What is the thermodynamic role of pyrophosphate (PPi) hydrolysis immediately after each nucleotide is added during DNA synthesis?
AIt provides the energy for the conformational change that ejects mismatched nucleotides
BIt makes the polymerization reaction thermodynamically irreversible, driving chain elongation forward
CIt regenerates the dNTP substrate by reattaching phosphates to the nucleoside
DIt activates the 3'-OH group, making it a better nucleophile for the next addition
The nucleophilic attack of the 3'-OH on the α-phosphate of the incoming dNTP releases pyrophosphate (PPi). If PPi were allowed to accumulate, the reverse reaction (depolymerization) would become thermodynamically favorable. Pyrophosphatase immediately cleaves PPi into two inorganic phosphates, making the reaction strongly exergonic and essentially irreversible. This thermodynamic pull is what commits each nucleotide addition and explains why replication proceeds unidirectionally. Option A describes proofreading, which uses a separate mechanism.
Question 2 Multiple Choice
A mutant DNA polymerase retains its polymerization activity but its 3'→5' exonuclease domain has been inactivated. What is the most likely consequence for DNA replication?
AThe polymerase cannot synthesize DNA at all, because the exonuclease domain is required to initiate polymerization
BReplication speed increases markedly because the polymerase no longer pauses to proofread
CThe error rate increases significantly because mismatched nucleotides cannot be excised before the next addition
DThe polymerase synthesizes DNA in the 3'→5' direction instead of 5'→3'
The 3'→5' exonuclease is the proofreading domain that clips out mismatched nucleotides after they are accidentally incorporated. Without it, mismatches persist in the template and the mutation rate rises by roughly 100-fold (from ~10⁻⁷ to ~10⁻⁵ per base). The polymerase can still synthesize DNA — these are two distinct activities in separate domains. Option D reflects a fundamental misconception: synthesis always proceeds 5'→3'; the 3'→5' label on the exonuclease refers to the direction it reads the strand while removing nucleotides from the 3' end.
Question 3 True / False
DNA polymerase can synthesize a new strand in both the 5'→3' and 3'→5' directions, depending on whether it is copying the leading or lagging strand template.
TTrue
FFalse
Answer: False
DNA polymerase always synthesizes DNA in the 5'→3' direction only — it can only add nucleotides to the 3'-OH end of a growing strand. This is an absolute constraint of the enzyme's chemistry. The lagging strand challenge is solved not by reversing synthesis direction but by synthesizing short Okazaki fragments in the 5'→3' direction that are individually oriented opposite to the overall replication fork movement. The 3'→5' exonuclease activity is for proofreading (removing nucleotides from the 3' end), not for synthesis.
Question 4 True / False
Geometric selection — the polymerase active site's shape complementarity to correct Watson-Crick base pairs — is the first and largest contributor to polymerase fidelity, reducing errors to roughly 1 in 10⁴–10⁵ before proofreading begins.
TTrue
FFalse
Answer: True
The polymerase active site undergoes a conformational change that closes tightly around an incoming dNTP only when it forms a correct Watson-Crick geometry. A mismatched base pair (wrong shape, misaligned hydrogen bonds) prevents this closing, reducing the rate of incorrect incorporation. This geometric filter alone achieves error rates near 1 in 10⁴–10⁵. Proofreading then improves this by another ~100-fold, and mismatch repair adds further correction, together achieving the final rate of ~10⁻¹⁰ per base.
Question 5 Short Answer
Explain how the three-level fidelity system — geometric selection, proofreading, and mismatch repair — achieves error rates far better than any single mechanism alone.
Think about your answer, then reveal below.
Model answer: Each mechanism catches a different subset of errors, and their effects multiply. Geometric selection filters out ~99.99% of mismatches at the point of insertion by requiring correct base-pair geometry for the polymerase to close and catalyze bond formation. Of the ~1 in 10⁴ that slip through, the 3'→5' exonuclease proofs the most recent addition: a mismatched 3' terminus slows the next polymerization step, giving the exonuclease time to clip it out, catching ~99% of remaining errors. Post-replicative mismatch repair proteins scan the newly synthesized strand for distortions left by the rare mismatches that survived proofreading, removing another large fraction. The combined effect is multiplicative: 10⁻⁵ × 10⁻² × 10⁻³ ≈ 10⁻¹⁰ per base.
The logic is cascading error correction: each stage exploits a different physical signature of a mismatch — wrong geometry (stage 1), distorted primer terminus (stage 2), distorted duplex topology (stage 3). No single mechanism could achieve 10⁻¹⁰ accuracy alone; the cascade is necessary because each individual filter is imperfect.