Question

In a separate experiment, researchers disrupted the block to polyspermy, generating embryos with 7 to 10 sperm nuclei. At the end of cleavage, these embryos had the same nucleus- to-cytoplasm ratio as the wild-type embryos, but cleavage ended at the 10th cell division rather than the 12th cell division. What do these results indicate about the timing of the end of cleavage?

Short answer

The results suggest that the ratio of nucleus to cytoplasm determines when cleavage ends.

Step-by-step solution

Outline of the experiment 

In the experiment, when researchers developed embryos with 7 to 10 sperm nuclei, they observed that the cleavage of the experimental embryo stopped at 10th cell division compared to wild-type embryos that stopped at 12th cell division.

Cleavage

Cleavage refers to a series of mitotic divisions that occur in an organism's zygote, resulting in many cells. The objective of cleavage is to create a multicellular embryo and organize its development areas.

Results and discussion 

The experimental embryo stopped at the 10th cell division, whereas the wild-type embryo stopped at the 12th division. However, the nucleus to cytoplasmic ratio of both types of embryos was the same.

Thus, the results suggest that the nucleus to cytoplasmic ratio determines the end of cleavage and cannot be determined by counting cell divisions.