S. cerevisiae and S. pombe are both members of the phylum Ascomycota, but they use dramatically different methods of reproduction. Befitting their popular names, S. cerevisiaereproduces by budding, while S. pombe reproduces by fission along a central plane. During each round of cell reproduction and division, the two yeasts show predictable changes in morphology.S. cerevisiae and S. pombe mutants that do not display these characteristic changes in morphology contributed to our current molecular understanding of the cell cycle - and the 2001 Nobel Prize.
As shown above, S. cerevisiae and S. pombe spend different fractions of each cell cycle in the G1, S, G2 and M phases, because the principal size check-point occurs at a different place in the cycle for each species (Turner et al., 2012). In S. cerevisiae (right), buds begin to form when cells enter S phase. The size of the bud, which will become the daughter cell, continues to grow until the cells divide in M phase. When the cell divides, the daughter cell is still smaller than the mother cell. The daughter cell will need to grow a bit before it enters another with permissionround of cell division.
By contrast, S. pombe (right; from Hochstenbach et al., 1998) divides by medial fission. Cells grow in length until they are 12-15 μm, when a septum forms and the cells divide. The unusually long G2 phase of S. pombe may reflect the fact that it is found primarily as a haploid in nature (unlike S. cerevisiae, which is found in both diploid and haploid forms). Because maintaining two copies of each gene during most of the cell cycle, S. pombe is able to survive a lethal mutation in one gene copy.
The cells in your yeast cultures will be in “log (short for logarithmic) phase,” when cells are dividing exponentially (Chapter 4). The cultures are not synchronized, however, and will contain cells at all points in the cell cycle. Our microscopes do not provide the same kinds of magnification as the electron micrographs shown above, but you should be able to see some subcellular compartments when you view the yeast at 1000X magnification.