,CHEM212/CHEM 212 Module 6 Exam Biochemistry
with Lab | Portage Learning | 26/27 Actual(PDF)
1. Which component is found in a nucleotide but not in a nucleoside?
A) A nitrogenous base
B) A pentose sugar
C) One or more phosphate groups
D) An N-glycosidic bond
Correct Answer: One or more phosphate groups
Rationale: A nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate
groups. A nucleoside contains only the base and sugar, lacking phosphate. The N-glycosidic bond
connects the base to the sugar in both molecules. This distinction is fundamental to nucleic acid
chemistry.
2. The purine bases found in both DNA and RNA are:
A) Cytosine and thymine
B) Adenine and guanine
C) Thymine and uracil
D) Adenine and cytosine
Correct Answer: Adenine and guanine
Rationale: Adenine and guanine are the two purine bases present in both DNA and RNA. Cytosine,
thymine, and uracil are pyrimidines. Purines have a fused two-ring structure, while pyrimidines have a
single ring, distinguishing them structurally and functionally.
3. Which structural feature distinguishes thymine from uracil?
A) Thymine contains an amino group
B) Thymine has an extra methyl group
,C) Thymine contains a sulfur atom
D) Thymine has a double ring structure
Correct Answer: Thymine has an extra methyl group
Rationale: Thymine and uracil are both pyrimidines, but thymine has a methyl group at carbon 5 that
uracil lacks. This methylation is the sole structural difference between the two bases. Thymine is
found in DNA, while uracil replaces it in RNA.
4. What distinguishes ribose from 2-deoxyribose?
A) Ribose has a hydroxyl group at carbon 2, while deoxyribose has a hydrogen
B) Ribose has a hydrogen at carbon 2, while deoxyribose has a hydroxyl group
C) Ribose contains an extra phosphate group
D) Ribose is a hexose, while deoxyribose is a pentose
Correct Answer: Ribose has a hydroxyl group at carbon 2, while deoxyribose has a hydrogen
Rationale: Ribose contains a hydroxyl group (–OH) at carbon 2, whereas 2-deoxyribose has a hydrogen
atom (–H) at the same position, hence the name deoxy. Both are pentose sugars. This difference
affects the stability and reactivity of RNA versus DNA.
5. The N-glycosidic bond in a nucleoside forms between which two atoms?
A) The anomeric carbon of the sugar and the nitrogen of the base
B) The phosphate group and the sugar
C) The base and the phosphate group
D) Carbon 5 of the sugar and the base
Correct Answer: The anomeric carbon of the sugar and the nitrogen of the base
, Rationale: The N-glycosidic bond connects the anomeric carbon (carbon 1 of ribose or deoxyribose) to
a nitrogen atom of the purine or pyrimidine base. Phosphate groups attach at carbon 5 via
phosphoester bonds. This linkage is essential for nucleotide structure.
6. Which of the following statements about the DNA double helix is correct?
A) The two strands run in parallel directions
B) The sugar-phosphate backbone is on the inside of the helix
C) The bases are on the outside of the helix
D) The two strands are antiparallel and the bases are on the inside
Correct Answer: The two strands are antiparallel and the bases are on the inside
Rationale: In the DNA double helix, the two polynucleotide strands run in opposite (antiparallel)
directions, and the nitrogenous bases project inward, forming hydrogen bonds with the
complementary strand. The sugar-phosphate backbone is on the exterior. This arrangement
maximizes stability.
7. How many hydrogen bonds form between guanine and cytosine?
A) One
B) Two
C) Three
D) Four
Correct Answer: Three
Rationale: Guanine and cytosine form three hydrogen bonds, while adenine and thymine form only
two. This difference contributes to the higher thermal stability of GC-rich DNA regions. Hydrogen
bonds between complementary bases stabilize the double helix.
8. Which form of DNA has a left-handed helix?
A) A-DNA
with Lab | Portage Learning | 26/27 Actual(PDF)
1. Which component is found in a nucleotide but not in a nucleoside?
A) A nitrogenous base
B) A pentose sugar
C) One or more phosphate groups
D) An N-glycosidic bond
Correct Answer: One or more phosphate groups
Rationale: A nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate
groups. A nucleoside contains only the base and sugar, lacking phosphate. The N-glycosidic bond
connects the base to the sugar in both molecules. This distinction is fundamental to nucleic acid
chemistry.
2. The purine bases found in both DNA and RNA are:
A) Cytosine and thymine
B) Adenine and guanine
C) Thymine and uracil
D) Adenine and cytosine
Correct Answer: Adenine and guanine
Rationale: Adenine and guanine are the two purine bases present in both DNA and RNA. Cytosine,
thymine, and uracil are pyrimidines. Purines have a fused two-ring structure, while pyrimidines have a
single ring, distinguishing them structurally and functionally.
3. Which structural feature distinguishes thymine from uracil?
A) Thymine contains an amino group
B) Thymine has an extra methyl group
,C) Thymine contains a sulfur atom
D) Thymine has a double ring structure
Correct Answer: Thymine has an extra methyl group
Rationale: Thymine and uracil are both pyrimidines, but thymine has a methyl group at carbon 5 that
uracil lacks. This methylation is the sole structural difference between the two bases. Thymine is
found in DNA, while uracil replaces it in RNA.
4. What distinguishes ribose from 2-deoxyribose?
A) Ribose has a hydroxyl group at carbon 2, while deoxyribose has a hydrogen
B) Ribose has a hydrogen at carbon 2, while deoxyribose has a hydroxyl group
C) Ribose contains an extra phosphate group
D) Ribose is a hexose, while deoxyribose is a pentose
Correct Answer: Ribose has a hydroxyl group at carbon 2, while deoxyribose has a hydrogen
Rationale: Ribose contains a hydroxyl group (–OH) at carbon 2, whereas 2-deoxyribose has a hydrogen
atom (–H) at the same position, hence the name deoxy. Both are pentose sugars. This difference
affects the stability and reactivity of RNA versus DNA.
5. The N-glycosidic bond in a nucleoside forms between which two atoms?
A) The anomeric carbon of the sugar and the nitrogen of the base
B) The phosphate group and the sugar
C) The base and the phosphate group
D) Carbon 5 of the sugar and the base
Correct Answer: The anomeric carbon of the sugar and the nitrogen of the base
, Rationale: The N-glycosidic bond connects the anomeric carbon (carbon 1 of ribose or deoxyribose) to
a nitrogen atom of the purine or pyrimidine base. Phosphate groups attach at carbon 5 via
phosphoester bonds. This linkage is essential for nucleotide structure.
6. Which of the following statements about the DNA double helix is correct?
A) The two strands run in parallel directions
B) The sugar-phosphate backbone is on the inside of the helix
C) The bases are on the outside of the helix
D) The two strands are antiparallel and the bases are on the inside
Correct Answer: The two strands are antiparallel and the bases are on the inside
Rationale: In the DNA double helix, the two polynucleotide strands run in opposite (antiparallel)
directions, and the nitrogenous bases project inward, forming hydrogen bonds with the
complementary strand. The sugar-phosphate backbone is on the exterior. This arrangement
maximizes stability.
7. How many hydrogen bonds form between guanine and cytosine?
A) One
B) Two
C) Three
D) Four
Correct Answer: Three
Rationale: Guanine and cytosine form three hydrogen bonds, while adenine and thymine form only
two. This difference contributes to the higher thermal stability of GC-rich DNA regions. Hydrogen
bonds between complementary bases stabilize the double helix.
8. Which form of DNA has a left-handed helix?
A) A-DNA
