Yeast

Yeast is a eukaryotic, unicellular organism and belongs to the kingdom of fungi. They can be spherical, cylindrical and filamentous. They usually replicate via budding. Budding is when a new cell grows out from the old cell and then separates from each other. Yeasts survive best in conditions with high sugar concentrations, and are able to survive both anaerobically and aerobically, meaning that they are faculative aerobes^[1].

Contents 1 Types 1.1 Saccharomyces cerevisae 1.2 Schizosaccharomyces pombe 2 Use in Genetic Research 2.1 Advantages 2.2 Disadvantages 3 References

Types

Saccharomyces cerevisae

Saccharomyces cerevisiae (or Baker's yeast) is a common form of yeast often used as a model organism due to its eukaryotic processes, similar to those of animal cells^[2]. The presence of mitochondria in S. cerevisae is key in this. This species of yeast contains 16 chromosomes and a genome sequence that is 12.2 mb long. In addition, much has been learned about the eukaryotic cell division cycle from the study of this organism, as the yeast can reproduce sexually as well as asexually.

This species of yeast replicates asexually by budding. Parent cells in cerevisiae can be differentiated by scars formed from budding, unlike in fission yeast where the daughter cells of budding yeast are smaller than the mother cell.

Schizosaccharomyces pombe

Schizosaccharomyces pombe (fission yeast) replicates by elongation then splitting and contains a genome sequence that is 12.5 Mb long (around 5123 genes) on 3 chromosomes. The species has an approximate homology with human disease genes of 6-7%. S. pombe is used in the production of African beer. Breakthroughs in science involve using this model organism to study the checkpoint chemicals used to control the cell cycle which is a major discovery in cancer chemistry.

In addition, sexual reproduction may occur, in which two haploids S. cerevisiae fuse to form a haploid cell.

Another species; Candidia albicans, responsible for some vaginial, lung and mouth infections, is a filamentous yeast. Its shape is what allows it to be pathogenic^[3].

Use in Genetic Research

Both S. cerevisiae and S. pombe are commonly used as a model organism for genetic research - particularly when looking into the cellular and genetic basis of human disease. As always with the use of model organisms in research, there are a number of advantages and disadvantages that should be considered:

Advantages

• Unicellular and able to be grown in a defined medium
• Fundamental processes are conserved in all Eukaryotes
• Easy to manipulate genome or knockout genes
• Has a stable haploid/diploid life cycle allowing complementation analysis to identify whether mutations are present in the same or different genes
• Complete genome sequence is available and genes identified. This can be merged with other information from other species and played a key role in lowering the cost of genome sequencing

Disadvantages

• Similar proteins may have organism-specific functions so may not function in the same way in other organisms of a different species
• Yeast genome contains many fewer genes than the human genome (around 6000 in yeast compared to around 30,000 in humans)
• Some processes in yeast are not found in other Eukaryotes
• Cells of different organisms live in different environments

References

1. ↑ Brock Biology of Microorganisms, Madigan at al, 11ed. Pearson Education, 1970, San Francisco
2. ↑ Alberts et al. (2008:33-34), Molecular Biology of the Cell, 5th edition, New York: Garland Science
3. ↑ B Alberts, A Johnson, J Lewis, M Raff, K Roberts, P Walter, 2008, Molecular biology of the cell, 5th edition, New York: Garland Science pp. 33-34