BIOCHEM 210 MOLECULAR BIOLOGY MODULE 5 EXAM
ACTUAL 2026/2027 | Geneva College | Complete Questions &
Verified Answers | Pass Guaranteed - A+ Graded
Section 1: DNA Structure & Replication (Questions 1-15)
Question 1
Which form of DNA represents the predominant right-handed double helix under normal
physiological conditions, characterized by approximately 10.5 base pairs per turn and
distinct major/minor grooves accessible for protein binding?
A. A-DNA
B. B-DNA [CORRECT]
C. Z-DNA
D. H-DNA
Rationale: B-DNA is the canonical right-handed double helix with ~10.5 bp/turn and
major/minor grooves that accommodate transcription factors and DNA-binding
proteins. A-DNA is dehydrated and wider; Z-DNA is left-handed and requires alternating
purine-pyrimidine tracts.
Correct Answer: B
Question 2
A researcher treats purified DNA with a dehydrating ethanol solution and observes a
conformational shift to a wider, shorter helix with 11 base pairs per turn. Which DNA
form has been induced?
A. B-DNA
B. A-DNA [CORRECT]
C. Z-DNA
,D. C-DNA
Rationale: A-DNA forms under dehydrated conditions with 11 bp/turn and a wider,
shorter helix compared to B-DNA. Z-DNA is left-handed and requires specific sequence
contexts; C-DNA is a rare low-humidity variant.
Correct Answer: B
Question 3
In the Meselson-Stahl experiment, E. coli grown for many generations in medium
containing heavy nitrogen (¹⁵N) is transferred to light nitrogen (¹⁴N) medium. After one
round of replication, centrifugation in a CsCl gradient reveals DNA at which position?
A. Only at the heavy (¹⁵N/¹⁵N) density position
B. Only at the light (¹⁴N/¹⁴N) density position
C. As a single band at intermediate (¹⁵N/¹⁴N) density [CORRECT]
D. As two distinct bands at heavy and light density positions
Rationale: Semi-conservative replication produces hybrid DNA molecules containing
one parental ¹⁵N strand and one newly synthesized ¹⁴N strand after one generation,
yielding a single intermediate-density band. Conservative replication would yield
separate heavy and light bands.
Correct Answer: C
Question 4
Which prokaryotic DNA polymerase possesses 5'→3' exonuclease activity essential for
the removal of RNA primers during DNA replication?
A. DNA polymerase II
B. DNA polymerase III
C. DNA polymerase I [CORRECT]
D. DNA polymerase IV
, Rationale: DNA polymerase I has 5'→3' exonuclease activity for primer removal and
3'→5' exonuclease proofreading. Pol III is the main replicative polymerase lacking 5'→3'
exonuclease activity; Pol II and IV are primarily repair polymerases.
Correct Answer: C
Question 5
During replication, the enzyme DnaB uses ATP hydrolysis to unwind the DNA double
helix at the replication fork. Which enzymatic activity does DnaB possess?
A. Topoisomerase activity
B. Helicase activity [CORRECT]
C. Primase activity
D. Ligase activity
Rationale: DnaB is the replicative helicase in E. coli that unwinds DNA using ATP
hydrolysis. Topoisomerases relieve supercoiling; primase (DnaG) synthesizes RNA
primers; ligase joins Okazaki fragments.
Correct Answer: B
Question 6
A missense mutation in the ε (epsilon) subunit of DNA polymerase III holoenzyme
abolishes its 3'→5' exonuclease activity. What is the most likely phenotypic
consequence for the bacterial cell?
A. Inability to initiate DNA replication at oriC
B. A significantly increased spontaneous mutation rate due to loss of proofreading
[CORRECT]
C. Failure to remove RNA primers from Okazaki fragments
D. Complete cessation of lagging strand synthesis
Rationale: The ε subunit provides 3'→5' exonuclease proofreading activity; its loss
eliminates correction of misincorporated nucleotides, causing a mutator phenotype.
