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Lecture 22: "Developmental Genetics"

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Biology 2581B
Jim Karagiannis

Genetics Lecture No. 22: Developmental Genetics I – Model Organisms st Monday April 1 , 2013 The Purpose Behind Model Organisms: -Model organisms are living things that exhibit qualities ideal for experimental study. Model organisms should be: Easily maintained under laboratory conditions (raising a large population under controlled environmental conditions), manageable as mutants (easy to generate, recognize and track), producing a large number of offspring in a short period of time (relative to each species), compatible with cloning and transformation procedures, sequenced by genome, present in genome databases and scientific literature, and be addressing interesting questions in order for research to be funded. Ideally, model organisms should encompass different scales (sort of as representatives of a particular group of organisms) so we can make cross-comparisons between species, answering different biological/genetic questions. Different Scales Of Model Organisms: -Different scales of model organisms span the basic universal processes/properties of life (transcription, translation, replication, etc.) which are present in all groups of organisms (bacteria, mammals, plants, fungi, etc.). With this in mind, we can compare how similar or different these processes through the process of evolution. We can compare certain model organisms like Drosophila and compare it to organisms within the same taxon (other insects) looking for taxon-specific properties. The same can be said of species-specific properties, where our Drosophila species would be compared with other species of that same Drosophila genus. Saccharomyces Cerevisiae As A Model Organism: -S. cerevisiae are also known as “the budding yeast” because of its mode of reproduction. It is selected as a typical model organism since: Humans have been in contact with this organism for hundreds of years (brewing for beer), it is a single-celled eukaryote (easy if you’re studying eukaryotic cells), it exists as a haploid and diploid (very easy for using genetic models), and it can be grown in large quantities. For transgenic purposes, S. cerevisiae can be transformed with plasmids followed by recombination. By using markers such as nutritional auxotrophy (inability of the organism to synthesize its own nutrients) or drug resistance (can survive in antibiotic environment), we can track these induced DNA changes. The main advantages to using S. cerevisiae as a model organism are that it involves all the basic functions of eukaryotic cells (secretory mechanisms, chromosome structure, protein trafficking, etc.). However, S. cereveisiae cells are limited in that they do not congregate to form tissues, meaning that extracellular signalling is not really developed as in higher eukaryotes (which also contain microRNAs that are less prevalent in S. cerevisiae). Recall that gene cassettes (modular DNA sequences encoding one or more genes for a single biochemical function) can be introduced into the nuclei of wild-type yeast strains and through recombination can become integrated into the genome of the new mutant (e.g. for Kanamycin resistance). Caenorhabditis Elegans As A Model Organism: -C. elegans is a worm model organism that is very interesting in that it has a defined cell lineage – an exact map for every cell division created. With this we can get information such as there being exactly 302 neurons in adult C. elegans, which simply cannot be done for any other organism in that level of detail. For transgenic purposes, we can inject double-stranded DNA into the worm so it is maintained as part of its genome (e.g. normal squiggling motion becoming mutant corkscrew motion. This injected DNA is usually radioactively-labelled in order to observe which worms have the mutant phenotype (usually markers that are morphological, related to shape and movement of the worm). The main advantages to using C. elegans as a model organism are that it involves many important aspects of the eukaryotic cell environment (programmed cell death, microRNA and RNA interference, signal transduction pathways, etc.). However C. elegans is limited as parts of its tissue cannot be grown separately (you have to grow the whole worm) and for this reason, targeted replacements cannot be done. C. elegans is notable for having two different sexes: a hermaphrodite that produces eggs and sperm and a male (male is morphologically smaller) that only produces sperm. Self-fertilization in hermaphrodites results in nearly 100% hermaphrodite progeny (some are males), while cross- fertilization between males and hermaphrodites results in a 50% male and 50% hermaphrodite progeny. Drosophila Melanogaster As A Model Organism: -D. melanogaster, the fruit f
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