PCB 3023C Exam 3: Molecular Biology and Cell
Cycle Questions & Answers | Grade A | 100%
Correct
1. During DNA replication, which enzyme is responsible for unwinding the
double helix at the replication fork?
A. DNA Polymerase III
B. Helicase
C. DNA Ligase
D. Topoisomerase
Answer: B
Explanation: Helicase breaks the hydrogen bonds between the two strands of DNA to
separate them, creating the replication fork.
2. Which of the following describes the Meselson-Stahl experiment’s conclusion
about DNA replication?
A. Semiconservative
B. Conservative
C. Dispersive
D. Non-conservative
Answer: A
Explanation: Meselson and Stahl used nitrogen isotopes to prove that each daughter DNA
molecule consists of one old strand and one newly synthesized strand.
,3. What is the function of topoisomerase during DNA replication?
A. To relieve torsional strain caused by overwinding
B. To seal nicks in the sugar-phosphate backbone
C. To prevent the re-annealing of single strands
D. To add RNA primers
Answer: A
Explanation: As DNA is unwound by helicase, the DNA ahead of the fork becomes
overwound; topoisomerase cuts and rejoins the strands to relieve this stress.
4. Okazaki fragments are synthesized on which strand?
A. Leading strand
B. Coding strand
C. Template strand
D. Lagging strand
Answer: D
Explanation: DNA polymerase can only synthesize in the 5’ to 3’ direction. Because the
lagging strand is oriented 3’ to 5’ relative to the fork movement, it must be synthesized in
short fragments.
5. Which enzyme replaces RNA primers with DNA nucleotides in E. coli?
A. DNA Polymerase III
B. DNA Polymerase I
C. Primase
D. DNA Ligase
Answer: B
Explanation: DNA Polymerase I has 5’ to 3’ exonuclease activity which allows it to remove
RNA primers and replace them with DNA.
, 6. What is the role of telomerase in eukaryotic cells?
A. To repair thymine dimers
B. To initiate transcription
C. To extend the 3’ ends of chromosomes to prevent shortening
D. To condense chromatin into chromosomes
Answer: C
Explanation: Telomerase uses an RNA template to extend the ends of chromosomes,
compensating for the ‘end-replication problem’.
7. In DNA replication, which protein prevents single-stranded DNA from re-
forming a double helix?
A. Clamp loader
B. Single-strand binding proteins (SSBPs)
C. Sliding clamp
D. Histones
Answer: B
Explanation: SSBPs bind to the exposed single strands to keep them stable and separated
until replication can occur.
8. Which of the following is true regarding the proofreading mechanism of DNA
Polymerase?
A. It uses 5’ to 3’ exonuclease activity
B. It occurs only after replication is fully complete
C. It uses 3’ to 5’ exonuclease activity
D. It involves the spliceosome
Answer: C
Explanation: DNA polymerases can backtrack and remove an incorrect nucleotide via 3’ to
5’ exonuclease activity before continuing synthesis.
Cycle Questions & Answers | Grade A | 100%
Correct
1. During DNA replication, which enzyme is responsible for unwinding the
double helix at the replication fork?
A. DNA Polymerase III
B. Helicase
C. DNA Ligase
D. Topoisomerase
Answer: B
Explanation: Helicase breaks the hydrogen bonds between the two strands of DNA to
separate them, creating the replication fork.
2. Which of the following describes the Meselson-Stahl experiment’s conclusion
about DNA replication?
A. Semiconservative
B. Conservative
C. Dispersive
D. Non-conservative
Answer: A
Explanation: Meselson and Stahl used nitrogen isotopes to prove that each daughter DNA
molecule consists of one old strand and one newly synthesized strand.
,3. What is the function of topoisomerase during DNA replication?
A. To relieve torsional strain caused by overwinding
B. To seal nicks in the sugar-phosphate backbone
C. To prevent the re-annealing of single strands
D. To add RNA primers
Answer: A
Explanation: As DNA is unwound by helicase, the DNA ahead of the fork becomes
overwound; topoisomerase cuts and rejoins the strands to relieve this stress.
4. Okazaki fragments are synthesized on which strand?
A. Leading strand
B. Coding strand
C. Template strand
D. Lagging strand
Answer: D
Explanation: DNA polymerase can only synthesize in the 5’ to 3’ direction. Because the
lagging strand is oriented 3’ to 5’ relative to the fork movement, it must be synthesized in
short fragments.
5. Which enzyme replaces RNA primers with DNA nucleotides in E. coli?
A. DNA Polymerase III
B. DNA Polymerase I
C. Primase
D. DNA Ligase
Answer: B
Explanation: DNA Polymerase I has 5’ to 3’ exonuclease activity which allows it to remove
RNA primers and replace them with DNA.
, 6. What is the role of telomerase in eukaryotic cells?
A. To repair thymine dimers
B. To initiate transcription
C. To extend the 3’ ends of chromosomes to prevent shortening
D. To condense chromatin into chromosomes
Answer: C
Explanation: Telomerase uses an RNA template to extend the ends of chromosomes,
compensating for the ‘end-replication problem’.
7. In DNA replication, which protein prevents single-stranded DNA from re-
forming a double helix?
A. Clamp loader
B. Single-strand binding proteins (SSBPs)
C. Sliding clamp
D. Histones
Answer: B
Explanation: SSBPs bind to the exposed single strands to keep them stable and separated
until replication can occur.
8. Which of the following is true regarding the proofreading mechanism of DNA
Polymerase?
A. It uses 5’ to 3’ exonuclease activity
B. It occurs only after replication is fully complete
C. It uses 3’ to 5’ exonuclease activity
D. It involves the spliceosome
Answer: C
Explanation: DNA polymerases can backtrack and remove an incorrect nucleotide via 3’ to
5’ exonuclease activity before continuing synthesis.
