: Transduction occurs when a virus (bacteriophage) accidentally packages
bacterial DNA into its capsid and delivers it to a new host cell. If the DNA is integrated into
the new host’s genome, horizontal gene transfer is achieved. There are two types:
generalized and specialized transduction.
Q&A with Rationale (MCB3020C Exam 3) | University of Central
Florida
1. Which enzyme is primarily responsible for synthesizing the new DNA strand during
replication in E. coli?
A. DNA Polymerase I
B. DNA Gyrase
C. DNA Ligase
D. DNA Polymerase III
Answer: D
Explanation: DNA Polymerase III is the main enzyme responsible for the elongation of the
DNA strand. It adds nucleotides to the 3’ end of the growing chain with high processivity.
This enzyme also possesses 3’ to 5’ exonuclease activity for proofreading.
2. The role of the sigma factor in bacterial transcription is to:
A. Recognize and bind to the promoter sequence
B. Terminate transcription at the rho site
C. Catalyze the formation of phosphodiester bonds
D. Unwind the DNA double helix
Answer: A
Explanation: The sigma factor is a subunit of the RNA polymerase holoenzyme that directs
the core enzyme to specific promoter regions. It ensures that transcription initiates at the
correct start site. Once transcription begins, the sigma factor is typically released.
3. In the lac operon, which molecule acts as the inducer by binding to the repressor protein?
A. Glucose
B. cAMP
C. Allolactose
D. Galactose
Answer: C
,Explanation: Allolactose is an isomer of lactose that binds to the lac repressor, causing a
conformational change. This change prevents the repressor from binding to the operator
site. Consequently, RNA polymerase can proceed with transcription of the structural genes.
4. What is the function of the enzyme aminoacyl-tRNA synthetase?
A. Synthesizing mRNA from a DNA template
B. Attaching the correct amino acid to its corresponding tRNA
C. Forming peptide bonds between amino acids
D. Binding the mRNA to the 30S ribosomal subunit
Answer: B
Explanation: Aminoacyl-tRNA synthetase is responsible for the ‘charging’ of tRNA
molecules. Each enzyme is specific for one amino acid and its corresponding tRNA. This
process is crucial for ensuring the accuracy of the genetic code during translation.
5. Which type of horizontal gene transfer involves the uptake of ‘naked’ DNA from the
environment?
A. Conjugation
B. Transduction
C. Transposition
D. Transformation
Answer: D
Explanation: Transformation is the process by which a competent bacterial cell takes up
free DNA fragments from its surroundings. This DNA can then be integrated into the host
genome through recombination. This phenomenon was first demonstrated by Frederick
Griffith in his experiments with Streptococcus pneumoniae.
6. During DNA replication, which enzyme relieves the torsional strain (supercoiling) ahead of
the replication fork?
A. Helicase
B. Single-strand binding proteins
C. Primase
D. DNA Gyrase (Topoisomerase II)
Answer: D
Explanation: DNA Gyrase introduces negative supercoils to counteract the positive
supercoiling caused by helicase unwinding the DNA. By breaking and rejoining DNA
, strands, it prevents the DNA from becoming overwound. This is an essential step to allow
the replication fork to continue moving.
7. A mutation that results in the replacement of a single amino acid with a different one is
known as a:
A. Silent mutation
B. Nonsense mutation
C. Missense mutation
D. Frameshift mutation
Answer: C
Explanation: A missense mutation occurs when a single nucleotide change results in a
codon that codes for a different amino acid. Depending on the properties of the new amino
acid, this may or may not affect protein function. This is a common type of point mutation
observed in bacterial genomes.
8. Which ribosomal site does the initiator tRNA-fMet occupy at the beginning of translation?
A. P site
B. A site
C. E site
D. Binding site
Answer: A
Explanation: In bacteria, the initiator tRNA carrying formylmethionine (fMet) binds
directly to the P (peptidyl) site of the ribosome. All subsequent tRNAs enter through the A
(aminoacyl) site. This positioning allows the formation of the first peptide bond during the
elongation phase.
9. The transfer of DNA from a donor to a recipient cell via a bacteriophage is called:
A. Transfection
D. Transformation
C. Conjugation
D. Transduction
Answer: D
Explanation: Transduction occurs when a virus (bacteriophage) accidentally packages
bacterial DNA into its capsid and delivers it to a new host cell. If the DNA is integrated into
the new host’s genome, horizontal gene transfer is achieved. There are two types:
generalized and specialized transduction.
