,Chapter 1 – The Molecular Basis of Heredity, Variation, and
Evolution
1.
If a double-stranded DNA molecule contains 24% thymine, what is
the expected percentage of guanine?
A) 26%
B) 24%
C) 50%
D) 76%
E) 13%
✅ Correct Answer: A
Rationale: By Chargaff’s rules, thymine = adenine, so T = 24% and A
= 24% → combined 48%. The remaining 52% consists of guanine and
cytosine in equal amounts → G = 26%.
Skill: Applying/Analyzing
2.
Which statement best reflects the central dogma of molecular biology?
A) RNA → DNA → Protein
B) DNA → RNA → Protein
C) Protein → RNA → DNA
D) DNA → Protein → RNA
E) RNA → Protein → DNA
✅ Correct Answer: B
Rationale: The central dogma describes the unidirectional flow of
genetic information from DNA (storage) to RNA (intermediate) to
protein (functional output), except in certain viruses.
Skill: Remembering/Understanding
,3.
Which type of bond stabilizes the base-pairing between
complementary strands in DNA?
A) Covalent bonds between phosphates
B) Hydrogen bonds between bases
C) Ionic bonds between bases
D) Disulfide bonds between bases
E) Peptide bonds between bases
✅ Correct Answer: B
Rationale: Complementary base pairs (A–T, G–C) are held together by
hydrogen bonds (2 for A–T, 3 for G–C), providing stability while
allowing strand separation during replication.
Skill: Remembering/Understanding
4.
In DNA replication, which enzyme synthesizes short RNA primers to
initiate DNA strand elongation?
A) DNA polymerase III
B) Primase
C) DNA ligase
D) Helicase
E) Topoisomerase
✅ Correct Answer: B
Rationale: Primase synthesizes RNA primers that provide a free 3′-
OH group for DNA polymerases to extend. DNA polymerase cannot
start synthesis de novo.
Skill: Remembering/Understanding
,5.
A point mutation changes an A–T base pair to a G–C base pair. Which
category best describes this mutation?
A) Frameshift mutation
B) Transition
C) Transversion
D) Nonsense mutation
E) Silent mutation
✅ Correct Answer: B
Rationale: A transition mutation is the substitution of a purine for a
purine (A ↔ G) or a pyrimidine for a pyrimidine (T ↔ C). Here, A
(purine) changes to G (purine).
Skill: Applying/Analyzing
6.
If DNA polymerase mistakenly incorporates a nucleotide, which
proofreading mechanism most directly corrects the error?
A) Mismatch repair
B) Excision repair
C) 3′→5′ exonuclease activity
D) SOS repair
E) Base excision repair
✅ Correct Answer: C
Rationale: DNA polymerases have intrinsic 3′→5′ exonuclease activity
that removes mispaired nucleotides immediately during replication,
enhancing fidelity.
Skill: Remembering/Understanding
7.
Which process increases genetic variation by physically exchanging
,DNA segments between nonsister chromatids?
A) Independent assortment
B) Crossing over
C) Nondisjunction
D) DNA replication
E) Reverse transcription
✅ Correct Answer: B
Rationale: Crossing over during prophase I of meiosis creates
recombinant chromosomes, increasing genetic diversity in offspring.
Skill: Understanding
8.
A retrovirus like HIV violates the classic central dogma by:
A) Encoding proteins directly from DNA
B) Using RNA as a template for DNA synthesis
C) Skipping transcription entirely
D) Using protein as a template for RNA synthesis
E) Replicating without nucleic acids
✅ Correct Answer: B
Rationale: Retroviruses use reverse transcriptase to make DNA from
an RNA genome, an exception to the traditional central dogma flow.
Skill: Applying/Analyzing
9.
Which experimental evidence most strongly demonstrated that DNA
is the genetic material in eukaryotes?
A) Avery, MacLeod, and McCarty transformation experiments
B) Hershey–Chase bacteriophage experiment
C) Griffith’s pneumococcus transformation
, D) Meselson–Stahl semi-conservative replication experiment
E) Beadle and Tatum’s one gene–one enzyme hypothesis
✅ Correct Answer: A
Rationale: Avery and colleagues purified DNA from virulent bacteria
and showed it could transform nonvirulent strains, directly
identifying DNA as the hereditary molecule.
Skill: Remembering/Understanding
10.
Which feature of the genetic code ensures that most amino acids are
encoded by more than one codon?
A) Degeneracy
B) Universality
C) Non-overlapping
D) Commaless nature
E) Wobble
✅ Correct Answer: A
Rationale: Degeneracy of the code allows multiple codons to specify
the same amino acid, contributing to mutation tolerance.
Skill: Understanding
11.
The wobble hypothesis primarily explains:
A) Synonymous mutations
B) How tRNA can recognize more than one codon
C) Degeneracy in the genetic code
D) Proofreading in translation
E) Codon–anticodon mismatches at the first base
Evolution
1.
