Теория и практика перевода. Терехова Г.В. - 84 стр.

characteristics—that is, in their genes —except for any differences caused by
mutation. Identical twins, for example, who originate from the division of a single
fertilized egg, are members of a clone; whereas nonidentical twins, derived from two
separate fertilized eggs, are not clones. Besides the organisms known as prokaryotes
(the bacteria and cyan bacteria), a number of other simple organisms, such as most
protozoans, many other algae, and some yeasts, also reproduce primarily by cloning,
as do certain higher organisms like the dandelion or aspen tree.
         Through recent advances in genetic engineering, scientists can isolate an
individual gene (or group of genes) from one organism and grow it in another
organism belonging to a different species.
          The species chosen as a recipient is usually one that can reproduce asexually,
such as a bacterium or yeast. Thus it is able to produce a clone of organisms, or cells,
that all contain the same foreign gene or genes. Because bacteria, yeasts, and other
cultured cells multiply rapidly, these methods make possible the production of many
copies of a particular gene. The copies can then be isolated and used for study (for
example, to investigate the chemical nature and structure of the gene) or for medical
and commercial purposes (for example, to make large quantities of a useful gene
product such as insulin, interferon, and growth hormone). This technique is called
cloning because it uses clones of organisms or cells. It has great economic and
medical potential and is the subject of active research.
         Identical-twin animals may be produced by cloning as well. An embryo in the
early stage of development is removed from the uterus and split, and then each
separate part is placed in a surrogate uterus. Mammals such as mice and sheep have
been produced by this method, which is generally called embryo splitting.
         Another development has been the discovery that a whole nucleus, containing
an entire set of chromosomes, can be taken from a cell and injected into a fertilized
egg whose own nucleus has been removed. The division of the egg brings about the
division of the nucleus, and the descendant nuclei can, in turn, be injected into eggs.
After several such transfers, the nuclei may be capable of directing the development
of the eggs into complete new organisms genetically identical to the organism from
which the original nucleus was taken. This cloning technique is in theory capable of
producing large numbers of genetically identical individuals. Experiments using this
technique have been successfully carried out with frogs and mice.
         Continue article...
         Progress in cloning higher mammals beyond an early embryonic stage
presents a much more formidable challenge. Genes in cells at the earliest stages of
embryonic life carry the encoded knowledge that enables cells to develop into any
part of the body. But skeptics theorized that once cells form into specific body
components, they thereafter lose the capability to reconstruct the entire organism
from the genetic contents of the nucleus.
         However, in July 1996, a team of Scottish scientists produced the first live
birth of a healthy sheep cloned from an adult mammal. The team scraped skin cells
from the udder of a donor sheep (sheep A) and these cells were temporarily starved of
nutrients to halt cell development. An unfertilized egg was removed from a second
sheep (sheep B) and its nuclear material was removed to eliminate genetic