Cloning

The cloning is a technique for obtaining an identical copy of a molecule, cell or organism already developed from DNA. Two clones are genetically identical individuals.

Genetic engineering allows the development of three types of cloning:

• Molecular cloning. It allows making copies of DNA through cells called host cells, although we have seen in the previous section another technique, PCR or polymerase chain reaction that is much faster.
  
• Cell cloning. Genetically identical cells can be obtained that can be used to repair diseased tissues without rejection.
• Cloning of whole organisms. Genetically identical individuals are obtained.

The cloning to obtain identical copies, is a natural and common process in organisms asexual reproduction. On the other hand, with sexual reproduction, the individuals generated are different from each other and from their parents. The only exception is that of monozygotic twins, which come from the same egg and the same sperm, which are genetically the same.

There are two types of cloning in animals:

• Reproductive cloning.
• Therapeutic cloning and stem cells.

Reproductive cloning

Basically, cloning consists of obtaining an individual identical to another by extracting the nucleus (2n) from a somatic cell of the original individual and introducing it into the ovum of another female individual of the same species from which the nucleus (n) has been extracted. Thus, the ovum has a nucleus (2n) behaves as if it had been fertilized and divides to form an embryo. This embryo implants itself in the uterus of another female. The embryo will give rise to an animal identical to the donor of the nucleus (2n) that was introduced into the ovum, since it will have the same DNA. This is how the first cloned mammal, Dolly the sheep, was born in 1996.

The cloning of animals can be used to recover endangered or even extinct species, improve livestock breeds or produce animals that generate human proteins or medicines.

Therapeutic cloning and stem cells

Human cloning is not legally allowed, but therapeutic cloning can be used to cure some diseases or in organ transplants. To do this, it is necessary to obtain stem cells, although there is also controversy over whether it is legitimate to obtain them from embryos that are not used in assisted reproduction or whether they should only be extracted from adult tissues and organs.

Stem cells are pluripotent, undifferentiated cells that do not yet have any specific function but can become different cells of the body. When a stem cell divides, the daughter cells can remain stem cells or change into another, more specialized type of cell, such as a red blood cell.

Stem cells can repair tissues damaged by heart attacks, burns, fractures, ..., and it is being investigated for its application in some diseases such as diabetes, Alzheimer's, or leukemia.

Interactive activity: Clone a mouse.