C785 Biochemistry Week 7 Study Guide 2026 |WGU
1. Which enzyme is responsible for unwinding the DNA double helix during
replication?
A. DNA Polymerase
B. Topoisomerase
C. Ligase
D. Helicase
Answer: D
Rationale: Helicase breaks the hydrogen bonds between the nitrogenous bases to separate
the two strands of the DNA double helix.
2. Which enzyme relieves the torsional strain (supercoiling) ahead of the
replication fork?
A. Primase
B. Topoisomerase
C. DNA Polymerase I
D. Helicase
Answer: B
Rationale: Topoisomerase (also known as DNA gyrase in prokaryotes) cuts and rejoins
DNA strands to prevent over-winding as the replication fork progresses.
,3. What is the function of Primase in DNA replication?
A. Joining Okazaki fragments
B. Synthesizing a short RNA primer
C. Proofreading the DNA strand
D. Removing RNA primers
Answer: B
Rationale: Primase synthesizes a short RNA primer that provides a 3’-OH group for DNA
polymerase to begin synthesis.
4. In which direction is a new DNA strand synthesized?
A. 3’ to 5’
B. N-terminus to C-terminus
C. 5’ to 3’
D. C-terminus to N-terminus
Answer: C
Rationale: DNA polymerase can only add new nucleotides to the 3’ end of a growing
strand, meaning synthesis always occurs in the 5’ to 3’ direction.
5. Which enzyme is responsible for joining Okazaki fragments on the lagging
strand?
A. DNA Polymerase III
B. DNA Polymerase I
C. RNA Polymerase
D. DNA Ligase
Answer: D
Rationale: DNA Ligase seals the nicks between Okazaki fragments by creating
phosphodiester bonds.
, 6. Which type of mutation results in the substitution of one amino acid for
another in a protein?
A. Silent mutation
B. Nonsense mutation
C. Missense mutation
D. Frameshift mutation
Answer: C
Rationale: A missense mutation is a point mutation that changes one codon so that it codes
for a different amino acid.
7. What is the effect of a nonsense mutation on a protein?
A. It changes one amino acid to another.
B. It has no effect on the protein sequence.
C. It creates a premature stop codon, shortening the protein.
D. It shifts the reading frame of the mRNA.
Answer: C
Rationale: A nonsense mutation converts a sense codon into a stop codon, resulting in a
truncated and usually nonfunctional protein.
8. Which DNA repair mechanism specifically fixes thymine dimers caused by UV
light?
A. Nucleotide Excision Repair (NER)
B. Mismatch Repair
C. Base Excision Repair (BER)
D. Homologous Recombination
Answer: A
Rationale: Nucleotide Excision Repair (NER) removes a short segment of DNA containing
bulky lesions like thymine dimers.
1. Which enzyme is responsible for unwinding the DNA double helix during
replication?
A. DNA Polymerase
B. Topoisomerase
C. Ligase
D. Helicase
Answer: D
Rationale: Helicase breaks the hydrogen bonds between the nitrogenous bases to separate
the two strands of the DNA double helix.
2. Which enzyme relieves the torsional strain (supercoiling) ahead of the
replication fork?
A. Primase
B. Topoisomerase
C. DNA Polymerase I
D. Helicase
Answer: B
Rationale: Topoisomerase (also known as DNA gyrase in prokaryotes) cuts and rejoins
DNA strands to prevent over-winding as the replication fork progresses.
,3. What is the function of Primase in DNA replication?
A. Joining Okazaki fragments
B. Synthesizing a short RNA primer
C. Proofreading the DNA strand
D. Removing RNA primers
Answer: B
Rationale: Primase synthesizes a short RNA primer that provides a 3’-OH group for DNA
polymerase to begin synthesis.
4. In which direction is a new DNA strand synthesized?
A. 3’ to 5’
B. N-terminus to C-terminus
C. 5’ to 3’
D. C-terminus to N-terminus
Answer: C
Rationale: DNA polymerase can only add new nucleotides to the 3’ end of a growing
strand, meaning synthesis always occurs in the 5’ to 3’ direction.
5. Which enzyme is responsible for joining Okazaki fragments on the lagging
strand?
A. DNA Polymerase III
B. DNA Polymerase I
C. RNA Polymerase
D. DNA Ligase
Answer: D
Rationale: DNA Ligase seals the nicks between Okazaki fragments by creating
phosphodiester bonds.
, 6. Which type of mutation results in the substitution of one amino acid for
another in a protein?
A. Silent mutation
B. Nonsense mutation
C. Missense mutation
D. Frameshift mutation
Answer: C
Rationale: A missense mutation is a point mutation that changes one codon so that it codes
for a different amino acid.
7. What is the effect of a nonsense mutation on a protein?
A. It changes one amino acid to another.
B. It has no effect on the protein sequence.
C. It creates a premature stop codon, shortening the protein.
D. It shifts the reading frame of the mRNA.
Answer: C
Rationale: A nonsense mutation converts a sense codon into a stop codon, resulting in a
truncated and usually nonfunctional protein.
8. Which DNA repair mechanism specifically fixes thymine dimers caused by UV
light?
A. Nucleotide Excision Repair (NER)
B. Mismatch Repair
C. Base Excision Repair (BER)
D. Homologous Recombination
Answer: A
Rationale: Nucleotide Excision Repair (NER) removes a short segment of DNA containing
bulky lesions like thymine dimers.
