CHEM 210 Module 6 Biochemistry Exam: Nucleotides, DNA, and
Transcription 2026/2027 UPDATE
1. Which of the following components is found in a nucleoside but not in a
phosphate group?
A. Nitrogenous base and phosphate
B. Nitrogenous base and pentose sugar
C. Pentose sugar and phosphate
D. Only the nitrogenous base
Answer: B
Rationale: A nucleoside consists of a nitrogenous base and a pentose sugar. Once a
phosphate group is added, it becomes a nucleotide.
2. What is the structural difference between ribose and deoxyribose?
A. Deoxyribose has an extra hydroxyl group at the 3’ carbon.
B. Ribose lacks a hydroxyl group at the 2’ carbon.
C. Deoxyribose lacks a hydroxyl group at the 2’ carbon.
D. Ribose has an extra phosphate group at the 5’ carbon.
Answer: C
Rationale: Deoxyribose is ‘de-oxygenated’ at the 2’ position, meaning it has a hydrogen
atom instead of a hydroxyl (-OH) group found in ribose.
,3. Which nitrogenous bases are classified as purines?
A. Cytosine and Thymine
B. Adenine and Thymine
C. Adenine and Guanine
D. Guanine and Cytosine
Answer: C
Rationale: Purines are double-ring structures including Adenine and Guanine. Pyrimidines
(Cytosine, Thymine, Uracil) are single-ring structures.
4. Which bond connects the 5’ phosphate group of one nucleotide to the 3’
hydroxyl group of the next?
A. Hydrogen bond
B. Glycosidic bond
C. Phosphodiester bond
D. Peptide bond
Answer: C
Rationale: The sugar-phosphate backbone of DNA and RNA is linked by 3’-5’
phosphodiester bonds.
5. According to Chargaff’s rules, if a DNA sample contains 20% Adenine, what is
the percentage of Guanine?
A. 20%
B. 80%
C. 40%
D. 30%
Answer: D
Rationale: If A=20%, then T=20% (Total A+T=40%). The remaining 60% must be G+C, so
G=30% and C=30%.
, 6. How many hydrogen bonds form between a Guanine-Cytosine (G-C) base
pair?
A. 3
B. 2
C. 1
D. 4
Answer: A
Rationale: G-C pairs form three hydrogen bonds, making them more stable than A-T pairs,
which form only two.
7. In the DNA double helix, the two strands run in opposite directions. This
orientation is called:
A. Parallel
B. Semiconservative
C. Antiparallel
D. Complementary
Answer: C
Rationale: Antiparallel refers to the 5’ to 3’ direction of one strand being opposite to the 5’
to 3’ direction of the other.
8. Which enzyme is responsible for unwinding the DNA double helix at the
replication fork?
A. DNA Polymerase
B. Topoisomerase
C. Helicase
D. RNA Primase
Answer: C
Rationale: Helicase breaks the hydrogen bonds between bases to ‘unzip’ the DNA strands
for replication.
Transcription 2026/2027 UPDATE
1. Which of the following components is found in a nucleoside but not in a
phosphate group?
A. Nitrogenous base and phosphate
B. Nitrogenous base and pentose sugar
C. Pentose sugar and phosphate
D. Only the nitrogenous base
Answer: B
Rationale: A nucleoside consists of a nitrogenous base and a pentose sugar. Once a
phosphate group is added, it becomes a nucleotide.
2. What is the structural difference between ribose and deoxyribose?
A. Deoxyribose has an extra hydroxyl group at the 3’ carbon.
B. Ribose lacks a hydroxyl group at the 2’ carbon.
C. Deoxyribose lacks a hydroxyl group at the 2’ carbon.
D. Ribose has an extra phosphate group at the 5’ carbon.
Answer: C
Rationale: Deoxyribose is ‘de-oxygenated’ at the 2’ position, meaning it has a hydrogen
atom instead of a hydroxyl (-OH) group found in ribose.
,3. Which nitrogenous bases are classified as purines?
A. Cytosine and Thymine
B. Adenine and Thymine
C. Adenine and Guanine
D. Guanine and Cytosine
Answer: C
Rationale: Purines are double-ring structures including Adenine and Guanine. Pyrimidines
(Cytosine, Thymine, Uracil) are single-ring structures.
4. Which bond connects the 5’ phosphate group of one nucleotide to the 3’
hydroxyl group of the next?
A. Hydrogen bond
B. Glycosidic bond
C. Phosphodiester bond
D. Peptide bond
Answer: C
Rationale: The sugar-phosphate backbone of DNA and RNA is linked by 3’-5’
phosphodiester bonds.
5. According to Chargaff’s rules, if a DNA sample contains 20% Adenine, what is
the percentage of Guanine?
A. 20%
B. 80%
C. 40%
D. 30%
Answer: D
Rationale: If A=20%, then T=20% (Total A+T=40%). The remaining 60% must be G+C, so
G=30% and C=30%.
, 6. How many hydrogen bonds form between a Guanine-Cytosine (G-C) base
pair?
A. 3
B. 2
C. 1
D. 4
Answer: A
Rationale: G-C pairs form three hydrogen bonds, making them more stable than A-T pairs,
which form only two.
7. In the DNA double helix, the two strands run in opposite directions. This
orientation is called:
A. Parallel
B. Semiconservative
C. Antiparallel
D. Complementary
Answer: C
Rationale: Antiparallel refers to the 5’ to 3’ direction of one strand being opposite to the 5’
to 3’ direction of the other.
8. Which enzyme is responsible for unwinding the DNA double helix at the
replication fork?
A. DNA Polymerase
B. Topoisomerase
C. Helicase
D. RNA Primase
Answer: C
Rationale: Helicase breaks the hydrogen bonds between bases to ‘unzip’ the DNA strands
for replication.
