Model Organisms

A model organism is a species that biologists choose to study, not necessarily because it has any inherent medical, agricultural, or economic value, but because it has certain traits that make it easy and convenient to work with. Studying model organisms enables researchers to perform experiments that might be impossible to carry out, due to logistical, financial, or ethical constraints, on organisms of more practical interest, such as humans.

This approach has been tremendously successful in the fields of genetics and molecular and cellular biology because, at their most fundamental levels, biological processes are remarkably similar across species. For example, the genetic code and much of the cellular machinery responsible for replication , transcription , translation , and gene regulation are essentially identical in all eukaryotic organisms. In many cases, genes have even been demonstrated to be functionally interchangeable between humans and baker's yeast.

Among the most commonly studied model organisms are: Escherichia coli (a bacterium), Saccharomyces cerevisiae (baker's yeast), Dictyostelium discoideum (slime mold), Drosophila melanogaster (a fruit fly), Caenorhabditis elegans (a soil roundworm), Brachydanio danio (zebrafish), Xenopus laevis (African clawed frog), Arabidopsis thaliana (a mustard weed), Zea mays (maize, or corn), and Mus musculus (mouse). Others include sea slugs, sea urchins, cyanobacteria, Chlamydomonas (an alga), puffer fish, Tetrahymena (a protozoan), and rats.

Useful Traits and Attributes

Most model organisms share a set of common features that make them amenable to study in the laboratory: They are generally small, easy, and inexpensive to rear in the lab, and reproduce quickly and prodigiously. In addition, the best genetic model organisms have small genome sizes, and many can reproduce sexually, allowing researchers to cross-breed individuals of different genotypes.

Beyond these common traits, most model organisms have one or several unique attributes that make them ideal for a particular line of research. For instance, zebrafish readily produce many large, transparent embryos, and are therefore a favorite research subject for developmental biologists. The roundworm, Caenorhabditis elegans, has a simple but nonetheless sophisticated nervous system, and displays simple behaviors, such as movement, feeding, and mating. These properties make it well suited for neurobiology and behavioral genetics.

The Model Mouse

Besides certain primates (such as monkeys and chimpanzees), which are costly and difficult to rear in the laboratory, the model organism most closely related to humans is the mouse, Mus musculus. The mouse genome is about the same size as the human genome, and the organization of genes (the order of genes on chromosomes) is strikingly similar between the two species. Findings from the nearly complete mouse genome sequencing project indicate that mice and humans share about 95 percent DNA sequence similarity. This means that any gene in humans is likely to have an identical or very similar counterpart (homologue) in the mouse genome.

In addition, it is much easier and less expensive to study genes in mice. A technology that has made the mouse an invaluable genetic model system is the ability to engineer "knockout" strains. These are mutant strains in which a single known gene has been selectively deleted from the genome of every cell. For human genes implicated in diseases, knocking out the homologous gene in mice can provide an excellent model system for studying the disease. The knockout mouse may show disease conditions similar to those of the human disease. Learning how the elimination of the gene in the mouse contributes to the mouse disease may then give important clues about the involvement of the homologous gene in the human disease. "Disease model" mouse strains are available for such disorders as cancer, Alzheimer's disease, arthritis, diabetes, heart disease, cystic fibrosis, and obesity.

see also Arabidopsis thaliana ; Escherichia coli : (E.coli bacterium); Fruit Fly: drosophila ; Maize; Rodent Models; Roundworm: Caenorhabditis elegans ; Transgenic Animals; Yeast; Zebrafish.

Paul J. Muhlra

Bibliography

Alberts, Bruce, et al. Molecular Biology of the Cell, 3rd ed. New York: Garland Publishing, 2002.

Pines, Maya, ed. The Genes We Share with Yeast, Flies, Worms, and Mice. Chevy Chase, MD: Howard Hughes Medical Institute, 2001. (Available from the Howard Hughes Medical Institute Web site: <www.hhmi.org>.)

Watson, James D., et al. Recombinant DNA, 2nd ed. New York: Scientific AmericanBooks, 1992.