Biochemistry Module 3 Comprehensive Review 2026 |WGU
1. Which component of a nucleotide is responsible for the negative charge of
DNA?
A. The pentose sugar
B. The phosphate group
C. The nitrogenous base
D. The hydrogen bond
Answer: B
Rationale: Phosphate groups in the DNA backbone carry a negative charge due to the
ionization of phosphoric acid at physiological pH.
2. In a DNA double helix, which type of bond holds the two strands together
between complementary bases?
A. Covalent bonds
B. Hydrogen bonds
C. Phosphodiester bonds
D. Ionic bonds
Answer: B
Rationale: Hydrogen bonds form between the nitrogenous bases (A with T, G with C) to
stabilize the double-stranded structure.
,3. Which enzyme is responsible for relieving torsional strain (supercoiling)
ahead of the replication fork?
A. DNA Helicase
B. DNA Ligase
C. Topoisomerase
D. DNA Polymerase III
Answer: C
Rationale: Topoisomerase (or DNA gyrase in prokaryotes) cuts and reseals the DNA
backbone to prevent overwinding during replication.
4. Which of the following describes the direction of DNA synthesis?
A. 3’ to 5’ on both strands
B. 3’ to 5’ on the leading strand and 5’ to 3’ on the lagging strand
C. 5’ to 3’ on the leading strand and 3’ to 5’ on the lagging strand
D. 5’ to 3’ on both strands
Answer: D
Rationale: DNA polymerase can only add nucleotides to the 3’ OH group of an existing
strand, meaning synthesis always occurs in the 5’ to 3’ direction.
5. What is the role of DNA Primase during DNA replication?
A. To join Okazaki fragments together
B. To synthesize a short RNA primer to provide a 3’-OH group
C. To remove RNA primers and replace them with DNA
D. To unwind the double helix
Answer: B
Rationale: DNA polymerase requires a free 3’-OH group to start synthesis; Primase
provides this by creating a short RNA sequence.
, 6. During DNA replication, which enzyme is responsible for joining the gaps
between Okazaki fragments on the lagging strand?
A. DNA Polymerase I
B. DNA Helicase
C. Single-strand binding proteins
D. DNA Ligase
Answer: D
Rationale: DNA Ligase catalyzes the formation of the phosphodiester bond that seals the
‘nick’ between DNA fragments.
7. Which nitrogenous base is found in RNA but not in DNA?
A. Thymine
B. Uracil
C. Guanine
D. Adenine
Answer: B
Rationale: RNA contains Uracil (U) which pairs with Adenine, whereas DNA contains
Thymine (T).
8. Chargaff’s rules state that in any sample of double-stranded DNA:
A. A = G and C = T
B. A = T and G = C
C. A + T = G + C
D. The amount of purines is twice the amount of pyrimidines
Answer: B
Rationale: Chargaff discovered that the amount of Adenine equals Thymine and Guanine
equals Cytosine due to specific base pairing.
1. Which component of a nucleotide is responsible for the negative charge of
DNA?
A. The pentose sugar
B. The phosphate group
C. The nitrogenous base
D. The hydrogen bond
Answer: B
Rationale: Phosphate groups in the DNA backbone carry a negative charge due to the
ionization of phosphoric acid at physiological pH.
2. In a DNA double helix, which type of bond holds the two strands together
between complementary bases?
A. Covalent bonds
B. Hydrogen bonds
C. Phosphodiester bonds
D. Ionic bonds
Answer: B
Rationale: Hydrogen bonds form between the nitrogenous bases (A with T, G with C) to
stabilize the double-stranded structure.
,3. Which enzyme is responsible for relieving torsional strain (supercoiling)
ahead of the replication fork?
A. DNA Helicase
B. DNA Ligase
C. Topoisomerase
D. DNA Polymerase III
Answer: C
Rationale: Topoisomerase (or DNA gyrase in prokaryotes) cuts and reseals the DNA
backbone to prevent overwinding during replication.
4. Which of the following describes the direction of DNA synthesis?
A. 3’ to 5’ on both strands
B. 3’ to 5’ on the leading strand and 5’ to 3’ on the lagging strand
C. 5’ to 3’ on the leading strand and 3’ to 5’ on the lagging strand
D. 5’ to 3’ on both strands
Answer: D
Rationale: DNA polymerase can only add nucleotides to the 3’ OH group of an existing
strand, meaning synthesis always occurs in the 5’ to 3’ direction.
5. What is the role of DNA Primase during DNA replication?
A. To join Okazaki fragments together
B. To synthesize a short RNA primer to provide a 3’-OH group
C. To remove RNA primers and replace them with DNA
D. To unwind the double helix
Answer: B
Rationale: DNA polymerase requires a free 3’-OH group to start synthesis; Primase
provides this by creating a short RNA sequence.
, 6. During DNA replication, which enzyme is responsible for joining the gaps
between Okazaki fragments on the lagging strand?
A. DNA Polymerase I
B. DNA Helicase
C. Single-strand binding proteins
D. DNA Ligase
Answer: D
Rationale: DNA Ligase catalyzes the formation of the phosphodiester bond that seals the
‘nick’ between DNA fragments.
7. Which nitrogenous base is found in RNA but not in DNA?
A. Thymine
B. Uracil
C. Guanine
D. Adenine
Answer: B
Rationale: RNA contains Uracil (U) which pairs with Adenine, whereas DNA contains
Thymine (T).
8. Chargaff’s rules state that in any sample of double-stranded DNA:
A. A = G and C = T
B. A = T and G = C
C. A + T = G + C
D. The amount of purines is twice the amount of pyrimidines
Answer: B
Rationale: Chargaff discovered that the amount of Adenine equals Thymine and Guanine
equals Cytosine due to specific base pairing.
