How is cloning done?
You may have first heard of cloning when Dolly the Sheep showed up on the scene in 1997. Cloning technologies have been around for much longer than Dolly, though.
How does one go about making an exact genetic copy of an organism? There are a couple of ways to do this: artificial embryo twinning and somatic cell nuclear transfer. How do these processes differ?
1. Artificial Embryo Twinning
Artificial embryo twinning is the relatively low-tech version of cloning. As the name suggests, this technology mimics the natural process of creating identical twins.
In nature, twins occur just after fertilization of an egg cell by a sperm cell. In rare cases, when the resulting fertilized egg, called a zygote, tries to divide into a two-celled embryo, the two cells separate. Each cell continues dividing on its own, ultimately developing into a separate individual within the mother. Since the two cells came from the same zygote, the resulting individuals are genetically identical.
Artificial embryo twinning uses the same approach, but it occurs in a Petri dish instead of in the mother's body. This is accomplished by manually separating a very early embryo into individual cells, and then allowing each cell to divide and develop on its own. The resulting embryos are placed into a surrogate mother, where they are carried to term and delivered. Again, since all the embryos came from the same zygote, they are genetically identical.
2. Somatic Cell Nuclear Transfer
Somatic cell nuclear transfer, (SCNT) uses a different approach than artificial embryo twinning, but it produces the same result: an exact clone, or genetic copy, of an individual. This was the method used to create Dolly the Sheep.
What does SCNT mean? Let's take it apart:
Somatic cell: A somatic cell is any cell in the body other than the two types of reproductive cells, sperm and egg. Sperm and egg are also called germ cells. In mammals, every somatic cell has two complete sets of chromosomes, whereas the germ cells only have one complete set.
Nuclear: The nucleus is like the cell's brain. It's an enclosed compartment that contains all the information that cells need to form an organism. This information comes in the form of DNA. It's the differences in our DNA that make each of us unique.
Transfer: Moving an object from one place to another.
To make Dolly, researchers isolated a somatic cell from an adult female sheep. Next, they transferred the nucleus from that cell to an egg cell from which the nucleus had been removed. After a couple of chemical tweaks, the egg cell, with its new nucleus, was behaving just like a freshly fertilized zygote. It developed into an embryo, which was implanted into a surrogate mother and carried to term.
The lamb, Dolly, was an exact genetic replica of the adult female sheep that donated the somatic cell nucleus to the egg. She was the first-ever mammal to be cloned from an adult somatic cell.