Model organismorganism is one that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. This works because evolution reuses fundamental biological principles and conserves metabolic, regulatory, and developmental pathways.
There are many model organisms. The first model organism for molecular biology was probably the bacterium Escherichia coli which is common in the human digestive system (and usually beneficial -- the dangerous is a rare strain). This also led to a study of many bacteriophages, particularly phage lambda.
In eukaryotes, several yeasts, particularly Saccharomyces cerevisiae ("baker's" or "budding" yeast), have been widely studied, largely because they are quick and easy to grow. The cell cycle in a simple yeast is very similar to the cell cycle in humans, and regulated by homologous proteins. The fruit fly Drosophila melanogaster was studied, again because it was easy to grow for a multicellular organism. The roundworm Caenorhabditis elegans is studied because it has very stereotyped development patterns and can be rapidly assayed for abnormalities.
When researchers look for an organism to use in their studies, they look for several traits. Common among these are size, lifespan, accessibility, manipulation, genetics, conservation of mechanisms, and potential economic benefit. As comparative molecular biology has become more common, some researchers have sought model organisms that represent assorted lineages of life.
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Important model organisms