If a double-stranded DNA molecule contains 24% thymine, what is
the expected percentage of guanine?
A) 26%
B) 24%
C) 50%
D) 76%
E) 13%
✅ Correct Answer: A
Rationale: By Chargaff’s rules, thymine = adenine, so T = 24% and A
= 24% → combined 48%. The remaining 52% consists of guanine and
cytosine in equal amounts → G = 26%.
Skill: Applying/Analyzing
2.
Which statement best reflects the central dogma of molecular biology?
A) RNA → DNA → Protein
B) DNA → RNA → Protein
C) Protein → RNA → DNA
D) DNA → Protein → RNA
E) RNA → Protein → DNA
✅ Correct Answer: B
Rationale: The central dogma describes the unidirectional flow of
genetic information from DNA (storage) to RNA (intermediate) to
protein (functional output), except in certain viruses.
Skill: Remembering/Understanding
,3.
Which type of bond stabilizes the base-pairing between
complementary strands in DNA?
A) Covalent bonds between phosphates
B) Hydrogen bonds between bases
C) Ionic bonds between bases
D) Disulfide bonds between bases
E) Peptide bonds between bases
✅ Correct Answer: B
Rationale: Complementary base pairs (A–T, G–C) are held together by
hydrogen bonds (2 for A–T, 3 for G–C), providing stability while
allowing strand separation during replication.
Skill: Remembering/Understanding
4.
In DNA replication, which enzyme synthesizes short RNA primers to
initiate DNA strand elongation?
A) DNA polymerase III
B) Primase
C) DNA ligase
D) Helicase
E) Topoisomerase
✅ Correct Answer: B
Rationale: Primase synthesizes RNA primers that provide a free 3′-
OH group for DNA polymerases to extend. DNA polymerase cannot
start synthesis de novo.
Skill: Remembering/Understanding
,5.
A point mutation changes an A–T base pair to a G–C base pair. Which
category best describes this mutation?
A) Frameshift mutation
B) Transition
C) Transversion
D) Nonsense mutation
E) Silent mutation
✅ Correct Answer: B
Rationale: A transition mutation is the substitution of a purine for a
purine (A ↔ G) or a pyrimidine for a pyrimidine (T ↔ C). Here, A
(purine) changes to G (purine).
Skill: Applying/Analyzing
6.
If DNA polymerase mistakenly incorporates a nucleotide, which
proofreading mechanism most directly corrects the error?
A) Mismatch repair
B) Excision repair
C) 3′→5′ exonuclease activity
D) SOS repair
E) Base excision repair
✅ Correct Answer: C
Rationale: DNA polymerases have intrinsic 3′→5′ exonuclease activity
that removes mispaired nucleotides immediately during replication,
enhancing fidelity.
Skill: Remembering/Understanding
7.
Which process increases genetic variation by physically exchanging
,DNA segments between nonsister chromatids?
A) Independent assortment
B) Crossing over
C) Nondisjunction
D) DNA replication
E) Reverse transcription
✅ Correct Answer: B
Rationale: Crossing over during prophase I of meiosis creates
recombinant chromosomes, increasing genetic diversity in offspring.
Skill: Understanding
8.
A retrovirus like HIV violates the classic central dogma by:
A) Encoding proteins directly from DNA
B) Using RNA as a template for DNA synthesis
C) Skipping transcription entirely
D) Using protein as a template for RNA synthesis
E) Replicating without nucleic acids
✅ Correct Answer: B
Rationale: Retroviruses use reverse transcriptase to make DNA from
an RNA genome, an exception to the traditional central dogma flow.
Skill: Applying/Analyzing
9.
Which experimental evidence most strongly demonstrated that DNA
is the genetic material in eukaryotes?
A) Avery, MacLeod, and McCarty transformation experiments
B) Hershey–Chase bacteriophage experiment
C) Griffith’s pneumococcus transformation
, D) Meselson–Stahl semi-conservative replication experiment
E) Beadle and Tatum’s one gene–one enzyme hypothesis
✅ Correct Answer: A
Rationale: Avery and colleagues purified DNA from virulent bacteria
and showed it could transform nonvirulent strains, directly
identifying DNA as the hereditary molecule.
Skill: Remembering/Understanding
10.
Which feature of the genetic code ensures that most amino acids are
encoded by more than one codon?
A) Degeneracy
B) Universality
C) Non-overlapping
D) Commaless nature
E) Wobble
✅ Correct Answer: A
Rationale: Degeneracy of the code allows multiple codons to specify
the same amino acid, contributing to mutation tolerance.
Skill: Understanding
11.
The wobble hypothesis primarily explains:
A) Synonymous mutations
B) How tRNA can recognize more than one codon
C) Degeneracy in the genetic code
D) Proofreading in translation
E) Codon–anticodon mismatches at the first base
