Question at position 2 In the organism you are studying, the DNA polymerase has an error rate of 1 mistake in every 106 bases copied. However, the overall mutation rate is much lower. What accounts for this difference?Not all mutations can be detected easilyThe polymerase is more careful in replicating regions where genes existDNA repair mechanisms correct errors made by the polymeraseDNA polymerase has no proofreading functionMutations do not occur if mutagens are not presentClear my selection

Question

Question at position 2 In the organism you are studying, the DNA polymerase has an error rate of 1 mistake in every 106 bases copied. However, the overall mutation rate is much lower. What accounts for this difference?Not all mutations can be detected easilyThe polymerase is more careful in replicating regions where genes existDNA repair mechanisms correct errors made by the polymeraseDNA polymerase has no proofreading functionMutations do not occur if mutagens are not presentClear my selection

BlackTom AI · Accepted Answer

The question presents a scenario where DNA polymerase makes one mistake per 10^6 bases copied, yet the organism’s overall mutation rate is much lower. Let's evaluate each option to see why the observed discrepancy arises.
Option 1: 'Not all mutations can be detected easily' This suggests that some mutations go unnoticed, which could contribute to an apparently lower observed mutation rate. However, while detection limits can affect measured rates, this does not account for the fundamental reduction in the rate relative to the polymerase error rate, since detectable vs undetectable mutations would still reflect the underlying error rate unless there is a compensatory mechanism. In short, detection difficulty affects measurement, not the actual occurrence.
Option 2: 'The polymerase is more careful in replicating regions where genes exist' This implies differential fidelity depending on genomic context. While some systems show variation in mutation rates across regions, the key point here is that the overall error rate is a property of the polymerase itself, and even if regional differences exist, they do not fully explain the substantial lowering of the overall mutation rate relative to the raw replication error rate given. This option oversimplifies or misattributes the primary mechanism behind the reduction.
Option 3: 'DNA repair mechanisms correct errors made by the polymerase' This directly addresses how a cell reconciles a high intrinsic error rate of replication with a much lower net mutation rate. Mismatch repair, excision repair, and proofreading by polymerases work together to detect and fix most replication errors, dramatically lowering the number of heritable mutations. This explanation aligns with fundamental molecular biology: replication errors are common, but repair systems correct most of them before fixation in the genome.
Option 4: 'DNA polymerase has no proofreading function' This is incorrect because it contradicts the premise that the polymerase makes errors at a certain rate; many DNA polymerases do have proofreading activity that reduces errors. If there were truly no proofreading, the error rate would not be mitigated so effectively by repair mechanisms, and the observed mutation rate would be higher, not lower.
Option 5: 'Mutations do not occur if mutagens are not present' This suggests that environmental mutagens are required for mutations to occur. While exposure to mutagens can increase mutation rates, mutations can and do arise spontaneously through replication errors and other endogenous processes even in the absence of external mutagens. Therefore, this statement does not explain why the intrinsic polymerase error rate is much higher than the observed mutation rate.
Overall, the most coherent explanation for the discrepancy between the polymerase error rate and the lower overall mutation rate is the action of DNA repair mechanisms that correct errors after they occur during replication, preventing those errors from becoming fixed mutations.

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