Saccharomyces cerevisiae is type of yeast. It is a small, oval, single- celled organism that divides by budding to reproduce. It can divide existing as a haploid or diploid, depending on environment. S. cerevisiae is commonly used as baker's and brewer's yeast. It is part of the fungi kingdom and has a thick, waxy cell wall. The cells contain mitachondria but no chloroplasts.
Its genome was sequenced in 1997 and was the first eukaryotic organism to have its genome sequenced. Its genome is approximately 13 Mb (Mb=millions of base pairs) and 6000 genes. 
It has key characteristics which make it a useful model organism, a well established experimental biological system, for work in laboratories:
- it is a unicellular eukaryote that can be studied under the microscope
- it is small so is cheap and easy store
- it grows rapidly in a simple nutrient medium, having an approximate generation time of 80 minutes
- it can be genetically modified relatively easily, and has been extensively studied in the past, so lots is known about its genes.
- it shares homology with humans and has been used specifically in the research of the cell division cycle, gene expression, DNA replication and repair and cell signalling
- it has a large mutant collection
- it is non-pathogenic
S. cerevisiae can produce in both a haploid and diploid state. In favourable conditions, S.cerevisiae will reproduce asexally in the haploid state by mitosis. This proliferation is brief in a plentiful nutrient supply and occurs only until they ecounter the opposite mating type. There is a alpha mating type and a a mating type of these haploid yeast cells. Each mating type sectretes a mating factor, alpha mating type secretes mating factor alpha and a mating type secretes mating factor a. When the encounter occurs between the mating factors of the two mating types, this triggers G1 arrest and mating. The resltant cell is diploid and the cells will reprduce in this way until starvation. Under unfavourable conditions, in limited nutrient supply the cells sporulate to form buds of blebs. The tetrad spores that form then germinate in improved conditions to form haploid cells, two of each mating type. The resulting cells then go on to proliferate and mate when conditions become even more favourable
- ↑ Alberts B., Johnson A., Lewis J., Raff M., Roberts K. and Walter P. (2008) Molecular Biology of the Cell, 5th Edition, New York: Garland Science. p33
- ↑ Hartl D.L and Ruvolo M. (2012) Genetics: Analysis of Genes and Genomes, 8th Edition, Burlington: Jones and Bartlett Learning. p34
- ↑ Roger W. Payne, Cletus P. Kurtzman, Jack W. Fell, Teun Boekhout, Yeast, Fifth edition, 2010
- ↑ David O Morgan, The Cell Cycle Principles of Control, London, New Science Press Ltd in association with Oxford University Press and Sinauer Associates Inc Publishers, 2007