In 1929, the famous physiologist and Nobel Laureate August Krogh remarked, “ For many problems, there is an animal on which it can be most conveniently studied”. In fact, many key discoveries in the field of Biology have been built upon the strong foundations laid out by the insights from studies made on such organisms. Ultimately the data and theories generated in them were applicable to other, more complex organisms giving them the apt title; Model Organisms.

From the tiny fruit fly aiding TH Morgan in his quest to understand the role of chromosomes in genetics to the Giant squid axon, which single-handedly established the biophysics of how our neurons communicate, the use of model organisms has only expanded and is continuing to shape numerous other discoveries to this day.

Cells are the basic unit of all life forms. The genetic material within each cell carries all the information specific to that organism. At the molecular level, all living forms on the earth use the same building blocks to store this information, DNA.

The DNA is composed of nucleotides A, T, G, and C; what differs from organism to organism is the order in which these nucleic acids are arranged in the DNA sequence of each organism. The whole DNA sequence in one organism is termed its genome, and subsets of the genome are genes. According to the central dogma of molecular biology, these genes are transcribed to RNA which is then translated into proteins.

At the cellular level, proteins perform most of the work - maintaining cell shape, cell mobility, etc. Many of these proteins are conserved across different species of organisms. These proteins are termed homologous proteins. For example, actin, a cytoskeletal protein important to maintain cell shape, shares ~85% of its sequence in even the most diverse organisms. The structure of actin in different organisms is also roughly the same. On account of the sequence and structure similarity, mutational analysis done on budding yeast (Saccharomyces cerevisiae) actin helped in the identification of disease-causing mutants in human actin.

Hence, in essence, despite the diversity observed at the organismal level, the similarity at the molecular level makes it possible to assume that the data and theories generated from the research on model organisms will be applicable to other 

Imagine you are a researcher and want to identify the function of a protein in humans, let's say - actin. Usually, the way it's done is - you knock down the gene of interest (actin) in the cell and observe the cell under the microscope to see the effect. Doing such experiments in live humans is out of the question as it's unethical as well as an uncontrolled experiment. To have control over the experimental conditions, researchers choose single cells for such experiments. So, hypothetically, if you choose a human cell, it has about 20 copies of actin genes in the genome. Which one do we delete? As opposed to this, if you choose fission yeast, a well-known eukaryotic model organism, there is only one actin gene. 

Such redundancy in terms of genes exists for a lot of proteins in complex organisms making it difficult to study them. But, model organisms are simpler in the sense that they have less redundancy in their genome. This simplicity of the model organisms allows them to be genetically tractable. 

Let's take an example of drug development. After the initial screening of the compounds, the chosen drug candidates are tested on animal models such as rats, mice, zebrafish, etc. before the clinical trials (Clinical trials involve human participants). These model organisms have shorter generation times and produce a large number of progeny as compared to humans. Both of these are important to take into consideration as they allow for a large sample size in the experiment. Adequate sample size is important as it directly affects our ability to reach an accurate conclusion, in this case, whether the drug should be tested in humans or not.