these two species diverged approximately 300 to 600 million years before present, and are significant tools in molecular and cellular biology. Some of the technical discriminants between these two species are: S. cerevisiae has approximately 5,600 open reading frames; S. pombe has approximately 5,070 open reading frames. Despite similar gene numbers, S. cerevisiae has only about 250 introns, while S. pombe has nearly 5,000. S. cerevisiae has 16 chromosomes, S. pombe has 3. S. cerevisiae is often diploid while S. pombe is usually haploid. S. pombe has a shelterin-like telomere complex while S. cerevisiae does not. S. cerevisiae is in the G1 phase of the cell cycle for an extended period (as a consequence, G1-S transition is tightly controlled), while S. pombe remains in the G2 phase of the cell cycle for an extended period (as a consequence, G2-M transition is under tight control). Both species share genes with higher eukaryotes that they do not share with each other. S. pombe has RNAi machinery genes like those in vertebrates, while this is missing from S. cerevisiae. S. cerevisiae also has greatly simplified heterochromatin compared to S. pombe. Conversely, S. cerevisiae has well-developed peroxisomes, while S. pombe does not. S. cerevisiae has small point centromere of 125 bp, and sequence-defined replication origins of about the same size. On the converse, S. pombe has large, repetitive centromeres (40–100 kb) more similar to mammalian centromeres, and degenerate replication origins of at least 1kb. S. pombe pathways and cellular processes S. pombe gene products (proteins and RNAs) participate in many cellular processes common across all life. The fission yeast GO slim provides a categorical high level overview of the biological role of all S. pombe gene products. Life cycle The fission yeast is a single-celled fungus with simple, fully characterized genome and a rapid growth rate. It has long been used in brewing, baking, and molecular genetics. S. pombe is a rod-shaped cell, approximately 3 μm in diameter, that grows entirely by elongation at the ends. After mitosis, division occurs by the formation of a septum, or cell plate, that cleaves the cell at its midpoint. The central events of cell reproduction are chromosome duplication, which takes place in S (Synthetic) phase, followed by chromosome segregation and nuclear division (mitosis) and cell division (cytokinesis), which are collectively called M (Mitotic) phase. G1 is the gap between M and S phases, and G2 is the