<P> Concerning organic requirements, most strains of S. cerevisiae require biotin . Indeed, a S. cerevisiae - based growth assay laid the foundation for the isolation, crystallisation, and later structural determination of biotin . Most strains also require pantothenate for full growth . In general, S. cerevisiae is prototrophic for vitamins . </P> <P> Yeast has two mating types, a and α (alpha), which show primitive aspects of sex differentiation . As in many other eukaryotes, mating leads to genetic recombination, i.e. production of novel combinations of chromosomes . Two haploid yeast cells of opposite mating type can mate to form diploid cells that can either sporulate to form another generation of haploid cells or continue to exist as diploid cells . Mating has been exploited by biologists as a tool to combine genes, plasmids, or proteins at will . </P> <P> The mating pathway employs a G protein - coupled receptor, G protein, RGS protein, and three - tiered MAPK signaling cascade that is homologous to those found in humans . This feature has been exploited by biologists to investigate basic mechanisms of signal transduction and desensitization . </P> <P> Growth in yeast is synchronised with the growth of the bud, which reaches the size of the mature cell by the time it separates from the parent cell . In well nourished, rapidly growing yeast cultures, all the cells can be seen to have buds, since bud formation occupies the whole cell cycle . Both mother and daughter cells can initiate bud formation before cell separation has occurred . In yeast cultures growing more slowly, cells lacking buds can be seen, and bud formation only occupies a part of the cell cycle . </P>

How many genes does baker's yeast have