1
BIOD 210 MIDTERM EXAM 2026/2027 | GENETICS –
PORTAGE LEARNING | EXPERT VERIFIED | 100
QUESTIONS & COMPLETE RATIONALES | PASS
GUARANTEED - A+ GRADED
SECTION 1: DNA STRUCTURE & REPLICATION (Questions 1-20)
Q1: Which of the following scientists first described the double helix structure of DNA?
A. Gregor Mendel
B. James Watson and Francis Crick
C. Friedrich Miescher
D. Erwin Chargaff
Correct Answer: B
Rationale: James Watson and Francis Crick first described the double helix structure of DNA in 1953,
using data from Rosalind Franklin's X-ray diffraction images. Mendel (A) described inheritance
patterns; Miescher (C) first isolated DNA; Chargaff (D) established base pairing rules. Key teaching
point: Watson and Crick used Franklin's X-ray data to propose the double helix model.
Q2: A DNA molecule is composed of two strands that run in opposite directions. This is referred to
as:
A. Complementary
B. Antiparallel
C. Semiconservative
D. Double-stranded
Correct Answer: B
Rationale: The two strands of DNA run in opposite directions (one 5'→3' and the other 3'→5'), which
is described as antiparallel. Complementary (A) refers to base pairing; semiconservative (C) describes
replication; double-stranded (D) describes the overall structure. Key teaching point: DNA strands are
antiparallel, which is essential for replication and transcription.
Q3: In DNA, which nitrogenous base pairs with adenine (A)?
A. Cytosine
B. Guanine
C. Thymine
D. Uracil
Correct Answer: C
,2
Rationale: In DNA, adenine (A) pairs with thymine (T) via two hydrogen bonds. In RNA, adenine pairs
with uracil (U). Key teaching point: DNA base pairing: A-T (2 H-bonds); G-C (3 H-bonds).
Q4: In DNA, which nitrogenous base pairs with guanine (G)?
A. Adenine
B. Thymine
C. Cytosine
D. Uracil
Correct Answer: C
Rationale: In DNA, guanine (G) pairs with cytosine (C) via three hydrogen bonds. Key teaching
point: G-C pairs have three hydrogen bonds, making them stronger than A-T pairs.
Q5: The DNA sequence 5'-ATGC-3' is complementary to which sequence?
A. 5'-GCAT-3'
B. 3'-TACG-5'
C. 5'-TACG-3'
D. 3'-GCAT-5'
Correct Answer: B
Rationale: Complementary base pairing and antiparallel orientation: 5'-ATGC-3' pairs with 3'-TACG-
5' (A-T, T-A, G-C, C-G). Key teaching point: Complementary DNA strands are antiparallel and follow
Chargaff's base pairing rules.
Q6: Which enzyme is responsible for unwinding the DNA double helix during replication?
A. DNA polymerase
B. Helicase
C. Primase
D. Ligase
Correct Answer: B
Rationale: Helicase unwinds the DNA double helix by breaking hydrogen bonds between base pairs,
creating a replication fork. DNA polymerase (A) synthesizes new DNA; primase (C) synthesizes RNA
primers; ligase (D) seals nicks. Key teaching point: Helicase is the "unwinding enzyme" that
separates DNA strands for replication.
Q7: Which enzyme synthesizes RNA primers during DNA replication?
A. DNA polymerase
B. Helicase
C. Primase
D. Ligase
, 3
Correct Answer: C
Rationale: Primase synthesizes short RNA primers that provide a free 3'-OH group for DNA
polymerase to begin DNA synthesis. Key teaching point: Primase creates RNA primers needed for
DNA polymerase to initiate replication.
Q8: DNA polymerase synthesizes new DNA in which direction?
A. 3'→5'
B. 5'→3'
C. Both directions
D. Randomly
Correct Answer: B
Rationale: DNA polymerase synthesizes new DNA in the 5'→3' direction by adding nucleotides to the
3'-OH group of the growing strand. Key teaching point: DNA synthesis always occurs in the 5'→3'
direction.
Q9: Which enzyme seals the nicks between Okazaki fragments on the lagging strand?
A. DNA polymerase
B. Helicase
C. Primase
D. Ligase
Correct Answer: D
Rationale: DNA ligase seals the nicks between Okazaki fragments by forming phosphodiester bonds,
creating a continuous DNA strand. Key teaching point: Ligase "glues" Okazaki fragments together on
the lagging strand.
Q10: The leading strand is synthesized:
A. Continuously in the 5'→3' direction
B. Discontinuously in the 5'→3' direction
C. Continuously in the 3'→5' direction
D. Discontinuously in the 3'→5' direction
Correct Answer: A
Rationale: The leading strand is synthesized continuously in the 5'→3' direction toward the
replication fork. The lagging strand is synthesized discontinuously as Okazaki fragments. Key
teaching point: Leading strand = continuous synthesis toward the fork; lagging strand =
discontinuous synthesis away from the fork.