ACTUAL 2026/2027 | Geneva College | Complete Questions &
Verified Answers | Pass Guaranteed - A+ Graded
Section 1: DNA Structure & Replication (Questions 1-15)
Question 1
Which form of DNA represents the predominant right-handed double helix under normal
physiological conditions, characterized by approximately 10.5 base pairs per turn and
distinct major/minor grooves accessible for protein binding?
A. A-DNA
B. B-DNA [CORRECT]
C. Z-DNA
D. H-DNA
Rationale: B-DNA is the canonical right-handed double helix with ~10.5 bp/turn and
major/minor grooves that accommodate transcription factors and DNA-binding
proteins. A-DNA is dehydrated and wider; Z-DNA is left-handed and requires alternating
purine-pyrimidine tracts.
Correct Answer: B
Question 2
A researcher treats purified DNA with a dehydrating ethanol solution and observes a
conformational shift to a wider, shorter helix with 11 base pairs per turn. Which DNA
form has been induced?
A. B-DNA
B. A-DNA [CORRECT]
C. Z-DNA
,D. C-DNA
Rationale: A-DNA forms under dehydrated conditions with 11 bp/turn and a wider,
shorter helix compared to B-DNA. Z-DNA is left-handed and requires specific sequence
contexts; C-DNA is a rare low-humidity variant.
Correct Answer: B
Question 3
In the Meselson-Stahl experiment, E. coli grown for many generations in medium
containing heavy nitrogen (¹⁵N) is transferred to light nitrogen (¹⁴N) medium. After one
round of replication, centrifugation in a CsCl gradient reveals DNA at which position?
A. Only at the heavy (¹⁵N/¹⁵N) density position
B. Only at the light (¹⁴N/¹⁴N) density position
C. As a single band at intermediate (¹⁵N/¹⁴N) density [CORRECT]
D. As two distinct bands at heavy and light density positions
Rationale: Semi-conservative replication produces hybrid DNA molecules containing
one parental ¹⁵N strand and one newly synthesized ¹⁴N strand after one generation,
yielding a single intermediate-density band. Conservative replication would yield
separate heavy and light bands.
Correct Answer: C
Question 4
Which prokaryotic DNA polymerase possesses 5'→3' exonuclease activity essential for
the removal of RNA primers during DNA replication?
A. DNA polymerase II
B. DNA polymerase III
C. DNA polymerase I [CORRECT]
D. DNA polymerase IV
, Rationale: DNA polymerase I has 5'→3' exonuclease activity for primer removal and
3'→5' exonuclease proofreading. Pol III is the main replicative polymerase lacking 5'→3'
exonuclease activity; Pol II and IV are primarily repair polymerases.
Correct Answer: C
Question 5
During replication, the enzyme DnaB uses ATP hydrolysis to unwind the DNA double
helix at the replication fork. Which enzymatic activity does DnaB possess?
A. Topoisomerase activity
B. Helicase activity [CORRECT]
C. Primase activity
D. Ligase activity
Rationale: DnaB is the replicative helicase in E. coli that unwinds DNA using ATP
hydrolysis. Topoisomerases relieve supercoiling; primase (DnaG) synthesizes RNA
primers; ligase joins Okazaki fragments.
Correct Answer: B
Question 6
A missense mutation in the ε (epsilon) subunit of DNA polymerase III holoenzyme
abolishes its 3'→5' exonuclease activity. What is the most likely phenotypic
consequence for the bacterial cell?
A. Inability to initiate DNA replication at oriC
B. A significantly increased spontaneous mutation rate due to loss of proofreading
[CORRECT]
C. Failure to remove RNA primers from Okazaki fragments
D. Complete cessation of lagging strand synthesis
Rationale: The ε subunit provides 3'→5' exonuclease proofreading activity; its loss
eliminates correction of misincorporated nucleotides, causing a mutator phenotype.
