Animal cloning

Background

Cloning has been going on in the natural world for thousands of years. A clone is simply one living thing made from another, leading to two organisms with the same set of genes. In that sense, identical twins are clones, because they have identical DNA. Sometimes, plants are self-pollinated, producing seeds and eventually more plants with the same genetic code. Some forests are made entirely of trees originating from one single plant; the original tree spread its roots, which later sprouted new trees. When earthworms are cut in half, they regenerate the missing parts of their bodies, leading to two worms with the same set of genes. However, the ability to intentionally create a clone in the animal kingdom by working on the cellular level is a very recent development.

Introduction

The first cloned animals were created by Hans Dreisch in the late 1800's. Dreich's original goal was not to create identical animals, but to prove that genetic material is not lost during cell division. Dreich's experiments involved sea urchins, which he picked because they have large embryo cells, and grow independently of their mothers. Dreich took a 2 celled embryo of a sea urchin and shook it in a beaker full of sea water until the two cells separated. Each grew independently, and formed a separate, whole sea urchin. In 1902, another scientist, embryologist Hans Spemman, used a hair from his infant son as a knife to separate a 2-celled embryo of a salamander, which also grow externally. He later separated a single cell from a 16-celled embryo. In these experiments, both the large and the small embryos developed into identical adult salamanders. Spemman went on to propose what he called a "fantastical experiment" -- to remove the genetic material from an adult cell, and use it to grow another adult. In this way, he theorized, he would be able to prove that no genetic material was lost as cells grew and divided.

Discussion

Animal clones are genetically identical. Natural clones occur in the form of identical twins but it is also possible to produce artificial clones by nuclear transfer. The nucleus is removed from a somatic (body) cell and placed in an egg whose own nucleus has been removed. The egg is then implanted in a surrogate mother and develops to term.

Key principles

Differentiated animal cells are unable to develop into complete animals but the nuclei of most differentiated cells retain all the necessary genetic information.

It is possible to transfer such a nucleus into an egg whose own nucleus has been removed.

Transfer to the environment of the egg reprograms the nucleus (makes it forget its history) and allows the full development of a viable animal that is genetically identical to the donor of the somatic cell.

Until 1997, cloning in mammals was only possible using nuclei obtained from very early embryos. A breakthrough was made when cloning was achieved using nuclei from adult cells.

Recent research suggests that animals produced by cloning from adult cells may age prematurely, but ...