Q11: DNA replication is described as semiconservative because:
BIOD 210 MIDTERM EXAM 2026/2027 | GENETICS –
PORTAGE LEARNING | EXPERT VERIFIED | 100
QUESTIONS & COMPLETE RATIONALES | PASS
GUARANTEED - A+ GRADED
SECTION 1: DNA STRUCTURE & REPLICATION (Questions 1-20)
Q1: Which of the following scientists first described the double helix structure of DNA?
A. Gregor Mendel
B. James Watson and Francis Crick
C. Friedrich Miescher
D. Erwin Chargaff
Correct Answer: B
Rationale: James Watson and Francis Crick first described the double helix structure of DNA in 1953,
using data from Rosalind Franklin's X-ray diffraction images. Mendel (A) described inheritance
patterns; Miescher (C) first isolated DNA; Chargaff (D) established base pairing rules. Key teaching
point: Watson and Crick used Franklin's X-ray data to propose the double helix model.
Q2: A DNA molecule is composed of two strands that run in opposite directions. This is referred to
as:
A. Complementary
B. Antiparallel
C. Semiconservative
D. Double-stranded
Correct Answer: B
Rationale: The two strands of DNA run in opposite directions (one 5'→3' and the other 3'→5'), which
is described as antiparallel. Complementary (A) refers to base pairing; semiconservative (C) describes
replication; double-stranded (D) describes the overall structure. Key teaching point: DNA strands are
antiparallel, which is essential for replication and transcription.
Q3: In DNA, which nitrogenous base pairs with adenine (A)?
A. Cytosine
B. Guanine
C. Thymine
D. Uracil
Correct Answer: C
,2
Rationale: In DNA, adenine (A) pairs with thymine (T) via two hydrogen bonds. In RNA, adenine pairs
with uracil (U). Key teaching point: DNA base pairing: A-T (2 H-bonds); G-C (3 H-bonds).
Q4: In DNA, which nitrogenous base pairs with guanine (G)?
A. Adenine
B. Thymine
C. Cytosine
D. Uracil
Correct Answer: C
Rationale: In DNA, guanine (G) pairs with cytosine (C) via three hydrogen bonds. Key teaching
point: G-C pairs have three hydrogen bonds, making them stronger than A-T pairs.
Q5: The DNA sequence 5'-ATGC-3' is complementary to which sequence?
A. 5'-GCAT-3'
B. 3'-TACG-5'
C. 5'-TACG-3'
D. 3'-GCAT-5'
Correct Answer: B
Rationale: Complementary base pairing and antiparallel orientation: 5'-ATGC-3' pairs with 3'-TACG-
5' (A-T, T-A, G-C, C-G). Key teaching point: Complementary DNA strands are antiparallel and follow
Chargaff's base pairing rules.
Q6: Which enzyme is responsible for unwinding the DNA double helix during replication?
A. DNA polymerase
B. Helicase
C. Primase
D. Ligase
Correct Answer: B
Rationale: Helicase unwinds the DNA double helix by breaking hydrogen bonds between base pairs,
creating a replication fork. DNA polymerase (A) synthesizes new DNA; primase (C) synthesizes RNA
primers; ligase (D) seals nicks. Key teaching point: Helicase is the "unwinding enzyme" that
separates DNA strands for replication.
Q7: Which enzyme synthesizes RNA primers during DNA replication?
A. DNA polymerase
B. Helicase
C. Primase
D. Ligase
, 3
Correct Answer: C
Rationale: Primase synthesizes short RNA primers that provide a free 3'-OH group for DNA
polymerase to begin DNA synthesis. Key teaching point: Primase creates RNA primers needed for
DNA polymerase to initiate replication.
Q8: DNA polymerase synthesizes new DNA in which direction?
A. 3'→5'
B. 5'→3'
C. Both directions
D. Randomly
Correct Answer: B
Rationale: DNA polymerase synthesizes new DNA in the 5'→3' direction by adding nucleotides to the
3'-OH group of the growing strand. Key teaching point: DNA synthesis always occurs in the 5'→3'
direction.
Q9: Which enzyme seals the nicks between Okazaki fragments on the lagging strand?
A. DNA polymerase
B. Helicase
C. Primase
D. Ligase
Correct Answer: D
Rationale: DNA ligase seals the nicks between Okazaki fragments by forming phosphodiester bonds,
creating a continuous DNA strand. Key teaching point: Ligase "glues" Okazaki fragments together on
the lagging strand.
Q10: The leading strand is synthesized:
A. Continuously in the 5'→3' direction
B. Discontinuously in the 5'→3' direction
C. Continuously in the 3'→5' direction
D. Discontinuously in the 3'→5' direction
Correct Answer: A
Rationale: The leading strand is synthesized continuously in the 5'→3' direction toward the
replication fork. The lagging strand is synthesized discontinuously as Okazaki fragments. Key
teaching point: Leading strand = continuous synthesis toward the fork; lagging strand =
discontinuous synthesis away from the fork.
Q11: DNA replication is described as semiconservative because:
