[Campbell Biology P.1098] Based on the "Data from the Experiments" graph, compare the rate of... | Practice Question

Based on the "Data from the Experiments" graph, compare the rate of DNA synthesis in the presence and absence of cytochalasin B. What conclusion can be drawn about the researchers' hypothesis that the toxin increases thymidine diffusion?

  • A: DNA synthesis is faster with cytochalasin B; the data support increased diffusion.
  • B: DNA synthesis is slower with cytochalasin B; the data contradict increased diffusion.
  • C: DNA synthesis rates are similar; the data neither support nor contradict.
  • D: RNA synthesis is affected, not DNA synthesis.

Explanation

The graph for 'DNA, cytochalasin B present' consistently shows higher values than the 'DNA' curve without cytochalasin B. This indicates that more labeled thymidine is being incorporated or taken up when cytochalasin B is present. Since cytochalasin B prevents cell division, it would lead to larger cells, potentially increasing the surface area for diffusion or simply maintaining the original large volume for uptake. The researchers' hypothesis that the toxin increases diffusion of thymidine into the embryos is supported by the observation of a higher apparent DNA synthesis rate.