The organisms selected for genome, projects were those that are mostly used in genetic and other scientific investigations. Thus, they may be regarded as model organisms. A model organism is an organism about which is a large amount of scientific knowledge is already available. These organisms include both prokaryotic and eukaryotic microorganisms as well as animals and plants. These organisms include E. coli, Bacillus subtilis, Archaeoglobus fulgidus, yeast {Saccharomyces cerevisiae), Arabidopsis thaliana, roundworm (Caenorhabditis elegans) and the fruitfly (Drosophila melanogaster). In addition, human genome project focussed on sequencing of the whole human genome. 

E. coli is without any doubt, the best studied microorganism. Many of the technological tools available today were developed for E. coli genome project. The E. coli genome sequencing was completed in 1997. B. subtislis is a Gram-positive bacterium that colonizes leaf surfaces. 

It is much used in industrial processes for both enzyme production and food supply fermentation. It is generally regarded as safe (GRAS). 

S. cerevisiae is perhaps the most important fungal species used in biotechnological processes. Its name derives from the fact that it can ferment saccharose (sugar). S. cerevisiae genome sequencing project began in 1989, and the sequencing was completed in 1999. Arabidopsis thaliana is a small plant belonging to the Cruciferae
family. It was recognized as a good organism for genetic studies in 1907, and has become a model plant for genetic studies. 

Since it has the smallest genome among higher plants. 

Further, its genome has a low amount of repetitive DNA. Arabidopsis genome sequencing project began in 1990 and was completed in 2000. The fruitfly, D. melanogaster, is fondly regarded the Queen of Genetics. Drosophila genome sequencing was completed in 2000. It has 180 x 106 bp and -16,000 genes. 

Thus, the number of genes in Drosophila is less than four-times as many as in the bacterium E.coli.