[The Leaven – exploring the relationship between science and religion (cont)]
The development of yeast molecular biology can literally be used to assess the impact that the application of scientific research has had on 21st century society. Scientific researchers often describe yeast as the workhorse of eukaryotic molecular biology with many laboratories devoted to studying this single-celled organism, as much of the information derived from it can be equally applied to the study of human cells.
Most modern laboratory strains of yeast originate from one particular Saccharomyces cerevisiae strain, EM93, isolated from dried figs in Merced, California in the 1930’s by Emil Mrak. This strain turned out to be heterothallic, meaning that cells existed as two types of sterile haploids, with a single copy of each gene, that when fused together formed a fertile diploid that could perform meiosis in a similar way to that seen in human cells. Up until this point most strains studied were homothallic, this meant that all haploid cells were of the same mating type and capable of fusing together to form a fertile cell known as a zygote. The emergence of a heterothallic strain meant that the genetic stability of a culture could be placed under greater control, as it would remain haploid until the other haploid type was introduced and then through the production of mating phermones followed by cell fusion a diploid cell could be created.
So why has yeast become such a popular organism to study molecular biology and why is this microbe chosen in favour of others microorganisms? Firstly, Saccharomyces is non-pathogenic and does not present a threat to human safety. Therefore laboratory workers do not require expensive protective equipment to practice research. Saccharomyces is also easy to contain as is not usually airborne unless transported involuntarily by animals and insects. Another reason is the ease by which it is cultured. Yeast can be grown easily and only requires a suitable carbon source, nutrients and appropriate physical conditions to continue multiplying. Additionally, these requirements can also be used to control the rate of cell division, for instance, by altering temperature or by creating metabolic mutants. Mutants are generated either through using a mutagen or by manipulating DNA through genetic engineering. Genes involved in yeast metabolism can be mutated and then used as molecular markers. For instance if the genes for the requirement of an essential amino acid are defective then the yeast will not grow without that amino acid added to its immediate environment. If the defective gene is artificially replaced by a functional one then the yeast cell will be able to continue growing without the need for that particular amino acid. Armed with this knowledge researchers are able to introduce fragments of DNA fused to these marker or reporter genes. If the yeast is able to grow without the selected amino acid this means that the DNA of interest to the researcher has been successfully introduced into the cell. This approach has led to the characterisation of countless genes and proteins in yeast and from other organisms.
Another reason why yeast is used as a molecular model system alongside other well-known microbes, such as Escherichia coli, is because it is a eukaryote. E. coli and other bacteria are prokaryotes, in contrast to eukaryotes they only have one chromosome housed in a cell without a nucleus. In yeast cells, DNA is packaged in chromosomes stored in a nucleus in a similar way as in human cells. Yeast has 16 individual chromosomes compared to 23 in humans. Surprisingly, there are only four chromosomes in the multi-celled fruit fly Drosophila another model organism used for biological research. Yeast also has the advantage of being able to grow just as happily with one set of chromosomes, in haploid cells, as with two or more sets of chromosomes, diploid and polyploid respectively. Additionally, as yeast is a single celled organism without the complexity of cellular differentiation it can be used to study the cell-cycle at a fundamental level. It can be used to study mitosis and meiosis. Many mutations that cause human disease are introduced during meiosis. Following cell fusion or mating, two haploid cells form diploids which can produce four individual haploid cells known collectively as an ascospore. After microscopic dissection of the ascospores, researchers can study recessive mutations and the complicated exchange of genetic material during meiosis by counting the numbers of surviving progeny. The information derived from yeast studies aids the study of genes involved in tissue development and cell differentiation in higher eukaryotes, such as Drosophila. Adding to all these factors many of the biochemical and cellular functions in yeast are conserved in human cells. Yeast therefore is a simple and practical system to study the mechanism of human cell division.