Portage Learning CHEM 210 Biochemistry
Module 7 Exam | Actual Nucleic Acids and
DNA Replication Assessment with Answers
DOMAIN: NUCLEOTIDE STRUCTURE & CHEMISTRY (20%)
Q1. Which of the following correctly identifies the complete components of a
nucleotide?
A. Nitrogenous base + pentose sugar
B. Nitrogenous base + phosphate group
C. Nitrogenous base + pentose sugar + phosphate group
D. Pentose sugar + phosphate group
Correct Answer: C
Rationale: C. [CORRECT] A nucleotide consists of three essential components: (1) a
nitrogenous base (purine or pyrimidine), (2) a pentose sugar (ribose in RNA, deoxyribose
in DNA), and (3) one or more phosphate groups attached to the 5' carbon of the sugar
via an ester linkage. This complete structure forms the monomeric unit of nucleic acids.
A is incorrect; this describes a nucleoside (base + sugar only), lacking the phosphate
group. B is incorrect; this lacks the essential pentose sugar backbone. D is incorrect;
this describes a sugar-phosphate backbone fragment without the nitrogenous base,
which carries the genetic information.
,Q2. Which of the following nitrogenous bases is found in DNA but not in RNA?
A. Adenine
B. Guanine
C. Cytosine
D. Thymine
Correct Answer: D
Rationale: D. [CORRECT] Thymine (5-methyluracil) is found exclusively in DNA, while
RNA contains uracil in its place. This structural difference provides stability to DNA
(thymine is less prone to mutation than uracil) and helps cells distinguish between DNA
and RNA molecules. A, B, and C are incorrect; adenine, guanine, and cytosine are found
in both DNA and RNA. Adenine and guanine are purines; cytosine is a pyrimidine present
in both nucleic acids.
Q3. [BASE PAIRING] In a DNA molecule, which of the following correctly describes the
pairing between adenine (A) and thymine (T)?
A. Guanine (G) via two hydrogen bonds
B. Thymine (T) via two hydrogen bonds
C. Thymine (T) via three hydrogen bonds
D. Cytosine (C) via three hydrogen bonds
Correct Answer: B
,Rationale: B. [CORRECT] In the Watson-Crick base pairing scheme, adenine (A) pairs
specifically with thymine (T) via two hydrogen bonds: one between the N6 of adenine
and O4 of thymine, and another between N1 of adenine and N3 of thymine. This specific
pairing is fundamental to DNA structure and replication fidelity. A is incorrect; adenine
does not pair with guanine; G pairs with C. C is incorrect; A-T pairs form two hydrogen
bonds, not three. Three hydrogen bonds characterize G≡C pairing. D is incorrect;
adenine does not pair with cytosine; this would violate complementary base pairing
rules.
Q4. The N-glycosidic bond in a nucleotide connects which two components?
A. Phosphate group to the pentose sugar
B. Nitrogenous base to the pentose sugar
C. Phosphate group to the nitrogenous base
D. Two nucleotides together in a chain
Correct Answer: B
Rationale: B. [CORRECT] The N-glycosidic bond is a covalent linkage between the
anomeric carbon (C1') of the pentose sugar and the N9 nitrogen of a purine (adenine,
guanine) or the N1 nitrogen of a pyrimidine (cytosine, thymine, uracil). This bond is
critical for nucleotide stability and is the site of attachment for the nitrogenous base. A
is incorrect; phosphate groups connect to the sugar via phosphoester bonds at the 5'
carbon (and 3' carbon in chain formation). C is incorrect; phosphate groups do not
directly bond to nitrogenous bases in nucleotide structure. D is incorrect; nucleotides
are connected via phosphodiester bonds between the 3'-OH of one sugar and the
5'-phosphate of the next nucleotide.
, Q5. Which of the following correctly distinguishes purines from pyrimidines?
A. Purines are single-ring structures; pyrimidines are double-ring structures
B. Purines contain adenine and cytosine; pyrimidines contain guanine and thymine
C. Purines are double-ring structures; pyrimidines are single-ring structures
D. Purines are found only in DNA; pyrimidines are found only in RNA
Correct Answer: C
Rationale: C. [CORRECT] Purines (adenine and guanine) are bicyclic, double-ring
structures consisting of a pyrimidine ring fused to an imidazole ring. Pyrimidines
(cytosine, thymine, uracil) are monocyclic, single-ring structures. This structural
difference affects their size and pairing geometry in the DNA double helix. A is incorrect;
this reverses the correct structural relationship. B is incorrect; purines contain adenine
and guanine; pyrimidines contain cytosine, thymine, and uracil. D is incorrect; both
purines and pyrimidines are found in DNA and RNA.
Q6. In the deoxyribose sugar found in DNA, which carbon position lacks a hydroxyl
group compared to ribose?
