Chapter 36 : Analyzing the Issues of Biotechnology: Cloning

Cloning is the creation of genetically identical copies. Livestock breeders have used this form of cloning for approximately 20 to 25 years. A fertilized egg is allowed to develop to the two- or four-cell stage and is divided into single cells. Each cell gives rise to an offspring that is genetically identical to the others.

Enucleation of eggs. Working at a microscope that is hooked to a computer monitor or television screen, scientists hold the egg steady with a pipette and then insert a much smaller pipette into the egg to withdraw the nucleus. (Photographs courtesy of Roslin Institute, Edinburgh, Scotland.)

Embryo splitting. At early stages of development, scientists use microsurgical blades to separate livestock embryos into separate cells. (Photograph courtesy of George Seidel, Colorado State University.)

Nuclear transfer. (A) Nuclei have been removed from either somatic or embryonic cells. (B) Note the nucleus in the small pipette. Each nucleus is injected into a separate enucleated egg cell. (C) The newly inserted nucleus is barely visible between the egg cell membrane and the cellular cytoplasm. A small tear in the cell membrane indicates the site where the pipette was inserted. (Photographs courtesy of Roslin Institute, Edinburgh, Scotland.)

Somatic cell nuclear transfer. Dolly was produced by a unique type of cloning. The nucleus in a fully differentiated somatic cell from breed A's udder was inserted into the enucleated egg of breed B by fusing the two cells. The resulting egg contained breed A's nuclear genetic material and mitochondrial DNA from breeds A and B. The egg developed into a blastula in tissue culture, and the blastula was inserted into the uterus of sheep C, the surrogate mother of Dolly. Note that Dolly's coloring is identical to that of female A, her genetic mother, and does not exhibit any markings of either her surrogate mother or breed B.

Human blastocyst. A fertilized egg develops into a blastocyst in approximately 4 to 5 days, whether it is in cell culture or the female reproductive tract. (Photograph courtesy of Michael Vernon, West Virginia Center of Reproductive Medicine.)