Saccharomyces cerevisiae, also known as the yeast cell, has been one of the most important organisms for the study of genetics. They are the simplest of eukaryotes and have similar organelles to human cells, conserving important gene functions and containing a nucleus, vacuoles, and a Golgi apparatus. At least fifty percent of yeast genes have at least one equivalent human gene (1). Due to these similarities in gene function and cell structure, yeast cells can be used in the study of Classical Genetics and Biochemistry, as well as in Recombinant Genetics. Yeast is relatively easy to grow, as they can grow either aerobically or anaerobically in simple media. They have an average doubling time of 1.5 hours in ideal conditions. This experiment explores these properties and uses them in the study of Mutagenesis (following the adenine pathway) and in the complementation of genes.
Yeast cells can exist in either the diploid or haploid state. They can produce sexually or asexually. In this experiment, two haploid cells (MATa or MATα) will be used to explore Yeast genetics. If these two haploid cell types are kept separate from each other, they will maintain their haploid state. In the event that they are brought together, they will fuse to form a diploid cell. The diploid state of the cell can be maintained as long as there is a sufficient amount of nutrition. In extreme cases when the cell’s nutrition is poor, the diploid cell will sporulate and form an ascus with four haploid spores (2). Yeast cells change their morphology as they go through their life cycle, as can be seen in Figure 1. The cell undergoes cell division by mitosis to form a small bud. The bud then enlarges as the cell progresses through G2 and mitosis. The bud then separates from the mother cell. It important to note that the mother cell does not change in size and that the budding process is the same in both haploid and diploid cells.
In order to fully explore the capabilities of yeast genetics, mutagenesis will be performed on the yeast cells. Mutagenesis is the inducing of a random mutation deliberately. It can be performed in several ways such as using chemicals, x-rays, and UV-radiation. These methods cause random changes in the sequence of DNA molecules and allow for the studying of concepts such as complementation. Specifically, the adenosine pathway will be studied using mutagenesis. Four haploid strands. ADE1a, ADE2a, ADE1α, and ADE2α will undergo mutagenesis and will then be placed on various media to observe growth and characteristics. The “1” mutants have a mutation in ADE1 and the “2” mutants will have a mutation in ADE2 in the adenine pathway. A mutant allele in either of these pathways will cause the buildup of P-ribosylamino imidazole, resulting in a red pigmented cell. The accumulation of P-ribosylamino imidazole also causes a decrease in the growth rate of ADE mutant cells.
These cells will be grown on one of three types of media, providing with a range of characteristics that can lead to the discovery of the genotype of the cell. The types of media are YED media (which contain all of the required nutrients for the yeast to grow), MV media (a minimal media which contains only the pure chemicals required for the growth of wild-type yeast, it is important to note that this type of media lacks adenine, causing haploid adenine mutants to die off when placed on this media), and YEKAC media, a starvation media with the purpose of inducing sporulation of diploid yeast cells. The growth on each of these types of media will allow us to determine whether two genes are complements of each other or not. Complementation occurs when two strains of an organism with the same mutation on different genes, allow for the growth of the cell and/or the wildtype phenotype to be observed.
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