Cells & Genes Tutorial Essay - Model Organisms PDF

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Cells & Genes
Model Organisms tutorial essay...

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‘Discuss the advantages and limitations of two model eukaryotic organisms used in the study of cells and genes (one vertebrate and one non-vertebrate), illustrating your answer with multiple examples of where these organisms have made important contributions to our understanding of general biological processes.’

Model Organisms used in the study of Cells and Genes by Kylie Dong Model organisms are those used in scientific studies because they have traits which are advantageous to specific experiments. These organisms can range from single celled prokaryotes like Escherichia coli, the most widely used organism in molecular genetics, to complicated large mammals like the dog, Canis lupus familiaris. Ideal model organisms for use in lab experiments are those that are: easy to keep, small in size and needing less resources, easy to breed or can be inbred with large numbers of offspring, have a simple reproductive biology, quick turnover between generations, provide predictable outcomes, and if for medical purposes are similar to humans. Many organisms fit these conditions and certain inconveniences can also be accommodated if such organisms have highly valued traits such as the expensive to keep but important Rhesus macaques which are as close to humans as model organisms used in labs can be. Certain traits can also be specifically selected like Lampreys, Petromyzon marinus, are used in spinal cord research because they have an easily accessible spinal cord that can be manipulated to observe the different effects. Pisum Sativum One famous model organism is Mendel’s garden peas Pisum Sativum. This was the first model organism used in genetic experiments and because of Mendel’s experiments we were able to gain the basic understanding of genetic needed for modern science today. However these peas were not used with a lot of intention as Mendel used them largely because they were readily available, cheap, and easy to grow. The advantages of using peas is that they grow easily in many climates and have short life cycles which means Mendel could generate many generations in a year. The male and female parts of the flower were easily distinguishable and large enough for manual crossing experiments. They could also be self or cross fertilized with self fertilized plants always breeding true and producing the same trait generation upon generation and hybrid plants producing offspring with mixed traits. Pea plants also produce a large number of flowers which could form many pea pods containing between two to eight peas each. This mean a large number of offspring could be grown and observed to negative the trends of chance in Mendel’s calculations. Mendel ended up making 287 crosses between 70 different purebred plants. Approximately 28,000 pea plants were used. Peas also show some clear-cut, qualitative traits for example height, pea colour, pea texture and one can unambiguously distinguish between two alternative forms. The seven traits that mendel chose were present on different loci and moreover they showed distinguished phenotypic characteristics. The seven traits that Mendel chose were also dominant traits which were easier to calculate results from (Capelli, 2016). The disadvantages to using peas, or more so the reason they are no longer commonly used as model organisms today is that they don’t have too many traits which are relevant for

more in depth or precise genetic experiments. They were perfect primarily as a simple model organism to decipher the basic principles of genetics which Mendel achieved. Mendel’s experiments were the first to explore the possibility of genes, paving the way for modern biology. He published his work in 1865 although it wasn’t properly regarded until 34 years later in 1902 when the chromosome theory of inheritance was proposed. Though he didn’t know about genes at the time and called them “factors” many of his ideas that he discovered with peas still stand today in the context of genes. Mendel was able to figure out a few important rules about genetics. Firstly he was able to disprove an idea at the time, blending which thought offspring were always a mix of their parental traits. He was able to argue that blending would lessen diversity and prevent the reappearance of lost traits which he showed did reappear as a recessive trait in a second generation from hybrid parents. His law of segregation stated that every individual carries two copies of each gene, one from each parent and that these segregate again during gamete formation. These then unite at random, one from each parent, in the offspring at fertilisation. Mendel’s repeated crosses of the plants confirms offsprings with phenotypic ratios that much the law of segregation. Mendel’s second law was that of independent assortment. This states that during gamete formation, the segregation of alleles at one locus is independent of the segregation of alleles of another locus. This law also showed results in predictable ratios of phenotypes in the second generation of breeding (Capelli, 2016). Rhesus macaques The Rhesus Macaque monkeys are another model organism, rather the opposite of Mendel’s peas in that they are used far more widely today than many years ago in science although they have been used in science quite a while. They are a type of old world monkey, brown or grey in colour with hairless pink faces, native to South, Central, and Southeast asia and inhabiting a wide variety of habitats from grasslands to arid and forest areas. They have many advantages as model organisms. They are listed as least concern on the IUCN Red List of Threatened Species which makes them a suitable and sustainable organism to use widely for science. They are sexually dimorphic with adult males weighing an average of 7.7.kg and females 5.3kg which means they are easily distinguisable and can be used in experiments researching sex variances. The monkeys have a lifespan of about 25 years which is an advantage because they can be used in longer term experiments. They are relatively easy to upkeep in captivity, having a variable omnivorous diet and being smaller than their primate counterparts the chimpanzee but still similar enough to humans for reliable experimental results. They have very similar genomes to humans making them suitable models to use for experiments that need to be extrapolated to humans. Their genome was sequenced and the results showed they share 93% DNA in common with humans, 200 genes appear to be key players in defining the differences between the monkeys and humans. This is less close than chimpanzees which share 98-99% of DNA to humans but Macaques are easier to maintain in lab environments. Macaques also have the same range of diversity in their populations when it comes to their genome as humans so by using them as a model we can more accurately predict the variable outcomes of e.g. drugs in people (Choi, 2007). The disadvantages to using Macaques comes mostly from the ethical implications involved with using more complex, intelligent organisms. Macaques have been shown to have highly developed cognition and are even able to recognise themselves in mirrors showing levels

