Cloning is the production of an organism with genetic material identical to that of another organism. Genetic material carries information concerning the physical and behavioral characteristics that living things inherit from their parents. The basic unit of genetic material is the gene. Genetic instructions are carried inside living cells in the structure of a molecule called DNA (deoxyribonucleic acid). Organisms that share an identical genetic makeup are called clones.
People often think of cloning as the use of laboratory techniques to create living copies of animals and other organisms. However, cloning also occurs widely in nature. In addition, scientists sometimes use the term cloning to mean the duplication of cells and genes in the laboratory.
Cloning animals
Complex animals, such as vertebrates (animals with backbones), consist of billions of cells. Nearly all these cells have a nucleus. The nucleus contains genetic information in the form of DNA. The information is the same for each nucleus in the animal. For example, the nucleus of a skin cell holds exactly the same genetic information as that of a cell in the liver. However, each cell uses only the part of the information necessary to its function. The rest of the DNA remains inactive. Despite this inactivity, the nucleus of any cell retains the information necessary to create the entire organism. This fact enables scientists to clone an entire animal from a single cell.
Nuclear transfer
is a technique that scientists use to clone animals. In this process, scientists remove the nucleus from a cell of the animal to be cloned. This donor nucleus may come from a variety of cells in the animal. Scientists inject the donor nucleus into an egg cell from the same species whose own nucleus has been destroyed. The egg cell, with its new nucleus, has the same genetic makeup as the donor animal. Nuclear transfer differs from sexual reproduction. In sexual reproduction, a fertilized egg receives half its DNA from a sperm cell and the other half from the egg itself.
After the egg cell has been injected with a new nucleus, the cell must be activated to develop into an embryo. An embryo is a mass of cells that can grow into a fully formed organism. In sexual reproduction, enzymes carried by the sperm cell activate the combined sperm and egg. Enzymes are molecules produced by living things that speed up chemical reactions. With nuclear transfer, scientists must artificially activate the egg cell. They usually do this by administering a small electric shock to the egg.
Next, the DNA of the donor nucleus must be reprogrammed by the egg. Reprogramming activates parts of the DNA not active in the parent cell. This enables the cell to develop into an entire organism rather than a mere copy of the donor cell. Scientists do not fully understand how the egg does such reprogramming. Furthermore, reprogramming techniques have a high rate of failure. They often result in the death of the cloned embryo. Scientists may place a cloned embryo into the womb of a surrogate (substitute) mother of the same species, who will carry the clone until birth.
Successes.
Scientists used nuclear transfer to clone such amphibians as frogs and salamanders as early as the 1950’s. In 1996, a group led by the British scientist Ian Wilmut used the procedure to clone a sheep. The sheep was the first mammal cloned from a donor cell from an adult mammal. They named the clone “Dolly.” Since the cloning of Dolly, scientists from many countries have used a similar technique to produce clones of mice, cattle, cats, and other mammals.
Further successes in cloning technology may yield important benefits to people. The cloning of such animals as cattle or sheep could enable scientists to create livestock with desirable genetic traits. Such livestock might produce higher quality meat, milk, and wool.
Differences among clones.
Even though they are genetically identical, cloned animals are usually not identical in physical appearance and behavior to the donor animal. To understand why, researchers have often studied identical twins. Identical twins are genetically exactly alike. Studies of these natural clones have shown that identical twins raised apart tend to vary more in their characteristics than identical twins raised together. This evidence shows that both genetic makeup and environment play important roles in an individual’s development. Because it is impossible to raise a cloned animal in exactly the same conditions as the donor, a clone usually appears and acts slightly different from the donor.
Another source of variation between clone and donor is the presence of a small amount of DNA in the mitochondria of the original egg cell. Mitochondria are small, rod-shaped structures found outside the nucleus that produce energy for the cell. Clones get their mitochondria—and the DNA inside them—from the egg cell rather than the donor nucleus. Some variation among clones may result from this mitochondrial DNA.
Further variation among clones may result from epigenetics. Epigenetics are changes in gene function that occur without any corresponding change in the DNA sequence. Examples of epigenetic variation include differences in color pattern in the hair of cloned mammals, such as cattle or cats. Complex factors during fetal development determine coat pattern in these animals. Genes that affect hair color are activated or inactivated in different cells.
Ethical concerns.
Experiments with cloning and cloned animals have provided scientists with a great deal of information on biological processes. This information can be used to develop treatments for a wide variety of diseases and conditions, including cancer and birth defects. However, many people worry that these procedures are not safe and may be used unethically to reproduce people.
Scientists have not perfected techniques for cloning mammals. They have found it difficult to consistently produce healthy clones. Most cloned embryos do not survive to reach birth. More troubling is that cloned animals have a much higher incidence of birth defects than those produced naturally. Some cloned animals suffer developmental problems and die soon after birth.
A few scientists have cloned human embryos to obtain stem cells. Stem cells that can develop into many different types of cells (see Stem cell ). This kind of cloning is known as therapeutic cloning. Doctors might be able to use stem cells cloned from a patient to replace damaged tissues and treat diseases. Many scientists think the use of therapeutic cloning to make body tissues for transplant—such as skin, liver, and pancreas tissue—could revolutionize medicine. However, therapeutic cloning remains controversial because human embryos are destroyed in the process of collecting the stem cells. Many people feel that destroying an embryo that has the potential to develop into a human being is unethical or against the principles of their religion.
Scientists soon may be able to use cloning techniques to reproduce human beings. However, virtually all cloning researchers reject the use of cloning to reproduce people. Most of them agree that the high incidence of abnormalities in cloned animals, along with other concerns, make it unethical to experiment with reproductive cloning in human beings. Many countries have enacted bans or other limits to discourage research that produces cloned human embryos for any purpose. In the United States, for example, the National Institutes of Health forbids laboratories that receive federal funding from conducting cloning research using human cells.
Clones in nature
Many examples of clones exist in nature. In human beings and other mammals, clones form naturally when identical twins or other genetically identical multiple births occur. Single-celled organisms, including bacteria, protozoa, and yeast, produce genetically identical offspring through asexual reproduction. These offspring develop from only one parent and thus are considered clones.
Plants can also reproduce asexually through a process called vegetative propagation. In this process, a piece of root or stem can generate a new plant that is genetically identical to the donor plant. Vegetative propagation helps people obtain plant clones with desired traits. For example, farmers and plant breeders use this technique to develop apples with unique flavors or roses of certain colors. Many crops, including nearly all potatoes, are produced in this way.
Gene and cell cloning
Scientists also use cloning techniques to reproduce specific genes or cells rather than entire organisms. Researchers may clone a single cell, such as a cancer cell, into an entire colony of cells that can be studied or used in experiments. Cloning individual genes commonly involves the use of plasmid DNA. Plasmid DNA occurs in certain bacteria and can be reproduced easily. Scientists first isolate a DNA fragment that contains a particular gene from a donor. They then combine that fragment with the plasmid DNA to form recombinant DNA. By implanting this DNA in bacteria, scientists can generate large quantities of the desired gene. Cell cloning and gene cloning have revolutionized the fields of biology and medicine, helping scientists acquire a better understanding of the structure and function of cells and genes.
