Conducting trials directly on humans is forbidden on humanitarian grounds. Since the human body is very complex direct involvement of humans as subjects of experiments may not always be possible. Therefore it is ideal to use prokaryotes, eukaryotes, and organisms other than humans. These helper organisms are known as model organisms. They contribute to scientific and industrial research. These organisms include bacteria, fungi, viruses, algae, nematodes, worms, and higher animals. Model organisms are selected based on criteria such as their ease of growing, short life cycle, well-defined cellular structures, easily analyzable properties, safe and ethically sound. Using hazardous toxin-producing bacteria or organisms for conducting experiments is forbidden and is accessed only in rare and unavoidable cases. Only trained, well-qualified professionals handle these organisms. Model organisms are involved in genome analysis, recombination experiments and sequence similarity studies. It requires a lot of care and caution to handle these organisms. Recombinant vaccines, peptides, and biomolecules are produced using model organisms. Biosatellites and spaceships use model organisms in their first launch.
1. Yeast:
Yeast is a very useful eukaryote with beneficial properties. Yeast is very easy to culture because it can easily grow on fruits, sugary and salty foods. It can easily proliferate from 120C to 400C. The environment of acidic to alkaline is suitable for its growth. Beer and wine industry widely make use of yeast. Preparation of bakery products also involves yeast. Division of yeast occurs due to a process is known as budding. Yeast cells are easy to study since yeast is a single-celled organism. In addition to studying yeast cells with ease, these cells are comparatively safe to handle.
Most of the experiments in genetic engineering and genome research involve the use of yeast as a model organism. It is possible to study cell cycle, DNA replication, protein and cell wall synthesis using yeast cells. Gene cloning can be achieved using vectors. Yeast artificial chromosomes (YACs) are yeast cloning vectors. Typical features of YACs include telomere (TEL), centromere (CEN), selectable markers on each arm, an origin of replication, and restriction sites or multiple cloning sites. YAC vectors can accommodate DNA fragments of several base pairs. Protein-protein interaction studies make use of the yeast two-hybrid system. Cancer genetic studies also involve yeast as model organisms.
2. Fruit fly:
Most of the genetics experiments involve the use of the fruit fly. It is scientifically known as Drosophila melanogaster. Drosophila is easy to grow since it does not require any specific conditions to grow. It can grow on a wide range of foodstuffs right from a piece of banana to a culture medium. There is a wide range of mutants observed in the fruit fly. Studying its genes is simpler as compared to other organisms. Drosophila is used to study developmental genes, mutant genes, and chromosomes. Morgan was the first scientist to use this model organism. The generation time of fruit fly is rapid as it takes less than two weeks to grow. Transgenic fruit flies are those that consist of foreign gene(s) inserted into their genome.
Drosophila is one such genus of the fruit flies that contains over 900 described species. The genic balance theory was used to identify the concept of sex determination. This theory worked out primarily in Drosophila. According to this theory, sex determination is the ratio of sex chromosomes and polytene chromosomes. A polytene chromosome is a giant chromosome in Drosophila produced by the endomitotic process. It consists of many chromatids. DNA replication and heterochromatin studies are carried out using Drosophila polytene chromosomes. They are also involved in gene mapping and linkage studies. The fruit fly is a model system for understanding human biology and disease. Findings suggest that this model organism has homologs for 177 out of 289 genes involved in human cancers.
Image: Model organisms
3. E.coli:
Escherichia coli are most widely used bacteria in the biotechnology industry. Genetic engineering principles make very high use of this bacteria for cloning and other experiments. The life cycle of E. coli is simple to understand. Hence they are easy to grow and safe to handle. These bacteria consist of extrachromosomal material known as plasmids. Genetic modifications are possible with this model organism. E.coli can be engineered to produce the desired product. For example, gene modification helps in synthesizing insulin. Plasmid cloning vectors include pUC 19 vectors that contain multiple cloning sites. Important applications in model organisms are as follows:
a. Gene cloning experiments: Pure samples of cloned DNA are obtained using plasmid vectors. Some of the plasmids with multiple cloning sites enable different types of restriction enzymes to act on them.
b. Bacteriophage studies: The viruses that infect the bacteria are known as bacteriophages. They follow lytic as well as the lysogenic cycle of infection.
c. Gene mapping: E.coli bacteria undergo a process of conjugation for transferring genes from one bacterium to another. Thus, gene mapping is possible with the help of conjugation experiments.
d. Study of DNA replication: Prokaryotic DNA replication study involves E. coli as a model organism.
e. Identification of mutant phages: E. coli are used to identify mutant phages in site-directed mutagenesis.
f. Bacterial Artificial Chromosomes (BACs): They are useful for cloning DNA fragments in E. coli. BACs consists of an origin of replication, multiple cloning sites, a selectable marker, and other features.
4. Nematode C. elegans:
Caenorhabditis elegans is the widely used nematode in zoology. This nematode exhibits anatomical simplicity. It is ideal to carry out gene diversity and cell cycle studies with this organism. The life cycle of this nematode is just three days. Early embryonic genes are clear to study with this organism. It is capable of learning simple tasks. C elegans is used to study the genetic and molecular aspects of embryonic development, morphogenesis, nerve systems, aging, and behavior.
5. Arabidopsis thaliana:
It is a small plant popular for studying genetic analysis. A complete genome sequence of A. thaliana is available. It is useful to understand processes including nutrient transport and flower development. CRISPR/Cas 9 gene editing also utilizes this plant.
6. Mus musculus:
The gene content in mice is similar to that of humans. The genome sequence of mice is known to us. Mouse models are used to study organ and immune systems. Mice are just like humans in developing diseases such as cancer, diabetes, atherosclerosis, hypertension and Alzheimer disease. Thus mouse models are far better to work. The mice help in studying various gene mutations, biochemical pathways, metabolism, pharmacokinetics, and pharmacodynamics. Thus, they are ideal for clinical trials and research.
7. Neurospora crassa:
It is a haploid fungus widely used in cellular processes. It is possible to grow this strain and use it for tetrad analysis. Typical findings using this model organism involve centromere distance, crossing over and poky mutants. The recessive traits easily show up in the offspring. Thus it is simple to study genetic analysis.
8. Danio rerio:
The common name for Danio rerio is zebrafish. It is a model organism in stem cell biology and developmental genetics. Its embryos are transparent and help in clearly identifying stages of development. It is easy to feed zebrafish by making a genetic cross.
9. Lambda bacteriophage:
It is a phage that infects E. coli cells. It replicates using bacterial machinery. A bacteriophage is an important tool for genetic analysis. It is a model system for studying genetic recombination, complementation, and cloning experiments. The lambda genome is capable of insertion into the bacterial chromosome.
10. Cavia porcellus:
The common name for this model organism is the guinea pig. However, this organism is not a pig but a rodent. Germ theory was established using guinea pigs as model organisms. They are used to study infectious diseases such as cholera, typhus, and brucellosis.
References:
[1] Biotechnology, David P. Clark, Nanette J. Pazdernik
[2] Introduction to Genetic Analysis, Anthony J.F. Griffiths, Susan R. Wessler, Richard C. Lewontin, Sean B. Carroll
[3] Genetic Analysis: Genes, Genomes, and Networks in Eukaryotes, Philip Mark Meneely
[4] Encyclopedia of Genetics, Eric C.R. Reeve
[5] Neurospora: Contributions of a Model Organism, Rowland H. Davis
[6] Model organisms- Wikipedia
[6] Model organisms- Wikipedia
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