of self awareness. Past experiments without ethical regulations have proved controversial and have been met with stricter rules for experiments now. This in turn has caused an increase in costs for buying individuals and maintaining them. Prices for a single monkey have risen from $1000 to $1500 in 1990 to $5000 to $12,000 in 2003. This causes reusing of individuals from experiment to experiment to save costs which can confuse results. Enclosures are also expensive to build as regulations increase to provide better living conditions for lab animals. Other regulations like having veterinarians on site to treat the animals if they get sick or injured also add to costs. They are also increasingly difficult to obtain, for a while there was a ban on exporting monkeys from India and inadequate breeding efforts in the US leading to a shortage of these animals, not being able to meet the increasing demand for scientific experiments. Because of this obtained individuals may not be entirely healthy with breeders giving the animals many antibiotics or hormones to hasten growth. Such individuals can be no longer resistant to many diseases and different to wild type monkeys (Marshall, 2004). The contributions that the Rhesus Macaques have already made to science are great and will continue to increase as they are used more widely in research. One such discovery was that of the Rhesus factor in blood. Discovered in 1937, the factor was named after the Rhesus Macaques as they were a crucial part of the experimentation process being the model organism. This was an significant discovery and is the substance thats presence or absence denotes a person’s blood type as positive or negative (Choi, 2007). The Rhesus monkey was also the first primate to be cloned. Tetra was a Rhesus monkey born in 2000, made from a split embryo in a similar process to the way twins form. This held large implications for science because it was demonstrated that monkeys could be cloned like mice for consistent experimental results and are more advantageous to use because of their similarity to humans (Ramsay, 2000). Then in 2001 Andi was another clone created to include a jellyfish luminescence gene. This was the first time a new gene had been inserted into a primate and researchers say the same technology could be used to create test primates that mimic humans with diseases more closely by inserting into them genes such as those that cause breast cancer (BBC, 2001). Rhesus Macaques have also been used as model organisms in the development of vaccines for many human diseases. Measles is one such disease and a recent experiment done in 2011 using Rhesus Macaques has developed a method to deliver a vaccine by an inhalable dry powder to remove the obstacles involved with injection vaccines. This method requires less equipment and medical expertise to deliver, providing the same coverage when tested in the monkeys (Lin et al, 2011). Vaccines for rabies, smallpox, and polio have also been developed using Rhesus Macaques as experimentation subjects. Also current research in the ongoing development of HIV and AIDs drugs also involve these crucial model organisms. In conclusion, the two model organisms, Pisum Sativum, the garden pea and Rhesus Macaque monkeys are both crucial to scientific research because of their unique properties related to the experiments in which they are used. The garden pea is used less commonly now but its impact on the history of scientific research continues to resonant into current work and the rules Mendel figured out are used as the basis of genetics. Rhesus macaques are used more as science moves to eradicate many diseases and their closeness to humans means we are able to more accurately predict the effect medical practices may have on humans. Model organisms and their contribution to science are as important as the work of the scientists conducting these tests and

as we move advance further it is important to recognise the life we use to aid our own. Sources: C.Q. Choi. 2007. Monkey DNA Points to Common Human Ancestor. Live Science. Retrieved 1 Feb 2015 from: http://www.livescience.com/1411-monkey-dna-points-common-humanancestor.html C. Marshall. 2004. Monkeys for Research: Much Coveted, and Hard to Come By. The New York Times. Retrieved 1 Feb 2015 from: http://www.nytimes.com/2004/04/06/science/monkeys-forresearch-much-coveted-and-hard-to-come-by.html?_r=0 BBC. 2001. GM Monkey First. BBC News. Retrieved 1 Feb 2015 from: http://news.bbc.co.uk/1/hi/sci/tech/1112171.stm S. Ramsay. 2000. Embryo splitting produces primate “clone”. The Lancet. 355: 205-205 A.W.S. Chan, M.C. Luetjens, T. Dominko, J. Ramalho-santos, C.R. Simerly, L. Hewitson, G. Schatten. 2000. TransgenICSI reviewed: Foreign DNA transmission by intracytoplasmic sperm injection in rhesus monkey. Molecular Reproduction and Development. Molecular Reproduction and Development. 56: 325-328 W.H. Lin, D.E, Griffin, P.A. Rota, M. Papania, S.P. Cape, D. Bennett, B. Quinn, R.E. Sievers, C. Shermer, K. Powell, R.J. Adams, S. Godin, S. Winston. 2011. Successful respiratory immunization with dry powder live-attenuated measles virus vaccine in rhesus macaques. Proceedings of the National Academy of Sciences of the United States of America. 108: 298792 Cristian Capelli, 19 January 2016, Oxford University Department of Plant Sciences Lecture: ‘Mendel meets CSI’...