In cloning animals, donor cells are typically taken before which developmental stage?

• A. Stage 8 development.
• B. Stage 3 development.
• C. Stage 12 development.
• D. Stage 20 development.

Answer: (A)

Donor nuclei must come from cells that can be reset to an embryonic, totipotent state by the egg’s cytoplasm. This is easiest when the donor cells are still undifferentiated, i.e., from an embryo before differentiation has progressed. In this context, that boundary is just before Stage 8 development, so donor cells are taken prior to Stage 8. If the donor cells come from later stages, the cells have begun to specialize, and reprogramming the nucleus becomes much harder, reducing the chances of a successful clone.