,BIOD101/BIOD 101 Module 4 Exam | Essential Biology I with
Lab | Portage Learning | Q & A | 2026/2027 Edition (PDF)
1. Which of the following best describes the structure of DNA?
A) A single-stranded molecule composed of nucleotides with ribose sugar
B) A double helix composed of two antiparallel strands held together by hydrogen bonds between
complementary bases
C) A triple-stranded molecule with adenine, guanine, cytosine, and uracil
D) A circular molecule found only in prokaryotic cells
Correct Answer: A double helix composed of two antiparallel strands held together by hydrogen bonds
between complementary bases.
Rationale: DNA is structured as a double helix with two antiparallel strands. The strands are held
together by hydrogen bonds between complementary nitrogenous bases: adenine pairs with thymine
(two hydrogen bonds), and guanine pairs with cytosine (three hydrogen bonds).
2. What are the four nitrogenous bases found in DNA?
A) Adenine, guanine, cytosine, and uracil
B) Adenine, guanine, cytosine, and thymine
C) Adenine, thymine, cytosine, and uracil
D) Guanine, cytosine, thymine, and uracil
Correct Answer: Adenine, guanine, cytosine, and thymine.
Rationale: The four nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).
RNA contains uracil (U) instead of thymine.
3. Which nitrogenous bases are classified as purines?
A) Adenine and thymine
B) Guanine and cytosine
,C) Adenine and guanine
D) Thymine and cytosine
Correct Answer: Adenine and guanine.
Rationale: Purines are double-ring structures that contain two carbon-nitrogen rings and four nitrogen
atoms. Adenine and guanine are purines. Pyrimidines (thymine, cytosine, and uracil) contain a single
carbon-nitrogen ring.
4. Which nitrogenous bases are classified as pyrimidines?
A) Adenine and guanine
B) Thymine and cytosine
C) Adenine and thymine
D) Guanine and cytosine
Correct Answer: Thymine and cytosine.
Rationale: Pyrimidines are single-ring structures that contain one carbon-nitrogen ring and two nitrogen
atoms. Thymine and cytosine are pyrimidines found in DNA; uracil is a pyrimidine found in RNA.
5. In DNA, adenine (A) pairs with which base?
A) Guanine (G)
B) Cytosine (C)
C) Thymine (T)
D) Uracil (U)
Correct Answer: Thymine (T).
, Rationale: In DNA, complementary base pairing occurs via hydrogen bonds: adenine (A) pairs with
thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three
hydrogen bonds.
6. In DNA, guanine (G) pairs with which base?
A) Adenine (A)
B) Thymine (T)
C) Cytosine (C)
D) Uracil (U)
Correct Answer: Cytosine (C).
Rationale: In DNA, guanine (G) pairs with cytosine (C) through three hydrogen bonds. This
complementary pairing is essential for the structure and replication of DNA.
7. What does the term "antiparallel" refer to in the context of DNA structure?
A) The two strands are oriented in the same direction
B) The two strands are oriented in opposite directions
C) The strands are parallel but not connected
D) The strands are circular
Correct Answer: The two strands are oriented in opposite directions.
Rationale: In the antiparallel arrangement of DNA, one strand is oriented in the 5'→3' direction and the
other strand in the 3'→5' direction. This orientation is essential for DNA replication and transcription.
8. What is a nucleoside?
A) A nitrogenous base plus a deoxyribose sugar
B) A nitrogenous base plus a deoxyribose sugar plus a phosphate group
C) A polymer of nucleotides
Lab | Portage Learning | Q & A | 2026/2027 Edition (PDF)
1. Which of the following best describes the structure of DNA?
A) A single-stranded molecule composed of nucleotides with ribose sugar
B) A double helix composed of two antiparallel strands held together by hydrogen bonds between
complementary bases
C) A triple-stranded molecule with adenine, guanine, cytosine, and uracil
D) A circular molecule found only in prokaryotic cells
Correct Answer: A double helix composed of two antiparallel strands held together by hydrogen bonds
between complementary bases.
Rationale: DNA is structured as a double helix with two antiparallel strands. The strands are held
together by hydrogen bonds between complementary nitrogenous bases: adenine pairs with thymine
(two hydrogen bonds), and guanine pairs with cytosine (three hydrogen bonds).
2. What are the four nitrogenous bases found in DNA?
A) Adenine, guanine, cytosine, and uracil
B) Adenine, guanine, cytosine, and thymine
C) Adenine, thymine, cytosine, and uracil
D) Guanine, cytosine, thymine, and uracil
Correct Answer: Adenine, guanine, cytosine, and thymine.
Rationale: The four nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).
RNA contains uracil (U) instead of thymine.
3. Which nitrogenous bases are classified as purines?
A) Adenine and thymine
B) Guanine and cytosine
,C) Adenine and guanine
D) Thymine and cytosine
Correct Answer: Adenine and guanine.
Rationale: Purines are double-ring structures that contain two carbon-nitrogen rings and four nitrogen
atoms. Adenine and guanine are purines. Pyrimidines (thymine, cytosine, and uracil) contain a single
carbon-nitrogen ring.
4. Which nitrogenous bases are classified as pyrimidines?
A) Adenine and guanine
B) Thymine and cytosine
C) Adenine and thymine
D) Guanine and cytosine
Correct Answer: Thymine and cytosine.
Rationale: Pyrimidines are single-ring structures that contain one carbon-nitrogen ring and two nitrogen
atoms. Thymine and cytosine are pyrimidines found in DNA; uracil is a pyrimidine found in RNA.
5. In DNA, adenine (A) pairs with which base?
A) Guanine (G)
B) Cytosine (C)
C) Thymine (T)
D) Uracil (U)
Correct Answer: Thymine (T).
, Rationale: In DNA, complementary base pairing occurs via hydrogen bonds: adenine (A) pairs with
thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three
hydrogen bonds.
6. In DNA, guanine (G) pairs with which base?
A) Adenine (A)
B) Thymine (T)
C) Cytosine (C)
D) Uracil (U)
Correct Answer: Cytosine (C).
Rationale: In DNA, guanine (G) pairs with cytosine (C) through three hydrogen bonds. This
complementary pairing is essential for the structure and replication of DNA.
7. What does the term "antiparallel" refer to in the context of DNA structure?
A) The two strands are oriented in the same direction
B) The two strands are oriented in opposite directions
C) The strands are parallel but not connected
D) The strands are circular
Correct Answer: The two strands are oriented in opposite directions.
Rationale: In the antiparallel arrangement of DNA, one strand is oriented in the 5'→3' direction and the
other strand in the 3'→5' direction. This orientation is essential for DNA replication and transcription.
8. What is a nucleoside?
A) A nitrogenous base plus a deoxyribose sugar
B) A nitrogenous base plus a deoxyribose sugar plus a phosphate group
C) A polymer of nucleotides
