[Campbell Biology P.1098] In a separate experiment, researchers created frog embryos with mul... | Practice Question

In a separate experiment, researchers created frog embryos with multiple sperm nuclei polyspermy , leading to a higher initial nucleus-to-cytoplasm ratio. These embryos ended cleavage at the 10th cell division, while wild-type embryos ended at the 12th cell division, both reaching the same final nucleus-to-cytoplasm ratio. What do these results suggest about the trigger for the end of cleavage?

  • A: The end of cleavage is primarily triggered by the achievement of a critical nucleus-to-cytoplasm ratio.
  • B: The number of cell divisions is the sole determinant of cleavage cessation.
  • C: Polyspermy directly extends the duration of cleavage.
  • D: Cytoplasmic volume dictates the timing of cleavage, independent of nuclei.

Explanation

The polyspermy experiment shows that embryos with a higher initial nucleus-to-cytoplasm ratio end cleavage earlier at fewer divisions but still reach the same critical final ratio as wild-type embryos. This strongly suggests that the end of cleavage is not determined by a fixed number of divisions, but rather by the achievement of a specific, critical nucleus-to-cytoplasm ratio. Once this ratio is met, the cell cycle changes, and cleavage ceases.