The question
Name: ______________________ Date: ____________ Density bands after rounds of replication (4 points)
Data table
DNA density in bacteria moved from light to heavy nitrogen
Bacteria were grown for many generations in a medium containing only light nitrogen (14N), then moved to a medium containing only heavy nitrogen (15N). All the cells divided once per generation. At each generation, DNA was extracted and spun in a density gradient, and the share of the DNA at each density was measured. The values for generation 3 were not recorded.
| Generations in 15N | Light (%) | Intermediate (%) | Heavy (%) |
|---|---|---|---|
| 0 | 100 | 0 | 0 |
| 1 | 2 | 96 | 2 |
| 2 | 1 | 51 | 48 |
| 3 | ? | ? | ? |
(a) Calculate the expected percentages of light, intermediate and heavy DNA at generation 3 if replication is semiconservative. [1 point]
(b) Describe how the data for generation 1 rule out the conservative model. [1 point]
(c) Predict the percentages of light, intermediate and heavy DNA at generation 2 under the dispersive model, and describe how the data differ from that prediction. [1 point]
(d) The measured values at generation 2 are 51% and 48%, not exactly 50% and 50%. Explain one reason the measured values could differ slightly from the predicted values. [1 point]