Experimental Organisms

Experimental Organisms

For over a century, research scientists have studied experimental, or model, organisms to examine the mechanisms of inheritance and development. In more recent times, researchers have analyzed the molecular biology and biochemistry of experimental organisms to gain knowledge of gene function. Sequencing of various genomes has revealed that many genes in bacteria and multicelled organisms are very similar to the human counterparts. Experimental organisms useful for study because much is known of their behavior and genetic composition. Their ease of handling, short life span (which permits the examination of many generations in a short time), and underlying similarity with human processes permits relatively easy study and manipulation in order to gain insight of various developmental or disease processes. For instance genetics is easier in small organisms that breed quickly than in larger organisms, such as humans. However, various underlying aspects of their biological processes are similar. Thus, information gleaned from study of the experimental organism can be germane to more complex organisms like humans. Similar studies in humans would be technically complex, and ethically repugnant.

Experimental organisms run the gamut from bacteria to mammals. The simplest experimental organism is the bacterium Escherichia coli. E. coli's whose well-known biochemical and structural behaviors make it the primary bacterial resource in labs worldwide. Studies in E. coli culminated in the 1950s with the discovery of DNA as genetic material and continued with the elucidation of the chemical details of replication and transcription. The genome sequence of two prominentE. coli strains, K12 and O157:H7, are now known. The genome of K12, the widely used lab strain, was published in September of 1997. The O157:H7 genome sequence was published in January of 2001. The genomes are very different from each other, with O157:H7 having at least 1400 proteins not produced by K-12. These genetic differences may translate into great differences in the functioning of the two strains. Thus, selection of an E. coli experimental organism may have to be made with care, depending on the experimental question being asked.

A different experimental organism is Saccharomyces cerevisiae. It is yeast, a single celled fungus, commonly known as baker's or budding yeast, in recognition of its important role in bread making. S. cerevisiae has been studied since antiquity. Through an international effort involving over 100 laboratories, the S. cerevisiae genome has also been sequenced, making it the first eukaryotic genome to be completely sequenced. S. cerevisiae is useful in studies of the processes of yeast cell growth and reproduction.

Caenorhabditis elegans is a nematode worm that is hermaphroditic, having male and female states during its life cycle. Sydney Brenner and colleagues developed it as a model organism in the 1960s. It is unique as a model organism in that the developmental pathway of each of its approximately 1,000 constituent cells is known. C. elegans is used primarily to study the genetic regulation of development and neurobiology processes such as learning and aging. It was the first multicellular animal genome to be sequenced.

Dictyostelium discoideum is a multicelled slime mold, which is especially useful for the study of cellular processes important in diseases such as cancer. These processes, fundamentally similar to those occurring in human cells, include movement, response to chemical signals, and cellular development.

A popular insect experimental organism is the fruit fly called Drosophila melanogaster. It has been used as a genetic system since early in the twentieth century, because it lends itself easily to classic breeding experiments. A short lifespan and ease of handling have made D. melanogaster an important tool in studies of developmental biology. Mutants are easily generated, allowing the development of various embryonic and adult structures to be probed in great detail. In the 1980s, researchers began characterizing the genes that corresponded with mutant phenotypes and discovered homeobox genes, which are involved in developmental patterning. Homeoboxes have since been discovered in other species, including invertebrates. In 2000, Craig Venter and colleagues used shotgun sequencing to sequence the Drosophila genome.

Experimental organisms also exist in the study of plants. The popularity of plants like tomato, tobacco and corn has bee supplanted by Arabidopsis thaliana, the mustard plant. It is now the main experimental model plant system for genetics. Sequencing of the A. thaliana genome was completed late in 2000.

Finally, the mouse is the closest model organism to humans. Although the mouse and humans diverge by 75 million years of evolution, their DNA is remarkably similar. Thus, mice are especially useful in studies of development, genetics and immunology. In particular, mice have been very useful in the study of the biology of genetic diseases. Many different mouse models exist, specifically bred and designed to mimic various disease states or immune system malfunctions. Some mutant mice have forms of human diseases, such as diabetes, and can be used as model organisms for such diseases. In the early 1980s researchers began to produce transgenic animals by inserting human genes into fertilized mouse eggs. Recently, the technique of homologous recombination has been used to target human genes into the mouse genome. These approaches have greatly increased the number of human diseases that can be modeled in mice.