CALIFORNIA UC SAN DIEGO MEDICAL BIOCHEMISTRY
AUTUMN MIDTERM EXAM
WITH ORIGINAL QUESTIONS AND VERIFIED ANSWERS,
PLUS EXPLAINED RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. The central dogma of molecular biology describes the flow of
genetic information as:
A. DNA → RNA → Protein
B. Protein → RNA → DNA
C. RNA → DNA → Protein
D. DNA → Protein → RNA
A. DNA → RNA → Protein
Rationale: The central dogma, articulated by Francis Crick, states that
genetic information flows from DNA to RNA (transcription) and from
RNA to protein (translation). Reverse transcription (RNA → DNA) is
an exception in retroviruses. The dogma does not permit information
flow from protein back to nucleic acid .
2. DNA replication is described as semi-conservative because:
A. Each daughter DNA molecule contains one original and one
newly synthesized strand
B. Both strands of the daughter DNA are newly synthesized
C. Both strands of the daughter DNA are original
D. DNA replication is random
A. Each daughter DNA molecule contains one original and one
newly synthesized strand
Rationale: The Meselson-Stahl experiment demonstrated that DNA
replication is semi-conservative: each daughter double helix contains
one parental strand and one newly synthesized strand. This ensures
accurate transmission of genetic information .
1
, 3. Which enzyme is responsible for relieving supercoiling ahead of
the DNA replication fork?
A. DNA topoisomerase
B. DNA helicase
C. DNA polymerase
D. DNA ligase
A. DNA topoisomerase
Rationale: DNA topoisomerases (e.g., gyrase in bacteria) relieve
supercoiling ahead of the replication fork by introducing transient
single- or double-strand breaks. Helicase unwinds the double helix;
polymerase synthesizes DNA; ligase seals nicks .
4. Which enzyme unwinds the DNA double helix at the replication
fork?
A. DNA helicase
B. DNA topoisomerase
C. DNA polymerase
D. DNA ligase
A. DNA helicase
Rationale: DNA helicase unwinds the double helix by breaking
hydrogen bonds between base pairs, creating the replication fork.
Topoisomerase relieves supercoiling; polymerase synthesizes DNA;
ligase seals nicks .
5. DNA polymerase synthesizes new DNA strands in which
direction?
A. 5′ to 3′
B. 3′ to 5′
C. Both directions simultaneously
D. Randomly
2
,A. 5′ to 3′
Rationale: DNA polymerase adds nucleotides to the 3′-OH end of the
growing strand, so synthesis proceeds in the 5′ to 3′ direction. The
template strand is read in the 3′ to 5′ direction .
6. The leading strand of DNA replication is synthesized:
A. Continuously in the 5′ to 3′ direction toward the replication fork
B. Discontinuously in short Okazaki fragments
C. In the 3′ to 5′ direction
D. Only after the lagging strand is completed
A. Continuously in the 5′ to 3′ direction toward the replication fork
Rationale: The leading strand is synthesized continuously in the 5′ to
3′ direction toward the replication fork. The lagging strand is
synthesized discontinuously in short Okazaki fragments away from the
fork .
7. Okazaki fragments are:
A. Short DNA segments synthesized discontinuously on the
lagging strand
B. Long continuous DNA segments on the leading strand
C. RNA primers
D. Proteins involved in replication
A. Short DNA segments synthesized discontinuously on the lagging
strand
Rationale: Okazaki fragments are short DNA segments synthesized
discontinuously on the lagging strand. They are later joined by DNA
ligase after RNA primers are removed and gaps are filled .
8. Which enzyme seals the nicks between Okazaki fragments?
A. DNA ligase
B. DNA polymerase
3
, C. DNA helicase
D. DNA topoisomerase
A. DNA ligase
Rationale: DNA ligase catalyzes the formation of phosphodiester
bonds between adjacent DNA fragments, sealing the nicks between
Okazaki fragments after RNA primers are removed and gaps are filled
by DNA polymerase .
9. RNA primers are synthesized by:
A. Primase
B. DNA polymerase
C. Helicase
D. Ligase
A. Primase
Rationale: Primase is an RNA polymerase that synthesizes short RNA
primers complementary to the DNA template, providing a 3′-OH end
for DNA polymerase to begin synthesis. DNA polymerase cannot
initiate synthesis de novo .
