ANSWERS
Process of Science (10.4)
What Is the Correct Model for DNA Replication?
Questions
1. Suppose you were a scientist evaluating these three models when they were first proposed. What would have been your hypothesis regarding which model was correct? Explain.
Answers and explanations will vary. For example, proponents of the semiconservative model might reason that the consistent base pairings enable each strand to reconstruct the complementary strand. Proponents of the conservative model might reason that for the double helix to replicate itself, the original would have to be conserved, or remain intact, as a blueprint. The dispersive model would be difficult to argue for. If the strands were divided into segments, it is hard to imagine how the complete sequence for each strand could be reconstructed.
2. Which two models have similar results after replication cycle 1?
In both the semiconservative and dispersive models, each daughter molecule is a combination of parental and new strands.
3. Why were culture samples taken after each replication cycle?
The density of the samples reflects the presence of parental and new strands after each replication, allowing comparison with each model.
4. Which model(s), if any, is ruled out by the experimental results of replication cycle 1? Explain.
The conservative model is ruled out because it predicts that some daughter molecules will have only parental (heavy) strands, while others will have only new (light) strands. The experimental results show all molecules in the intermediate range, indicating a mix of parental and new strands. The other two models predict this mixture.
5. Which model is proved correct by the experimental results of replication cycle 2? Explain.
The semiconservative model is proved correct because the results show light and intermediate bands, corresponding to the prediction that some daughter molecules will have only new (light) strands, while others combine parental (heavy) and new strands, resulting in intermediate density. If the dispersive model had been correct, there would have been a single intermediate band because that model predicts all daughter molecules will combine parental (heavy) and new (light) strands.
____________________________________________________________________________________
Copyright © 2010 Pearson Education, Inc., publishing as Pearson Benjamin Cummings