What is Genetic Engineering?

A Summary of Commonly-Used Terms
By Michael E. Vayda and John T. Singer, Professors of Biochemistry, Microbiology and Molecular Biology, University of Maine

SUMMARY

Many scientists consider genetic engineering as just "one more tool" in the toolbox of selective "breeding." To understand how scientists use genetic engineering, it helps to know how the process works. This paper outlines the process and defines some of the terms commonly used in the popular press.

Selective Breeding

Selective Breeding is the process of mating selected plants or animals to obtain varieties with desired characteristics such large wholesome fruits, flowers of particular colors, or docile animals with high meat content. Selective breeding can be traced back 10,000 years to the first attempts to domesticate wild plants and animals, but scientific advances of the last century have dramatically enhanced our ability to produce organisms with specific desired characteristics.

Physical characteristics or "traits" are governed by expression of genetic information.

"Genetic information" is encoded in the order of chemical units which make up DNA molecules, similar to the way the meaning of a sentence is encoded in the order of letters which make up words. DNA is called the "hereditary material" because the immensely long DNA molecules (millions- to- billions of chemical units long) passed from parents to offspring contain the instructions to synthesize all of the proteins made by that organism.

A "gene" is a segment of DNA (hundreds-to-thousands of chemical units long) which is the coded information to make a single protein. Proteins are the major structural components of our cells and bodies, and are the principle chemical machinery ("enzymes") which direct chemical reactions. Thus, these proteins, which scientists call "gene products," largely determine the genetic characteristics of an organism. If we consider each gene a sentence, the entire collection of gene sentences comprise the story of a book. Changes in the DNA sequence of a gene can change the structure or action of a protein, just like the changes in the spelling of a word can change the meaning of a sentence. Changes in the genetic information may alter the inherited characteristics of an organism, just as a mistake in a master copy will be present in all printed copies of a book.

Offspring differ from their parents Offspring differ from their parents because they typically are "hybrids" receiving half of their genetic information (DNA) from each parent. Animals or plants with desired characteristics are obtained by mating parents with desired characteristics and selecting those offspring with the highest complement of desired agronomic traits. Such "selective breeding" has resulted in the crop plants and animals common to agriculture. However, since each mating is the mixing of thousands of gene combinations, sorting of "desirable" from "undesirable" traits is a time consuming, imprecise process spanning many generations.

For example, resistance to a specific pathogen might be introduced into a crop plant by breeding with a wild, non-crop relative which expresses such resistance. Some of the offspring will express the resistance trait, but will also express many of the "undesirable" characteristics of the "wild" parent which made it unsuitable as a crop plant. It usually takes 15 generations or more to breed out these "undesirable" traits from a genetic line.

The process of genetic engineering allows a more precise modification of one or a few specific genetic traits without affecting the other desirable characteristics of an organism. The approach is to identify that segment of DNA which encodes the desirable trait, and transfer just that segment without any of the other "genetic baggage" of a typical genetic cross. Genetically modified organisms — commonly referred to as "GMO’s" or "transgenics" — are any plant, animal, yeast or bacterium whose genetic complement has been modified using "recombinant DNA technology" (also known as "genetic engineering").

The process of genetic engineering uses molecular techniques to identify, cut out, and make many copies of that comparatively small segment of DNA (thousands of chemical units long). The DNA segment that codes for the desired trait is linked to a carrier DNA molecule called a "vector" or a "cassette". The result, which has DNA segments originating from two or more sources, is called a "recombinant DNA molecule" or "DNA construct". The additional segments of the recombinant DNA molecule are necessary in order to make copies of this DNA, transfer it to a target animal, plant or bacterium, and identify which target cells have taken up the desired DNA information. The recombinant DNA molecule is transferred to the target cells, which can give rise to an animal or plant which maintains the initial desired characteristics but also expresses the additional trait or traits encoded by the recombinant DNA molecule. The "new" traits can be the increased or decreased expression of proteins commonly found in that species, or the introduction of novel proteins found only in other species. To continue the analogy, genetic engineering is like inserting three or four sentences from Uncle Henry’s into War and Peace; the extra information will provide new details without changing the original story.

"Selectable markers" "Selectable markers" are genes contained in the "recombinant DNA molecule" which allow researchers to distinguish those individual cells that have incorporated the recombinant DNA from those which have not. "Selectable marker" genes typically encode proteins which allow the recipient cells to survive in the presence of an antibiotic, a herbicide or a toxin, or to grow in the absence of an essential nutrient. Since only one out of hundreds of cells take up the recombinant DNA, selectable markers are usually required during the early stages of genetic engineering.

But processes such as "somatic transformation", which require that only a fraction of the cells of organism express the recombinant DNA, do not need selectable markers. Somatic transformation is the basis of "gene replacement therapy" recently proven successful in the treatment of human cystic fibrosis. Somatic transformation is also the basis of novel vaccination strategies. However, somatic transformation only affects the treated individual and is not heritable. The recombinant DNA molecule and the traits it encodes are not passed to subsequent generations. For this reason, this process is also called "transient expression".

For further information, contact:
Michael E. Vayda, Professor
Biochemistry, Microbiology & Molecular Biology
University of Maine
Phone: 581-2821 or E-mail: vayda@maine.maine.edu

or

John T. Singer, Chair,
Department of Biochemistry, Microbiology and Molecular Biology,
University of Maine
Phone: (207) 581-2808 or E-mail: jsinger@maine.maine.edu