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A Glossary of Cloning Definitions

CC, the cat clone.
Photo: College of Veterinary Medicine, Texas A & M University

Clone: An exact genetic copy of another organism. Normal sexual reproduction doesn't produce an exact copy. An unfertilized egg has one set of chromosomes. Sperm delivers the other set during fertilization. So an animal created through sexual reproduction gets a set of chromosomes from each parent, making a unique new individual. A clone gets both sets of chromosomes from one individual, through a process called somatic cell nuclear transfer. See how a clone is made through nuclear transfer.

Chromosomes: Structures composed of very long DNA molecules that carry most of the hereditary information of an organism. Chromosomes are divided into functional units called genes, each of which contains the genetic instructions for making a specific protein. A normal human body cell (somatic) contains 46 chromosomes; a normal human reproductive cell (gamete) contains 23 chromosomes.

DNA: A chemical, deoxyribonucleic acid, found primarily in the nucleus of cells. DNA carries the genetic instructions for making all the structures and materials the body needs to function.

A "somatic" nerve cell.
Photo: NIH

Somatic Cell: Any cell in the body other than an egg or sperm cell.

Germ Cell: A sperm or egg, or a cell that can develop into a sperm or egg; all other body cells are called somatic cells.

Somatic Cell Nuclear Transfer: The main method of cloning. The nucleus, containing two sets of each chromosome, is taken from a somatic cell, such as a skin cell or cheek cell, and inserted into an egg from which the chromosomes have been removed. The process is sometimes referred to as "nuclear transplantation." See how a clone is made through nuclear transfer.

Removing the maternal nucleus during the process of nuclear transfer.
Photo: Roslin Institute

Therapeutic Cloning: Also called research cloning and nuclear transplantation. Cultivating stem cells from a cloned embryo with the intention of curing a disease. For instance, creating new heart muscle cells or insulin-producing cells that could be "injected" back into a person and replace damaged cells or tissue. The embryo is never implanted into a woman, and it never develops beyond a few hundred cells.

Blastocyst: A few days after a mammal egg is fertilized or, in the case of cloning, chemically activated, it forms a blastocyst, a tiny structure shaped like a hollow ball. The ball is made up of about 30 to 150 cells, and consists of an outer layer of cells, a fluid-filled cavity, and cluster of cells on the inside, called the inner cell mass (ICM). The ICM cells can develop into embryonic stem cells if grown in culture. If the blastocyst is implanted into a uterus, the ICM cells give rise to a fetus.

A blastocyst.
Graphic: FASEB

Stem cells: Cells that have the ability to renew themselves (divide for an indefinite period of time) and to give rise to many different types of specialized cells that make up an organism, like a nerve cell or heart cell.

Differentiated cell: A cell that has developed into a specialized cell, such as a heart, liver or muscle cell.

Undifferentiated cell: A cell that hasn't developed into a specialized cell or tissue type.

Adult Stem Cell: Undifferentiated cells found in tissue other than the egg, sperm or early embryos. These cells can renew themselves and can also turn into specialized cell types as needed by the tissue where they're found.

Embryonic Stem Cell: Undifferentiated cells from the embryo that have the potential to become a wide variety of specialized cell types. They are derived from the inner cell mass of the blastocyst. Embryonic stem cells are not embryos; by themselves, they cannot produce the necessary cell types in an organized fashion so as to give rise to a complete organism. Embryonic stem cells can also be derived through nuclear transfer.

Sources: NIH, The National Academies, and the National Bioethics Advisory Council