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Most model organisms are small, easy to care for and inexpensive to keep. Size is a limiting factor when considering an organism for study in a laboratory. Most research is done on a considerable sample size and if the organism is much larger than a mouse, it will require a large amount of space to house them. Also, a model organism must be easy and inexpensive to care for. Time and money are finite resources that can evaporate quickly if the organism of study requires extensive care and a special diet.
A2. Model organisms reproduce quickly, many by sexual reproduction, with high fecundity and have a short life cycle. Fast reproduction allows genetic studies to cover many generations in a short period of time. It also replenishes the inventory of study organisms rapidly. Sexual reproduction allows for crossbreeding different genotypes. High fecundity creates both a large sample size and great variations in genotypes due to sexual reproduction. Quick development allows researches to rapidly observe phenotype expression in differing stages of an organism's life cycle.
A3. Model organisms have a thorough catalogue of biological data; most with their genomes sequenced. Due to the common genetic material and the preservation through evolution of similar developmental and metabolic pathways in all living organisms, model organisms can be used to study other organisms that are not amenable to experimentation. Most model organisms have small genomes, little junk DNA and are genetically easy to manipulate and induce mutation.
B1. Escherichia coli is a rod-shaped Gram negative bacterium. It is a facultative anaerobic prokaryote with a simple genome which has been sequenced and is cheap and easy to cultivate on a variety of substrates. It is approximately 2 μm in size, has a generation time of 30 minutes and has an easily manipulated genome. The use of plasmids to create recombinant DNA and inserting them into E. coli for mass production of proteins, including human insulin, is the underpinning of biotechnology. E. coli has been fundamental in the study of phage genetics and bacterial conjugation, can express and store foreign DNA and has many genetic mutations available.
B2. Schizosaccharomyces pombe is a unicellular eukaryotic rod-shaped yeast with 3 chromosomes and approximately 5,000 protein coding genes, whose genome has been sequenced. It is cheap and easy to grow, about 10 μm in length, has a generation time of 2 hours and can be genetically crossed due to its two sexes. S. pombe produces two equal sized daughter cells by medial fission which benefits cell cycle research and by combining genetics, Paul Nurse, Lee Hartwell and Tim Hunt won the Nobel Prize in 2001 for research in cell cycle regulation. Many homologous human disease genes have been identified in S. pombe. S. pombe has become important in studying DNA replication, responses to DNA cellular damage and has RNAi genes like those found in vertebrates. S. pombe can express and store foreign DNA and has many mutants available.
B3. Drosophila melanogaster is a fly about 2.5 mm long and requires little space, cost and equipment to care for. D. melanogaster's genome has been sequenced and is easily manipulated. They have four chromosome pairs, three autosomes and an X/Y sex pair containing and estimated 14,000 genes which encode approximately 13,700 proteins, with sixty percent of the genome functioning as non-coding DNA used in expression control. Development time under ideal conditions is 8.5 days with females laying up to 100 eggs each day. D. melanogaster is the most used model organism out of all eukaryotes, contributing to the study of transcription, replication, heredity, multiple alleles and epistasis and gene mapping. The genome of fruit flies contain approximately 75% of known disease genes in humans making them invaluable in genetic medical research.
B4. Arabidopsis thaliana is a small flowering plant, growing to approximately 25 cm tall. They are easy to grow in the lab, have a quick life cycle, between eight to ten weeks and can produce thousands of progeny. They are able to self and sexually reproduce, have a small genome which has been sequenced and have about 27,000 genes that encode 35,000 proteins. Arabidopsis was the first genome sequenced for a plant and has been mapped for many biochemical and phenotypic mutations. It is considered a model for the study of flower development and light sensing in plants.
C1. Humans are sometimes used in genetic study, but are not good model organisms. Humans are large, expensive to care for and are almost impossible to sequester for long periods of time and thus it is difficult to control environmental variables. Humans take many years to develop to sexual maturity, nine months to reproduce and have a very low fecundity. Furthermore, humans have a long life cycle and can take years to show mutational phenotypes. While there is a lot of biological research on humans and their genome is sequenced, it is often unethical to experiment on humans, especially when experimenting in an invasive way or infecting humans with pathogens.
C2. Populus trichocarpa is a deciduous tree often used as a model organism. It does not make a good model organism because it is large, growing to 50 meters and is dioecious requiring at least two trees to reproduce, thus a large area to cultivate. P. trichocarpa is slow to reproduce, requiring 4-6 years to reach reproductive maturity and though it grows fast for a tree, its life cycle is slow for studying genetic mutations and phenotypes. The P. trichocarpa genome is relatively large for a plant, containing 45,500 putative genes, making it a genetically complex model organism for use in plant biology.
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2 Model Organisms. Biology Animation Library. Dolan DNA Learning Center. Retrieved February 23, 2011, from http://www.dnalc.org/resources/animations/model_organisms.html
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6 Drosophila melanogaster. Wikipedia, the free encyclopedia. Retrieved February 23, 2011, from http://en.wikipedia.org/wiki/Drosophila
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9 Arabidopsis thaliana. Wikipedia, the free encyclopedia. Retrieved February 23, 2011, from http://en.wikipedia.org/wiki/Arabid
10 Populus trichocarpa. Wikipedia, the free encyclopedia. Retrieved February 23, 2011, from http://en.wikipedia.org/wiki/Populus