Cloning

I

Introduction

Cloning, process of creating an exact copy of a single gene, cell, or organism. The copies produced through cloning have identical genetic makeup and are known as clones. Many organisms in nature reproduce by cloning. Scientists use cloning techniques in the laboratory to create copies of cells or organisms with valuable traits. Their work aims to find practical applications for cloning that will produce advances in medicine, biological research, and industry. Gene cloning, for example, is often used to study human disease.

II

Overview

Farmers started cloning plants thousands of years ago in simple ways, such as taking a cutting of a plant and letting it root to make another plant. Early farmers also devised breeding techniques to reproduce plants with such characteristics as faster growth, larger seeds, or sweeter fruits. They combined these breeding techniques with cloning to produce many plants with desired traits. These early forms of cloning and breeding were slow and sometimes unpredictable. By the late 20th century scientists developed genetic engineering, in which they manipulate deoxyribonucleic acid (DNA), the genetic material of living things, to more precisely modify a plant’s genes. Scientists combine genetic engineering with cloning to quickly and inexpensively produce thousands of plants with a desired characteristic.

Cloning techniques can also be applied to animals. Scientists generate genetically modified animals with new traits, such as the ability to resist disease, and they use cloning techniques to reproduce these genetically modified animals. In the near future scientists hope to bolster populations of endangered species by cloning members from existing populations. Someday scientists may even resurrect extinct species by cloning cells from preserved specimens.

Industry also utilizes cloning technology. For example, some bacteria eat toxic substances, such as gasoline or industrial chemicals, that are common pollutants. These bacteria can be cloned to make legions of bacteria with the ability to clean up environmental contamination (see Bioremediation). Likewise, cloned animals can be used to make a variety of ingredients, such as proteins, that are used in many commercial products.

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Perhaps most important from a human perspective, cloning promises great advances in medicine. Scientists have already inserted fragments of DNA containing the human gene for a blood-clotting protein into cells of a sheep. Through cloning techniques, scientists have generated new sheep whose milk contains the protein, which is needed by people with the blood-clotting disorder known as hemophilia. In the near future, researchers hope to use cloning to develop animals with human diseases and use these cloned animals to test the safety and effectiveness of new treatments devised for humans. Biomedical scientists hope to take cells from an ill patient, genetically modify them, and clone the modified cells to grow exactly the cells that the patient needs to regain health. Some scientists even imagine a day when cloning could be part of a process that grows entire organs for transplants.

Despite the current and potential benefits of cloning, the process fuels a fiery battle. Little controversy ever surrounded plant cloning. In fact, few people even think of making plants from cuttings as cloning at all, but it is. Many people fight against the creation and cloning of genetically modified plants. But that worry generally involves the manipulation of the plant’s DNA, not the cloning process.

Animal cloning, on the other hand, stirs heartfelt controversy. Critics argue that the science of cloning is in its infancy and, in order to achieve success, mistakes may be made along the way. This could result in the development of cloned animals or humans with serious defects. Opponents to human cloning argue that without proper regulation, cloning could result in such questionable practices as designing babies with chosen genetic qualities so that they are more athletic, beautiful, or intelligent. Others fear that cloning tampers with God’s will. As a result of so much controversy, the future of cloning remains uncertain.

III

Cloning in Nature

Despite all of the modern concerns over cloning in laboratories, cloning started in nature. Many organisms replicate themselves through the process of asexual reproduction. Genetic information is encoded and transmitted from generation to generation in DNA, a coiled molecule organized into structures called chromosomes within cells. Segments along the length of a DNA molecule form genes. In asexual reproduction, an organism’s DNA copies itself, and a new body grows around one copy of the DNA to form an offspring that is genetically identical to its parent.

Organisms composed of just one cell, such as bacteria, reproduce through a type of asexual reproduction called fission. During fission, a cell duplicates its DNA to form two complete sets of DNA. This parent cell then divides to form two daughter cells. Each daughter cell receives one set of DNA. The two newly formed daughter cells are genetically identical to each other and to the parent cell. In another type of asexual reproduction known as budding, used by yeasts and some multicellular animals such as hydra, a little bump grows on the parent cell. The parent’s DNA duplicates and a copy goes into the bump. The bump grows and eventually splits off as a clone of the parent.

Many plants—including strawberries and some grasses—clone themselves by producing runners, which are stems that grow on top of the ground. A runner forms roots at any place that it hits the ground. The roots form a new plant that is genetically identical to the original. Other plants—including ferns, irises, and some grasses and trees—produce underground stems called rhizomes that generate new plants genetically identical to the parent plant.

Some animals, including aphids, brine shrimp, and some species of fish, frogs, and lizards, reproduce asexually through a process called parthenogenesis—a word that derives from the Greek words parthenos (“virgin”) and genesis (“birth”). In parthenogenesis a female’s egg develops without fertilization from male sperm.

Sometimes mammals also produce clones, but unlike the clones of other organisms the resulting offspring arise from sexual reproduction, in which a father’s sperm fertilizes a mother’s egg. In such cases, a mammal’s fertilized egg divides in the womb and forms two or more embryos. These offspring are clones of each other, sharing exactly the same genes. They are not clones of the mother or father, however, since the offspring only have half of their genes in common with either parent.

IV

How Scientists Clone Cells

Scientists initially made cloned cells in the laboratory by letting a single cell divide into a population of genetically identical cells. In this process scientists put the original cell in a laboratory dish containing culture medium (nutrients needed to keep a cell alive). The cell’s natural process of mitosis (cell division) then produces genetically identical offspring. This process mimics how cells multiply, for instance, in plants and in the human body.

Scientists later developed more complex cloning techniques using animal embryos. Every cell in an animal arises from a fertilized egg. The fertilized egg divides to form an embryo, and each cell in the embryo has the same genetic makeup. At some point in the embryo’s growth and development, cells differentiate and become specialized. For instance, a heart cell only functions in the heart and not the liver, even though the genes of a heart cell and liver cell are the same.

In the 1950s scientists began to experiment with embryo cells that were undifferentiated—that is, they had not yet specialized into a particular type of cell. Scientists found that such embryo cells are totipotent (able to give rise to all the different cell types in the body). Exploiting this characteristic, scientists developed three techniques to clone embryo cells: blastomere separation, blastocyst division, and somatic cell nuclear transfer.

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