Cloning

The Science and Technology of Cloning

The term cloning was first introduced at the beginning of the 20th century. Its root is the ancient Greek term κλων, meaning “twig” and referring to the process whereby a new plant can be created from a twig. A basic characteristic of cloning is that sexual reproduction is bypassed. The production of cloned DNA fragments, cells, or embryos is an important technique in the toolbox of molecular biotechnology. Cloning requires knowledge of embryology, genetics, and reproductive technology. It complements two other key technologies, genetic engineering (the transfer of genetic material from one organism to another) and genomics (the mapping, sequencing, and functional analysis of the entire genetic makeup of an organism).

Scientists today apply a variety of cloning strategies. Every biology student clones genes by transferring and amplifying DNA fragments in unicellular organisms such as bacteria. This strategy generates multiple copies of a DNA fragment and can be used to introduce a gene into the genetic makeup of a different species.

Cloning a cell or an embryo, achieved by splitting embryos, is more demanding. Basic research in animal embryology dates back as far as the late 19th century, when the German embryologist Hans Driech produced multiple individuals from split sea urchin embryos. In the early 20th century, Hans Spemann successfully manipulated salamander eggs to remove the cell nucleus, the part that contains most of the genetic material in a cell. The eggs were then renucleated with the nucleus from an early embryo. Spemann was replacing the egg cell nucleus with another embryonic one but was already thinking ahead to using the nucleus of a more specialized cell. Such experiments were realized in the 1950s when U.S. embryologists Robert Briggs and Thomas King transferred the nuclei of frog cells in different phases of tadpole development into frog eggs.

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