10. Telomeres are:
A. Repetitive DNA sequences at the ends of chromosomes that
protect against degradation and fusion
B. Origins of replication
C. Centromeres
D. Introns
A. Repetitive DNA sequences at the ends of chromosomes that
protect against degradation and fusion
Rationale: Telomeres are repetitive DNA sequences (TTAGGG in
humans) at chromosome ends that protect against degradation, fusion,
and recognition as DNA damage. They are maintained by telomerase
4
AUTUMN MIDTERM EXAM
WITH ORIGINAL QUESTIONS AND VERIFIED ANSWERS,
PLUS EXPLAINED RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. The central dogma of molecular biology describes the flow of
genetic information as:
A. DNA → RNA → Protein
B. Protein → RNA → DNA
C. RNA → DNA → Protein
D. DNA → Protein → RNA
A. DNA → RNA → Protein
Rationale: The central dogma, articulated by Francis Crick, states that
genetic information flows from DNA to RNA (transcription) and from
RNA to protein (translation). Reverse transcription (RNA → DNA) is
an exception in retroviruses. The dogma does not permit information
flow from protein back to nucleic acid .
2. DNA replication is described as semi-conservative because:
A. Each daughter DNA molecule contains one original and one
newly synthesized strand
B. Both strands of the daughter DNA are newly synthesized
C. Both strands of the daughter DNA are original
D. DNA replication is random
A. Each daughter DNA molecule contains one original and one
newly synthesized strand
Rationale: The Meselson-Stahl experiment demonstrated that DNA
replication is semi-conservative: each daughter double helix contains
one parental strand and one newly synthesized strand. This ensures
accurate transmission of genetic information .
1
, 3. Which enzyme is responsible for relieving supercoiling ahead of
the DNA replication fork?
A. DNA topoisomerase
B. DNA helicase
C. DNA polymerase
D. DNA ligase
A. DNA topoisomerase
Rationale: DNA topoisomerases (e.g., gyrase in bacteria) relieve
supercoiling ahead of the replication fork by introducing transient
single- or double-strand breaks. Helicase unwinds the double helix;
polymerase synthesizes DNA; ligase seals nicks .
4. Which enzyme unwinds the DNA double helix at the replication
fork?
A. DNA helicase
B. DNA topoisomerase
C. DNA polymerase
D. DNA ligase
A. DNA helicase
Rationale: DNA helicase unwinds the double helix by breaking
hydrogen bonds between base pairs, creating the replication fork.
Topoisomerase relieves supercoiling; polymerase synthesizes DNA;
ligase seals nicks .
5. DNA polymerase synthesizes new DNA strands in which
direction?
A. 5′ to 3′
B. 3′ to 5′
C. Both directions simultaneously
D. Randomly
2
,A. 5′ to 3′
Rationale: DNA polymerase adds nucleotides to the 3′-OH end of the
growing strand, so synthesis proceeds in the 5′ to 3′ direction. The
template strand is read in the 3′ to 5′ direction .
6. The leading strand of DNA replication is synthesized:
A. Continuously in the 5′ to 3′ direction toward the replication fork
B. Discontinuously in short Okazaki fragments
C. In the 3′ to 5′ direction
D. Only after the lagging strand is completed
A. Continuously in the 5′ to 3′ direction toward the replication fork
Rationale: The leading strand is synthesized continuously in the 5′ to
3′ direction toward the replication fork. The lagging strand is
synthesized discontinuously in short Okazaki fragments away from the
fork .
7. Okazaki fragments are:
A. Short DNA segments synthesized discontinuously on the
lagging strand
B. Long continuous DNA segments on the leading strand
C. RNA primers
D. Proteins involved in replication
A. Short DNA segments synthesized discontinuously on the lagging
strand
Rationale: Okazaki fragments are short DNA segments synthesized
discontinuously on the lagging strand. They are later joined by DNA
ligase after RNA primers are removed and gaps are filled .
8. Which enzyme seals the nicks between Okazaki fragments?
A. DNA ligase
B. DNA polymerase
3
, C. DNA helicase
D. DNA topoisomerase
A. DNA ligase
Rationale: DNA ligase catalyzes the formation of phosphodiester
bonds between adjacent DNA fragments, sealing the nicks between
Okazaki fragments after RNA primers are removed and gaps are filled
by DNA polymerase .
9. RNA primers are synthesized by:
A. Primase
B. DNA polymerase
C. Helicase
D. Ligase
A. Primase
Rationale: Primase is an RNA polymerase that synthesizes short RNA
primers complementary to the DNA template, providing a 3′-OH end
for DNA polymerase to begin synthesis. DNA polymerase cannot
initiate synthesis de novo .
10. Telomeres are:
A. Repetitive DNA sequences at the ends of chromosomes that
protect against degradation and fusion
B. Origins of replication
C. Centromeres
D. Introns
A. Repetitive DNA sequences at the ends of chromosomes that
protect against degradation and fusion
Rationale: Telomeres are repetitive DNA sequences (TTAGGG in
humans) at chromosome ends that protect against degradation, fusion,
and recognition as DNA damage. They are maintained by telomerase
4
