C785 Biochemistry Module 4: DNA and RNA Study Guide 2026 |WGU
1. Which of the following describes the backbone of a DNA molecule?
A. Nitrogenous bases linked by hydrogen bonds
B. Fatty acid chains
C. Amino acids linked by peptide bonds
D. Alternating sugar and phosphate groups
Answer: D
Rationale: The DNA backbone consists of alternating deoxyribose sugar and phosphate
groups held together by phosphodiester bonds.
2. Which nitrogenous bases are classified as purines?
A. Adenine and Thymine
B. Cytosine and Thymine
C. Adenine and Guanine
D. Guanine and Cytosine
Answer: C
Rationale: Adenine and Guanine are purines, which have a double-ring structure, while
Cytosine and Thymine are pyrimidines with a single-ring structure.
3. How many hydrogen bonds form between a Cytosine and Guanine pair in
DNA?
A. One
B. Two
C. Three
D. Four
Answer: C
,Rationale: G-C base pairs are held together by three hydrogen bonds, making them slightly
stronger than A-T pairs, which have two.
4. In DNA replication, which enzyme is responsible for unwinding the double
helix?
A. Topoisomerase
B. Ligase
C. Primase
D. Helicase
Answer: D
Rationale: Helicase breaks the hydrogen bonds between the nitrogenous bases to separate
the DNA strands for replication.
5. In what direction is a new DNA strand always synthesized?
A. 3’ to 5’
B. C-terminus to N-terminus
C. N-terminus to C-terminus
D. 5’ to 3’
Answer: D
Rationale: DNA Polymerase can only add nucleotides to the 3’ end of a growing strand,
resulting in 5’ to 3’ synthesis.
6. What is the function of the enzyme Primase during replication?
A. To seal gaps between DNA fragments
B. To remove mismatched bases
C. To prevent DNA from supercoiling
D. To synthesize a short RNA primer
Answer: D
Rationale: DNA polymerase requires an existing 3’ hydroxyl group to start; Primase
provides this via a short RNA sequence.
, 7. Okazaki fragments are found on which of the following?
A. Leading strand
B. RNA transcript
C. Lagging strand
D. Amino acid chain
Answer: C
Rationale: Because DNA is antiparallel, the lagging strand is synthesized discontinuously
in short segments called Okazaki fragments.
8. Which enzyme joins Okazaki fragments together?
A. DNA Polymerase I
B. Helicase
C. Telomerase
D. Ligase
Answer: D
Rationale: DNA ligase creates the final phosphodiester bond to join the sugar-phosphate
backbones of the fragments.
9. Which enzyme relieves the torsional strain (supercoiling) ahead of the
replication fork?
A. SSB proteins
B. Topoisomerase
C. Primase
D. Exonuclease
Answer: B
Rationale: Topoisomerase (or DNA Gyrase in bacteria) cuts and reseals DNA to prevent it
from becoming overwound during unwinding.
1. Which of the following describes the backbone of a DNA molecule?
A. Nitrogenous bases linked by hydrogen bonds
B. Fatty acid chains
C. Amino acids linked by peptide bonds
D. Alternating sugar and phosphate groups
Answer: D
Rationale: The DNA backbone consists of alternating deoxyribose sugar and phosphate
groups held together by phosphodiester bonds.
2. Which nitrogenous bases are classified as purines?
A. Adenine and Thymine
B. Cytosine and Thymine
C. Adenine and Guanine
D. Guanine and Cytosine
Answer: C
Rationale: Adenine and Guanine are purines, which have a double-ring structure, while
Cytosine and Thymine are pyrimidines with a single-ring structure.
3. How many hydrogen bonds form between a Cytosine and Guanine pair in
DNA?
A. One
B. Two
C. Three
D. Four
Answer: C
,Rationale: G-C base pairs are held together by three hydrogen bonds, making them slightly
stronger than A-T pairs, which have two.
4. In DNA replication, which enzyme is responsible for unwinding the double
helix?
A. Topoisomerase
B. Ligase
C. Primase
D. Helicase
Answer: D
Rationale: Helicase breaks the hydrogen bonds between the nitrogenous bases to separate
the DNA strands for replication.
5. In what direction is a new DNA strand always synthesized?
A. 3’ to 5’
B. C-terminus to N-terminus
C. N-terminus to C-terminus
D. 5’ to 3’
Answer: D
Rationale: DNA Polymerase can only add nucleotides to the 3’ end of a growing strand,
resulting in 5’ to 3’ synthesis.
6. What is the function of the enzyme Primase during replication?
A. To seal gaps between DNA fragments
B. To remove mismatched bases
C. To prevent DNA from supercoiling
D. To synthesize a short RNA primer
Answer: D
Rationale: DNA polymerase requires an existing 3’ hydroxyl group to start; Primase
provides this via a short RNA sequence.
, 7. Okazaki fragments are found on which of the following?
A. Leading strand
B. RNA transcript
C. Lagging strand
D. Amino acid chain
Answer: C
Rationale: Because DNA is antiparallel, the lagging strand is synthesized discontinuously
in short segments called Okazaki fragments.
8. Which enzyme joins Okazaki fragments together?
A. DNA Polymerase I
B. Helicase
C. Telomerase
D. Ligase
Answer: D
Rationale: DNA ligase creates the final phosphodiester bond to join the sugar-phosphate
backbones of the fragments.
9. Which enzyme relieves the torsional strain (supercoiling) ahead of the
replication fork?
A. SSB proteins
B. Topoisomerase
C. Primase
D. Exonuclease
Answer: B
Rationale: Topoisomerase (or DNA Gyrase in bacteria) cuts and reseals DNA to prevent it
from becoming overwound during unwinding.
