Biochemical Techniques and Instrumentation

6.2.3.3.2. Reproductive cloning

Reproductive cloning is used for the creation of an animal that has the same DNA as another animal. It has been successfully used to clone Dolly, the sheep. In this type of cloning, the DNA from an ovum is removed and replaced with the DNA from a somatic cell removed from an adult animal. The fertilized ovum or pre-embryo, is then implanted in a uterus and allowed to develop into a new animal. This technique is known as Somatic Cell Nuclear Transfer (SCNT). In these techniques, the resulting offspring will be genetically identical to the donor and not the surrogate, unless the donated nucleus is taken from a somatic cell of the surrogate.

There are two variations in this technique:

1. Roslin technique
2. Honolulu technique

SCNT refers to the transfer of the nucleus from a somatic cell to an egg cell. A somatic cell is any cell in the body other than the two types of reproductive germ cells ie. sperm and egg. Somatic cell could be a blood cell, heart cell, skin cell, etc. In mammals, every somatic cell has two complete sets of chromosomes, whereas the reproductive germ cells have only one complete set of chromosomes. The nucleus in a cell is an enclosed compartment that contains all information in the form of DNA that each cell needs to form an organism.

In SCNT process, the egg from a female donor is first taken and its nucleus is removed to create an enucleated egg. A somatic cell which contains DNA is taken from the person to be cloned. Then, the enucleated egg is fused together with the somatic cell. The embryo thus created begins to divide normally. It is then implanted into the uterus of the surrogate mother. The clone produced will be an exact genetic replica of the adult that donated the somatic cell nucleus to the egg. The difference between fertilization and SCNT lies in origin of two sets of chromosomes. In fertilization, the sperm and egg both contain one set of chromosomes; and the resulting zygote ends up with two sets - one from the father (sperm) and one from the mother (egg). In SCNT, the egg cell's single set of chromosomes is removed and replaced by the nucleus from a somatic cell, which already contains two complete sets of chromosomes.