Unfortunately the science is wrong in this episode. The story is that the couple need good mitochondria from the donor egg and not the nuclear DNA, but the donor egg has already been fertilized. Once fertilized, the donor nuclear DNA cannot be removed and the wife's nuclear DNA added, as they are trying to pretend in this episode!
What a couple in this situation - i.e. needing undamaged mitochondrial DNA - would actually need is for the nucleus to be removed from the donor egg, the nucleus of an egg from the wife to be put in its place, and THEN for the egg to be fertilized with the husband's sperm. The donor egg is only needed for its healthy cytoplasm which is where the mitochondria are, and mitochondrial DNA is something very different from nuclear DNA which is what makes us what we are.
The process would need to be done before fertilization and this story is impossible!
What a couple in this situation - i.e. needing undamaged mitochondrial DNA - would actually need is for the nucleus to be removed from the donor egg, the nucleus of an egg from the wife to be put in its place, and THEN for the egg to be fertilized with the husband's sperm. The donor egg is only needed for its healthy cytoplasm which is where the mitochondria are, and mitochondrial DNA is something very different from nuclear DNA which is what makes us what we are.
The process would need to be done before fertilization and this story is impossible!