In somatic cell nuclear transfer, the donor nucleus is taken from:

• A. An adult somatic cell.
• B. A fertilized ovum.
• C. A stem cell from the embryo.
• D. A gamete from a donor.

Answer: (A)

In somatic cell nuclear transfer, the donor nucleus comes from a mature, non-gamete cell—the somatic cell of the donor. The nucleus from that adult or differentiated cell is inserted into an egg that has had its own nucleus removed, and the egg cytoplasm reprograms the donor genome to an embryonic, totipotent state. This is why Dolly the sheep’s clone used a nucleus from an adult mammary (somatic) cell. The other options don’t fit because fertilized eggs already contain a zygotic nucleus from two gametes, embryonic stem cell nuclei aren’t used as the standard donor in cloning, and a gamete nucleus would be haploid and not suitable for forming a normal diploid embryo.