Yeast, a type of fungus, is an excellent model for studying many basic cellular processes. Its cellular machinery, with the DNA in a structure called the nucleus, is shared with virtually all other organisms — including humans.
Antidote Europe (AE): Most people associate yeast with being the essential ingredient required in the baking of bread or the fermentation of wine. Relatively few people know what an important role yeasts have played in helping us to understand genetics. Could you provide our readers with a brief history of yeast research in the field of genetics? Are yeasts considered to be animals, plants, or something else?
Anonymous researcher (AR): ‘Bakers Yeast’ Saccharomyces cerevisiae has been widely used as a model organism for many years. Yeast is a type of fungus, and is an excellent model for studying many basic cellular processes, since its cellular machinery is shared with virtually all other organisms in which the DNA is found in a structure called the nucleus (including humans). These processes include cell division, gene expression and DNA replication and repair. The yeast genome was one of the first to be sequenced. Because many yeast genes have human counterparts or ‘homologues’, knowledge of how these yeast genes work can inform us of their roles in human cells. About 20 per cent of human disease genes have homologues in yeast, so studying these genes in yeast has helped to inform us about what goes wrong in these diseases at a cellular level.
AE: How similar, or how different are yeast from human cells, in terms of chromosome number and DNA content? Can you describe in plain language to our readers the most important discoveries made from yeast, and how it has been possible to learn so much about human genetics from such a simple organism?
AR: Yeast cells are much simpler than human (and other mammalian) cells and this is the key to the success of yeast as a model organism. Yeast has 16 chromosomes (compared to human 46) and only 1 copy of each gene (humans have 2 copies of each) . The yeast genome is just over 12 million base pairs in length and contains about 6000 genes. The human genome is much larger at 3 billion base pairs and 20-25000 genes. The small size of the yeast genome means that its full sequence has been known for a number of years. The fact that yeast only has one copy of each gene means that it is very easy to delete genes and see if this has any effect on the cells. Furthermore, many tools to expedite yeast research are commercially available, including collections of yeast strains with seperate deletions of each gene. In the human genome there are often multiple genes with the same role which means that determining the function of genes can be difficult.
Although the yeast genome is much simpler than the human genome, yeast have proved to be a very powerful tool for informing us about human cells. Many proteins important in human biology were first discovered by studying their homologues in yeast including cell cycle proteins, signaling proteins, and protein-processing enzymes. In 2001 the nobel prize in physiology or medicine was shared between 3 yeast researchers for their work which showed which genes were involved in cell division. Many of these have human homologues and because cell division is affected in cancer, this has important wider implications.
AE: How accurately can one extrapolate data obtained in yeast experiments to human beings? We realise of course that no species – whether plant or animal – can reliably substitute for a different species. Would you say that yeast studies fall under the category of “basic research” rather than “applied research”, with respect to human genetics?
AR: Bakers yeast is a basic yet powerful tool that has provided information about the probable function of many human genes. But until the function of any such gene is proved in human cells it remains a hypothesis and I do not know of any (good) yeast researcher (or indeed researcher using any non-human model) who would directly extrapolate their results to humans without further evidence.
The simplicity of yeast cells that makes them amenable to research can also limit the types of comparison that can be made with human cells, where the sheer complexity of the genome means e.g. that genes are more likely to interect with others and may play different roles to those defined in a simple model organism.