,BIOD 101 Module 4 | Essential Biology I | Portage Learning |
Questions & Verified Answers | 2026 Edition
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. Adenine and guanine are purines, while thymine and
cytosine are pyrimidines.
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 nitrogenous bases with a double-ring structure. Adenine and guanine are purines.
Pyrimidines (thymine and cytosine) have a single-ring structure. In DNA, a purine always pairs with a
pyrimidine.
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 nitrogenous bases with a single-ring structure. Thymine and cytosine are
pyrimidines. Adenine and guanine are purines. This structural difference is fundamental to the
complementary base pairing in DNA.
5. What is a nucleoside?
A) A nitrogenous base plus a phosphate group
B) A nitrogenous base plus a deoxyribose sugar
C) A nitrogenous base plus a deoxyribose sugar plus a phosphate group
D) Two complementary nitrogenous bases held together by hydrogen bonds
Correct Answer: A nitrogenous base plus a deoxyribose sugar
, Rationale: A nucleoside consists of a nitrogenous base chemically linked to the sugar deoxyribose. When
a phosphate group is added, it becomes a nucleotide (a base plus deoxyribose plus a phosphate).
6. What is a nucleotide?
A) A nitrogenous base plus a deoxyribose sugar
B) A nitrogenous base plus a phosphate group
C) A nitrogenous base plus a deoxyribose sugar plus a phosphate group
D) Two complementary nitrogenous bases held together by hydrogen bonds
Correct Answer: A nitrogenous base plus a deoxyribose sugar plus a phosphate group
Rationale: A nucleotide is composed of three components: a nitrogenous base, a deoxyribose sugar, and
a phosphate group. Nucleotides are the monomeric building blocks of nucleic acids like DNA and RNA.
7. How many hydrogen bonds form between adenine and thymine in DNA?
A) One hydrogen bond
B) Two hydrogen bonds
C) Three hydrogen bonds
D) Four hydrogen bonds
Correct Answer: Two hydrogen bonds
Rationale: Adenine pairs with thymine via two hydrogen bonds. Guanine pairs with cytosine via three
hydrogen bonds. This difference in hydrogen bond number is important for DNA stability and the
energetics of strand separation during replication and transcription.
8. How many hydrogen bonds form between guanine and cytosine in DNA?
A) One hydrogen bond
B) Two hydrogen bonds
C) Three hydrogen bonds
Questions & Verified Answers | 2026 Edition
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. Adenine and guanine are purines, while thymine and
cytosine are pyrimidines.
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 nitrogenous bases with a double-ring structure. Adenine and guanine are purines.
Pyrimidines (thymine and cytosine) have a single-ring structure. In DNA, a purine always pairs with a
pyrimidine.
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 nitrogenous bases with a single-ring structure. Thymine and cytosine are
pyrimidines. Adenine and guanine are purines. This structural difference is fundamental to the
complementary base pairing in DNA.
5. What is a nucleoside?
A) A nitrogenous base plus a phosphate group
B) A nitrogenous base plus a deoxyribose sugar
C) A nitrogenous base plus a deoxyribose sugar plus a phosphate group
D) Two complementary nitrogenous bases held together by hydrogen bonds
Correct Answer: A nitrogenous base plus a deoxyribose sugar
, Rationale: A nucleoside consists of a nitrogenous base chemically linked to the sugar deoxyribose. When
a phosphate group is added, it becomes a nucleotide (a base plus deoxyribose plus a phosphate).
6. What is a nucleotide?
A) A nitrogenous base plus a deoxyribose sugar
B) A nitrogenous base plus a phosphate group
C) A nitrogenous base plus a deoxyribose sugar plus a phosphate group
D) Two complementary nitrogenous bases held together by hydrogen bonds
Correct Answer: A nitrogenous base plus a deoxyribose sugar plus a phosphate group
Rationale: A nucleotide is composed of three components: a nitrogenous base, a deoxyribose sugar, and
a phosphate group. Nucleotides are the monomeric building blocks of nucleic acids like DNA and RNA.
7. How many hydrogen bonds form between adenine and thymine in DNA?
A) One hydrogen bond
B) Two hydrogen bonds
C) Three hydrogen bonds
D) Four hydrogen bonds
Correct Answer: Two hydrogen bonds
Rationale: Adenine pairs with thymine via two hydrogen bonds. Guanine pairs with cytosine via three
hydrogen bonds. This difference in hydrogen bond number is important for DNA stability and the
energetics of strand separation during replication and transcription.
8. How many hydrogen bonds form between guanine and cytosine in DNA?
A) One hydrogen bond
B) Two hydrogen bonds
C) Three hydrogen bonds
