Cloning Organisms

The first cloning of a vertebrate by nuclear transfer was reported by John Gurdon of the University of Cambridge in the 1950s. In nuclear The process used to clone Dolly. Adapted from Gurdon, J. B., 1999. transplantation, the nucleus of an unfertilized donor egg is either mechanically removed or it is destroyed by ultraviolet light in a process called enucleation. The original nucleus is then replaced by a nucleus containing a full set of genes that has been taken from a body cell of an organism. This procedure eliminates the need for the fertilization of an egg by a sperm.

The most successful nuclear transplants have been achieved after serially transferring donor intestinal nuclei, that is, putting an adult nucleus from an intestinal cell into an egg whose nucleus was destroyed, allowing the egg to divide only a certain number of times, removing nuclei from these cells, and repeating this process several times before allowing the embryo to complete development. Eventually, transplantation of nuclei from albino Scanning electron microscope scan of human embryo, day three, at the eight-cell stage. embryonic frog cells into enucleated eggs from a dark green female frog led to the production of adult albino frog clones, demonstrating that a properly treated adult nucleus could support the full development of an egg into an adult clone. Later experiments demonstrated that nuclei from cells of other tissues, even quiescent cells such as blood cells, could also be used if properly treated. Despite these successes, no adult frog has been cloned when a nucleus from an adult cell was used without serial transfer. Without serial transfer of the nuclei, the animals would only develop to the tadpole stage, and then they would die.