lec 2 - Lecture 2 2 In this lecture we are going to...

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Lecture 2 Lecture 2 In this lecture we are going to consider experiments on yeast, a very useful organism for genetic study. Yeast is more properly known as Saccharomyces cerevisiae, which is the single-celled microbe used to make bread and beer. Yeast can exist as haploids of either mating type α (MAT α ) or mating type a (MAT a ). Haploid cells of different mating type when mixed together will mate to make a diploid cell. Haploids and diploids are isomorphic – meaning that a given mutation will cause essentially the same change in haploid and diploid cells. This allows us to look at the effect of having two different alleles in the same (diploid) cell. All yeast needs to grow are salts, minerals, and glucose (minimal medium). From these compounds, yeast cells can synthesize all of the molecules such as amino acids and nucleotides that are needed to construct a cell. The synthesis of complicated molecules requires many enzymatic steps. When combined, these enzymatic reactions constitute a biochemical pathway Consider the pathway for the synthesis of the amino acid histidine. A B C D histidine Protein Enzyme: 1 2 3 4 Each intermediate compound in the pathway is converted to the next by an enzyme. For example, if there is a mutation in the gene for enzyme 3 then intermediate C can not be converted to D and the cell can not make histidine. Such a mutant will only grow if histidine is provided in the growth medium.
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This type of mutation is known as an auxotrophic mutation auxotrophic mutation auxotrophic mutation and is very useful for genetic analysis. growth on minimal growth on minimal + histidine His + (wild type) + + His + Phenotype: Phenotype: All traits of an organism (with an emphasis on trait under investigation) Homozygote: Homozygote: Homozygote: diploid with two like alleles of same gene Heterozygote: diploid with two different alleles of same gene Recessive Allele: Recessive Allele: trait not expressed in heterozygote genotype phenotype Mate to : diploid genotype diploid phenotype MAT a His3 His MAT α His3 His3 /His3 His MAT a His3 His MAT α His3 + His3 /His3 + His + Based on the His phenotype of the His3 /His3 + heterozygote, we would say that His3 is recessive to wild type.
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lec 2 - Lecture 2 2 In this lecture we are going to...

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