notable model system to study basic principles of a cell that can be used to understand more complex organisms like mammals and in particular humans. this single cell eukaryote is nonpathogenic and easily grown and manipulated in the lab. fission yeast contains one of the smallest numbers of genes of a known genome sequence for a eukaryote, and has only three chromosomes in its genome. many of the genes responsible for cell division and cellular organization in fission yeast cell are also found in the human's genome. cell cycle regulation and division are crucial for growth and development of any cell. fission yeast's conserved genes has been heavily studied and the reason for many recent biomedical developments. fission yeast is also a practical model system to observe cell division because fission yeast's are cylindrically shaped single celled eukaryotes that divide and reproduce by medial fission. this can easily be seen using microscopy. fission yeast also have an extremely short generation time, 2 to 4 hours, which also makes it an easy model system to observe and grow in the laboratory fission yeast's simplicity in genomic structure yet similarities with mammalian genome, ease of ability to manipulate, and ability to be used for drug analysis is why fission yeast is making many contributions to biomedicine and cellular biology research, and a model system for genetic analysis. genome schizosaccharomyces pombe is often used to study cell division and growth because of conserved genomic regions also seen in humans including: heterochromatin proteins, large origins of replication, large centromeres, conserved cellular checkpoints, telomere function, gene splicing, and many other cellular processes. s. pombe's genome was fully sequenced in 2002, the sixth eukaryotic genome to be sequenced as part of the genome project. an estimated 4,979 genes were discovered within three chromosomes containing about 14mb of dna. this dna is contained within 3 different chromosomes in the nucleus with gaps in the centromeric (40kb) and telomeric (260kb) regions. after the initial sequencing of the fission yeast's genome, other previous non-sequenced regions of the genes have been sequenced. structural and functional analysis of these gene regions can be found on large scale fission yeast databases such as pombase. forty-three percent of the genes in the genome project were found to contain introns in 4,739 genes. fission yeast does not have as many duplicated genes compared to budding yeast, only containing 5%, making fission yeast a great model genome to observe and gives researchers the ability to create more functional research approaches. s. pombe's having a large number of introns gives opportunities for an increase of range of protein types produced from alternative splicing and genes that code for comparable genes in human. 81% of the three centromeres in fission yeast have been sequenced. the lengths of the three centromeres were found to be 34, 65, and 110 kb. this is 300–100 times longer than the centromeres of budding yeast. an extremely high level of conservation (97%) is also seen over 1,780-bp region in the dgs regions of the centromere. this elongation of centromeres and its conservative sequences makes fission yeast a practical model system to use to observe cell division and in humans because of their likeness. pombase reports over 69% of protein coding genes have human orthologs and over 500 of these are associated with human disease . this makes s. pombe a great system to use to study human genes and disease pathways, especially cell cycle and dna checkpoint systems. genetic diversity biodiversity and evolutionary study of fission yeast was carried out on 161 strains of schizosaccharomyces pombe collected from 20 countries. modeling of the evolutionary rate showed that all strains derived from a common ancestor that has lived since ~2,300 years ago. the study also identified a set of 57 strains of fission yeast that each differed by ≥1,900 snps;, and all detected 57 strains of fission yeast were prototrophic (able to grow on the same minimal medium as the reference strain). a number of studies on s.pombe genome support the idea that the genetic diversity of fission yeast strains is slightly less than budding yeast. indeed, only limited variations of s.pombe occur in proliferation in different environments. in addition, the amount of phenotypic variation segregating in fission yeast is less than that seen, in s. cerevisiae. since most strains of fission yeast were isolated from brewed beverages, there is no ecological or historical context to this dispersal. cell cycle analysis dna replication in yeast has been increasingly studied by many researchers. further understanding of dna replication, gene expression, and conserved mechanisms in yeast can provide researchers with information on how these systems operate in mammalian cells in general and human cells in particular. other stages, such as cellular growth and aging, are also observed in yeast in order to understand these mechanisms in more complex systems. s. pombe stationary phase cells undergo chronological aging due to production of reactive oxygen species that cause dna damages. most such damages can ordinarily be repaired by dna base excision repair and nucleotide excision repair. defects in these repair processes lead to reduced survival. cytokinesis is one of the components of cell division that is often observed in fission yeast. well-conserved components of cytokinesis are observed in fission yeast and allow us to look at various genomic scenarios and pinpoint mutations. cytokinesis is a permanent step and very crucial to the wellbeing of the cell. contractile ring formation in particular is heavily studied by researchers using s. pombe as a model system. the contractile ring is highly conserved in both fission yeast and human cytokinesis. mutations in cytokinesis can result in many malfunctions of the cell including cell death and development of cancerous cells. this is a complex process in human cell division, but in s. pombe simpler experiments can yield results that can then be applied for research in higher-order model systems such as humans. one of the safety precautions that the cell takes to ensure precise cell division occurs is the cell-cycle checkpoint. these checkpoints ensure any mutagens are eliminated. this is done often by relay signals that stimulate ubiquitination of the targets and delay cytokinesis. without mitotic check points such as these, mutagens are created and replicated, resulting in multitudes of cellular issues including cell death or tumorigenesis seen in cancerous cells. paul nurse, leland hartwell, and tim hunt were awarded the nobel prize in physiology or medicine in 2001. they discovered key conserved checkpoints that are crucial for a cell to divide properly. these findings have been linked to cancer and diseased cells and are a notable finding for biomedicine. researchers using fission yeast as a model system also look at organelle dynamics and responses and the possible correlations between yeast cells and mammalian cells. mitochondria diseases, and various organelle systems such as the golgi apparatus and endoplasmic reticulum, can be further understood, by observing fission yeast's chromosome dynamics and protein expression levels and regulation. biomedical tool however, there are limitations with using fission yeast as a model system: its multidrug resistance. "the mdr response involves overexpression of two types of drug efflux pumps, the atp-binding cassette (abc) family... and the major facilitator superfamily". paul nurse and some of his colleagues have recently created s. pombe strains sensitive to chemical inhibitors and common probes to see whether it is possible to use fission yeast as a model system of chemical drug research. for example, doxorubicin, a very common chemotherapeutic antibiotic, has many adverse side-effects. researchers are looking for ways to further understand how doxorubicin works by observing the genes linked to resistance by using fission yeast as a model system. links between doxorubicin adverse side-effects and chromosome metabolism and membrane transport were seen. metabolic models for drug targeting are now being used in biotechnology, and further advances are expected in the future using the fission yeast model system. experimental approaches fission yeast is easily accessible, easily grown and manipulated to make mutants, and able to be maintained at either a haploid or diploid state. s. pombe is normally a haploid cell but, when put under stressful conditions, usually nitrogen deficiency, two cells will conjugate to form a diploid that later form four spores within a tetrad ascus. this process is easily visible and observable under any microscope and allows us to look at meiosis in a simpler model system to see how this phenomenon operates. virtually any genetics experiment or technique can, therefore, be applied to this model system such as: tetrad dissection, mutagens analysis, transformations, and microscopy techniques such as frap and fret. new models, such as tug-of-war (gtow), are also being used to analyze yeast robustness and observe gene expression. making knock-in and knock-out genes is fairly easy and with the fission yeast's genome being sequenced this task is very accessible and well known.