In DNA Cloning (a.k.a. "recombinant DNA technology," "molecular
cloning,"or "gene cloning") only the DNA of a cell
is replicated. A DNA from an organism is transferred to a self-replicating
genetic element such as a bacterial plasmid.
In other words, a small piece of the DNA strand is removed and united
with a plasmid which reproduces itself to create multiple copies of
the same DNA code. This plasmid is also known as a vector.
This copied DNA can then be propagated in a foreign host cell. After
it is introduced into a suitable host cell, the recombinant vector
can then be reproduced along with the host cell DNA
Reproductive cloning is a technology used to generate an animal that
has the same nuclear DNA as another currently or previously existing
animal. Human cloning also falls into this category. Dolly was created
by reproductive cloning technology. In a process called "somatic
cell nuclear transfer" (SCNT), the DNA information from the nucleus
of a donor adult cell is copied into a cell whose nucleus (thus also
its genetic material) has been removed. Chemicals or electric current
are used to stimulate cell division. Once the cells start dividing
and the embryo reaches a suitable stage, it is planted into the uterus
of a female host where it develops until birth.
There is one important fact to know about reproductive cloning. Even
though the DNA from the nucleus in the cloned cells is identical to
the original cells, the whole animal (or human) is not identical.
This is because some DNA is stored in the mitochondria of the cell
which is not cloned and is thus unique. The DNA in mitochondria is
believed to play an important role in the aging process.
Therapeutic cloning is like reproductive cloning, except that the
embryos are not allowed to develop fully. The purpose of therapeutic
cloning is to extract the stem cells from the embryos and study them.
When the egg has been cloned and divided for 5 days, the stem cells
are extracted from it. The embryos are destroyed due to the extraction
process, which raises ethical concerns.
But why the need for stem cells? The answer lies in the composition
of these cells. Stem cells are unspecialized cells
which can transform into any of the 220 cell types that are in the
human body. Many researchers hope that one day stem cells can be used
to serve as replacement cells to treat heart disease, Alzheimer's,
cancer, and various other diseases.
In November 2001, scientists from Advanced Cell Technologies (ACT),
a biotechnology company in Massachusetts, announced that they had
cloned the first human embryos for the purpose of advancing therapeutic
research. They had taken cells from a woman's ovaries and removed
their nucleus. Skin cells were then inserted into the ovary cells
to serve as the new nucleus. The cells began dividing, but the results
were limited in success. The process was carried out with eight eggs
and only one of them successfully divided into six cells before stopping.