A. 1' carbon
B. 2' carbon
C. 3' carbon
D. 5' carbon
Correct Answer: B
Module 7 Exam | Actual Nucleic Acids and
DNA Replication Assessment with Answers
DOMAIN: NUCLEOTIDE STRUCTURE & CHEMISTRY (20%)
Q1. Which of the following correctly identifies the complete components of a
nucleotide?
A. Nitrogenous base + pentose sugar
B. Nitrogenous base + phosphate group
C. Nitrogenous base + pentose sugar + phosphate group
D. Pentose sugar + phosphate group
Correct Answer: C
Rationale: C. [CORRECT] A nucleotide consists of three essential components: (1) a
nitrogenous base (purine or pyrimidine), (2) a pentose sugar (ribose in RNA, deoxyribose
in DNA), and (3) one or more phosphate groups attached to the 5' carbon of the sugar
via an ester linkage. This complete structure forms the monomeric unit of nucleic acids.
A is incorrect; this describes a nucleoside (base + sugar only), lacking the phosphate
group. B is incorrect; this lacks the essential pentose sugar backbone. D is incorrect;
this describes a sugar-phosphate backbone fragment without the nitrogenous base,
which carries the genetic information.
,Q2. Which of the following nitrogenous bases is found in DNA but not in RNA?
A. Adenine
B. Guanine
C. Cytosine
D. Thymine
Correct Answer: D
Rationale: D. [CORRECT] Thymine (5-methyluracil) is found exclusively in DNA, while
RNA contains uracil in its place. This structural difference provides stability to DNA
(thymine is less prone to mutation than uracil) and helps cells distinguish between DNA
and RNA molecules. A, B, and C are incorrect; adenine, guanine, and cytosine are found
in both DNA and RNA. Adenine and guanine are purines; cytosine is a pyrimidine present
in both nucleic acids.
Q3. [BASE PAIRING] In a DNA molecule, which of the following correctly describes the
pairing between adenine (A) and thymine (T)?
A. Guanine (G) via two hydrogen bonds
B. Thymine (T) via two hydrogen bonds
C. Thymine (T) via three hydrogen bonds
D. Cytosine (C) via three hydrogen bonds
Correct Answer: B
,Rationale: B. [CORRECT] In the Watson-Crick base pairing scheme, adenine (A) pairs
specifically with thymine (T) via two hydrogen bonds: one between the N6 of adenine
and O4 of thymine, and another between N1 of adenine and N3 of thymine. This specific
pairing is fundamental to DNA structure and replication fidelity. A is incorrect; adenine
does not pair with guanine; G pairs with C. C is incorrect; A-T pairs form two hydrogen
bonds, not three. Three hydrogen bonds characterize G≡C pairing. D is incorrect;
adenine does not pair with cytosine; this would violate complementary base pairing
rules.
Q4. The N-glycosidic bond in a nucleotide connects which two components?
A. Phosphate group to the pentose sugar
B. Nitrogenous base to the pentose sugar
C. Phosphate group to the nitrogenous base
D. Two nucleotides together in a chain
Correct Answer: B
Rationale: B. [CORRECT] The N-glycosidic bond is a covalent linkage between the
anomeric carbon (C1') of the pentose sugar and the N9 nitrogen of a purine (adenine,
guanine) or the N1 nitrogen of a pyrimidine (cytosine, thymine, uracil). This bond is
critical for nucleotide stability and is the site of attachment for the nitrogenous base. A
is incorrect; phosphate groups connect to the sugar via phosphoester bonds at the 5'
carbon (and 3' carbon in chain formation). C is incorrect; phosphate groups do not
directly bond to nitrogenous bases in nucleotide structure. D is incorrect; nucleotides
are connected via phosphodiester bonds between the 3'-OH of one sugar and the
5'-phosphate of the next nucleotide.
, Q5. Which of the following correctly distinguishes purines from pyrimidines?
A. Purines are single-ring structures; pyrimidines are double-ring structures
B. Purines contain adenine and cytosine; pyrimidines contain guanine and thymine
C. Purines are double-ring structures; pyrimidines are single-ring structures
D. Purines are found only in DNA; pyrimidines are found only in RNA
Correct Answer: C
Rationale: C. [CORRECT] Purines (adenine and guanine) are bicyclic, double-ring
structures consisting of a pyrimidine ring fused to an imidazole ring. Pyrimidines
(cytosine, thymine, uracil) are monocyclic, single-ring structures. This structural
difference affects their size and pairing geometry in the DNA double helix. A is incorrect;
this reverses the correct structural relationship. B is incorrect; purines contain adenine
and guanine; pyrimidines contain cytosine, thymine, and uracil. D is incorrect; both
purines and pyrimidines are found in DNA and RNA.
Q6. In the deoxyribose sugar found in DNA, which carbon position lacks a hydroxyl
group compared to ribose?
A. 1' carbon
B. 2' carbon
C. 3' carbon
D. 5' carbon
Correct Answer: B
