MCB 3020C Exam 3 V3 | MCB 3020C General Microbiology | Actual
Q&A with Rationale (MCB3020C Exam 3) | University of Central
Florida
1. During DNA replication, which enzyme is responsible for relieving the torsional stress
(supercoiling) ahead of the replication fork?
A. DNA Helicase
B. DNA Gyrase (Topoisomerase II)
C. Primase
D. DNA Polymerase I
Answer: B
Explanation: DNA Gyrase, a type of topoisomerase, introduces negative supercoils to
neutralize the positive supercoiling caused by unwinding. This enzymatic activity prevents
the DNA from becoming overwound and stalling the replication process. It is a critical
target for antibiotics such as quinolones.
2. In prokaryotic transcription, which component of the RNA polymerase holoenzyme is
specifically required for promoter recognition?
A. Alpha subunit
B. Beta prime subunit
C. Sigma factor
D. Omega subunit
Answer: C
Explanation: The sigma factor is an initiation factor that enables specific binding of RNA
polymerase to gene promoters. Once transcription is initiated and a short stretch of RNA is
synthesized, the sigma factor usually dissociates from the core enzyme. Different sigma
factors allow the cell to respond to various environmental stresses by directing the
polymerase to different sets of genes.
3. What is the function of the Shine-Dalgarno sequence in bacterial translation?
A. Ribosome binding site positioning on mRNA
B. Termination of the polypeptide chain
C. Initiation of DNA replication
D. Encoding the first amino acid
,Answer: A
Explanation: The Shine-Dalgarno sequence is a ribosomal binding site in bacterial
messenger RNA, generally located around 8 bases upstream of the start codon AUG. It helps
recruit the 30S ribosomal subunit to the messenger RNA to initiate protein synthesis by
aligning it correctly. This sequence is complementary to a section at the 3’ end of the 16S
rRNA.
4. Under which condition is the Lac operon expressed at the highest levels?
A. High glucose and high lactose
B. High glucose and no lactose
C. Low glucose and high lactose
D. Low glucose and no lactose
Answer: C
Explanation: Maximum expression of the lac operon requires both the presence of the
inducer (allolactose) and high levels of cAMP. When glucose is low, cAMP levels rise,
allowing the cAMP-CAP complex to bind and recruit RNA polymerase. Lactose must be
present to remove the lac repressor from the operator site.
5. Which type of horizontal gene transfer requires direct physical contact between a donor
and a recipient cell?
A. Conjugation
B. Transduction
C. Transformation
D. Transposition
Answer: A
Explanation: Conjugation involves the transfer of genetic material via a sex pilus or direct
cell-to-cell contact. This process is often mediated by plasmids such as the F plasmid in
Escherichia coli. It is one of the primary methods for the rapid spread of antibiotic
resistance among bacterial populations.
6. Which enzyme catalyzes the synthesis of DNA from an RNA template in retroviruses?
A. DNA-dependent DNA polymerase
B. RNA Polymerase II
C. RNA Replicase
D. Reverse Transcriptase
Answer: D
, Explanation: Reverse transcriptase is an enzyme used by retroviruses like HIV to generate
complementary DNA (cDNA) from an RNA genome. This process violates the traditional
central dogma of molecular biology which states information flows from DNA to RNA. The
resulting DNA can then be integrated into the host cell’s genome.
7. What describes a mutation that results in the replacement of one amino acid with a
different amino acid in the resulting protein?
A. Silent mutation
B. Nonsense mutation
C. Missense mutation
D. Frameshift mutation
Answer: C
Explanation: A missense mutation is a point mutation in which a single nucleotide change
results in a codon that codes for a different amino acid. This can affect the protein’s folding
and function depending on the properties of the new amino acid. If the change occurs in a
critical site, such as an enzyme’s active site, it can lead to a loss of function.
8. In the tryptophan (trp) operon, what role does tryptophan play in regulation?
A. Corepressor
B. Inducer
C. Apo-repressor
D. Activator
Answer: A
Explanation: Tryptophan acts as a corepressor by binding to the inactive trp repressor
protein. When tryptophan levels are high, the complex binds to the operator to block
transcription. This is a classic example of negative feedback regulation in biosynthetic
pathways.
9. Which of the following describes a ‘temperate’ bacteriophage?
A. A virus capable of lysogeny and integration into the host genome
B. A virus that only undergoes the lytic cycle
C. A virus that infects only eukaryotic cells
D. A virus that lacks a protein capsid
Answer: A
Explanation: Temperate phages can choose between the lytic cycle, where they replicate
and burst the cell, or the lysogenic cycle. In lysogeny, the phage DNA (prophage) integrates
Q&A with Rationale (MCB3020C Exam 3) | University of Central
Florida
1. During DNA replication, which enzyme is responsible for relieving the torsional stress
(supercoiling) ahead of the replication fork?
A. DNA Helicase
B. DNA Gyrase (Topoisomerase II)
C. Primase
D. DNA Polymerase I
Answer: B
Explanation: DNA Gyrase, a type of topoisomerase, introduces negative supercoils to
neutralize the positive supercoiling caused by unwinding. This enzymatic activity prevents
the DNA from becoming overwound and stalling the replication process. It is a critical
target for antibiotics such as quinolones.
2. In prokaryotic transcription, which component of the RNA polymerase holoenzyme is
specifically required for promoter recognition?
A. Alpha subunit
B. Beta prime subunit
C. Sigma factor
D. Omega subunit
Answer: C
Explanation: The sigma factor is an initiation factor that enables specific binding of RNA
polymerase to gene promoters. Once transcription is initiated and a short stretch of RNA is
synthesized, the sigma factor usually dissociates from the core enzyme. Different sigma
factors allow the cell to respond to various environmental stresses by directing the
polymerase to different sets of genes.
3. What is the function of the Shine-Dalgarno sequence in bacterial translation?
A. Ribosome binding site positioning on mRNA
B. Termination of the polypeptide chain
C. Initiation of DNA replication
D. Encoding the first amino acid
,Answer: A
Explanation: The Shine-Dalgarno sequence is a ribosomal binding site in bacterial
messenger RNA, generally located around 8 bases upstream of the start codon AUG. It helps
recruit the 30S ribosomal subunit to the messenger RNA to initiate protein synthesis by
aligning it correctly. This sequence is complementary to a section at the 3’ end of the 16S
rRNA.
4. Under which condition is the Lac operon expressed at the highest levels?
A. High glucose and high lactose
B. High glucose and no lactose
C. Low glucose and high lactose
D. Low glucose and no lactose
Answer: C
Explanation: Maximum expression of the lac operon requires both the presence of the
inducer (allolactose) and high levels of cAMP. When glucose is low, cAMP levels rise,
allowing the cAMP-CAP complex to bind and recruit RNA polymerase. Lactose must be
present to remove the lac repressor from the operator site.
5. Which type of horizontal gene transfer requires direct physical contact between a donor
and a recipient cell?
A. Conjugation
B. Transduction
C. Transformation
D. Transposition
Answer: A
Explanation: Conjugation involves the transfer of genetic material via a sex pilus or direct
cell-to-cell contact. This process is often mediated by plasmids such as the F plasmid in
Escherichia coli. It is one of the primary methods for the rapid spread of antibiotic
resistance among bacterial populations.
6. Which enzyme catalyzes the synthesis of DNA from an RNA template in retroviruses?
A. DNA-dependent DNA polymerase
B. RNA Polymerase II
C. RNA Replicase
D. Reverse Transcriptase
Answer: D
, Explanation: Reverse transcriptase is an enzyme used by retroviruses like HIV to generate
complementary DNA (cDNA) from an RNA genome. This process violates the traditional
central dogma of molecular biology which states information flows from DNA to RNA. The
resulting DNA can then be integrated into the host cell’s genome.
7. What describes a mutation that results in the replacement of one amino acid with a
different amino acid in the resulting protein?
A. Silent mutation
B. Nonsense mutation
C. Missense mutation
D. Frameshift mutation
Answer: C
Explanation: A missense mutation is a point mutation in which a single nucleotide change
results in a codon that codes for a different amino acid. This can affect the protein’s folding
and function depending on the properties of the new amino acid. If the change occurs in a
critical site, such as an enzyme’s active site, it can lead to a loss of function.
8. In the tryptophan (trp) operon, what role does tryptophan play in regulation?
A. Corepressor
B. Inducer
C. Apo-repressor
D. Activator
Answer: A
Explanation: Tryptophan acts as a corepressor by binding to the inactive trp repressor
protein. When tryptophan levels are high, the complex binds to the operator to block
transcription. This is a classic example of negative feedback regulation in biosynthetic
pathways.
9. Which of the following describes a ‘temperate’ bacteriophage?
A. A virus capable of lysogeny and integration into the host genome
B. A virus that only undergoes the lytic cycle
C. A virus that infects only eukaryotic cells
D. A virus that lacks a protein capsid
Answer: A
Explanation: Temperate phages can choose between the lytic cycle, where they replicate
and burst the cell, or the lysogenic cycle. In lysogeny, the phage DNA (prophage) integrates
