CHEM210 Biochemistry
Final Assessment Review
Module 6 (Questions & Solutions)
2025
©2025
, 1. Case Study – DNA Double-Helix Stability:
A researcher studying the structure of double-stranded DNA performs
thermal denaturation experiments. The melting temperature (Tm)
increases markedly when the GC content of the DNA is higher.
Question: What is the primary explanation for this observation?
A. GC pairs form additional van der Waals bonds compared to AT pairs.
B. GC pairs form three hydrogen bonds, whereas AT pairs form only two.
C. GC pairs increase the DNA’s backbone rigidity due to extra covalent
bonds.
D. GC-rich sequences adopt a different helical form that is less flexible.
ANS: B. GC pairs form three hydrogen bonds, whereas AT pairs form
only two.
Rationale: Higher GC content increases the number of hydrogen bonds
per base pair (three for GC vs. two for AT), thereby providing greater
thermal stability and a higher melting temperature.
---
2. Case Study – Noncanonical Base Pairing:
In crystallographic studies, researchers found that under certain
conditions, a segment of DNA adopts an alternative base-pairing mode
(Hoogsteen pairing) in which adenine forms two hydrogen bonds with
thymine.
Question: What is a key functional implication of Hoogsteen base
pairing in DNA?
A. It increases the overall negative charge of the DNA strand.
B. It facilitates temporary structural flexibility during protein binding.
C. It permanently denatures the double helix.
D. It enhances the Watson–Crick hydrogen bonding pattern.
ANS: B. It facilitates temporary structural flexibility during protein
binding.
©2025
, Rationale: Hoogsteen base pairs provide an alternative hydrogen
bonding scheme that can transiently alter the groove dimensions of DNA,
which may help accommodate the binding of proteins or facilitate
processes such as transcription and repair.
---
3. Case Study – DNA Replication Fidelity:
A molecular biologist investigates the error rate of DNA polymerase and
finds that intrinsic proofreading reduces replication errors by several
orders of magnitude.
Question: Which activity of DNA polymerase is responsible for this
increased fidelity?
A. 5′→3′ exonuclease activity
B. 3′→5′ exonuclease activity
C. Ligase activity
D. Helicase activity
ANS: B. 3′→5′ exonuclease activity
Rationale: The 3′→5′ exonuclease proofreading function of DNA
polymerase excises incorrectly incorporated nucleotides, thereby
ensuring high fidelity during DNA replication.
---
4. Case Study – Variations in RNA Structure:
Researchers comparing various RNA molecules note that certain RNA
species adopt extensive secondary structures, including hairpin loops and
pseudoknots, which are critical for their catalytic activity.
Question: Which type of RNA is best exemplified by having intrinsic
ribozyme activity?
A. Messenger RNA (mRNA)
B. Transfer RNA (tRNA)
C. Ribosomal RNA (rRNA)
D. Small nuclear RNA (snRNA)
©2025
Final Assessment Review
Module 6 (Questions & Solutions)
2025
©2025
, 1. Case Study – DNA Double-Helix Stability:
A researcher studying the structure of double-stranded DNA performs
thermal denaturation experiments. The melting temperature (Tm)
increases markedly when the GC content of the DNA is higher.
Question: What is the primary explanation for this observation?
A. GC pairs form additional van der Waals bonds compared to AT pairs.
B. GC pairs form three hydrogen bonds, whereas AT pairs form only two.
C. GC pairs increase the DNA’s backbone rigidity due to extra covalent
bonds.
D. GC-rich sequences adopt a different helical form that is less flexible.
ANS: B. GC pairs form three hydrogen bonds, whereas AT pairs form
only two.
Rationale: Higher GC content increases the number of hydrogen bonds
per base pair (three for GC vs. two for AT), thereby providing greater
thermal stability and a higher melting temperature.
---
2. Case Study – Noncanonical Base Pairing:
In crystallographic studies, researchers found that under certain
conditions, a segment of DNA adopts an alternative base-pairing mode
(Hoogsteen pairing) in which adenine forms two hydrogen bonds with
thymine.
Question: What is a key functional implication of Hoogsteen base
pairing in DNA?
A. It increases the overall negative charge of the DNA strand.
B. It facilitates temporary structural flexibility during protein binding.
C. It permanently denatures the double helix.
D. It enhances the Watson–Crick hydrogen bonding pattern.
ANS: B. It facilitates temporary structural flexibility during protein
binding.
©2025
, Rationale: Hoogsteen base pairs provide an alternative hydrogen
bonding scheme that can transiently alter the groove dimensions of DNA,
which may help accommodate the binding of proteins or facilitate
processes such as transcription and repair.
---
3. Case Study – DNA Replication Fidelity:
A molecular biologist investigates the error rate of DNA polymerase and
finds that intrinsic proofreading reduces replication errors by several
orders of magnitude.
Question: Which activity of DNA polymerase is responsible for this
increased fidelity?
A. 5′→3′ exonuclease activity
B. 3′→5′ exonuclease activity
C. Ligase activity
D. Helicase activity
ANS: B. 3′→5′ exonuclease activity
Rationale: The 3′→5′ exonuclease proofreading function of DNA
polymerase excises incorrectly incorporated nucleotides, thereby
ensuring high fidelity during DNA replication.
---
4. Case Study – Variations in RNA Structure:
Researchers comparing various RNA molecules note that certain RNA
species adopt extensive secondary structures, including hairpin loops and
pseudoknots, which are critical for their catalytic activity.
Question: Which type of RNA is best exemplified by having intrinsic
ribozyme activity?
A. Messenger RNA (mRNA)
B. Transfer RNA (tRNA)
C. Ribosomal RNA (rRNA)
D. Small nuclear RNA (snRNA)
©2025
