= Solution
Essential <cell cycle> genes cannot generally be discovered by selecting viable complete deletions alone. A <temperature-sensitive mutation> solves this problem: its product supports growth under permissive conditions but fails under restrictive conditions. Screen conditional mutants for a reproducible division defect, then distinguish a specific cycle block from nonspecific loss of viability. A <cdc mutant> is defined by its cycle <phenotype>, not simply by poor growth at high temperature.
In <Saccharomyces cerevisiae>, bud size and nuclear morphology provide convenient stage markers. After shifting an asynchronous culture, mutants defective at a particular step tend to accumulate with a characteristic morphology. <Leland H. Hartwell> and colleagues used time-lapse observations to distinguish the time a function must act from the stage at which its absence becomes visible. Temperature-shift timing, DNA-content measurements and recovery after return to permissive conditions strengthen the interpretation. A delayed terminal <phenotype> need not identify the moment of the original molecular defect.<Genetic control of the cell-division cycle in yeast. I. Detection of mutants by Leland H. Hartwell and others>[Conditional mutants, execution points and termination phenotypes]{ref}
In <Schizosaccharomyces pombe>, elongated cells that continue growing without dividing are especially useful candidates; nuclear number, septum formation and <DNA> content separate blocks in replication, mitosis and division. <Paul Nurse>, <Pierre Thuriaux> and <Kim Nasmyth> isolated conditional mutants defining multiple essential functions. <Complementation testing> groups recessive <mutations> affecting the same function, and segregation analysis distinguishes linked from independent loci. Molecular identification, rescue and biochemical tests then establish what the affected products do.<Genetic control of the cell division cycle in the fission yeast Schizosaccharomyces pombe by Paul Nurse and others>[Conditional mutants affecting DNA synthesis, nuclear division and septation]{ref}
Three contrasting examples explain why similar-looking division blocks need not have the same cause. In fission yeast, <Cdc2>, the <cyclin-dependent kinase 1> catalytic subunit, is required for key cycle transitions. A restrictive loss-of-function allele can leave cells growing without entering mitosis; some alleles also reveal the earlier requirement at Start. <Cdc25> is an activating <protein phosphatase> that removes inhibitory phosphorylation from <Cdc2>. Loss of <Cdc25> therefore leaves the mitotic kinase inadequately activated and commonly produces an elongated G2-arrested <phenotype>. In budding yeast, <Cdc6> participates in <DNA replication licensing>: failure to establish competent origins prevents successful genome duplication and hence normal division. Species and allele must be stated: identically numbered CDC names in different yeasts are not automatically homologous <proteins>.
The importance for <cancer> is that an unbiased genetic screen exposed conserved mechanisms connecting growth, <DNA> duplication and division. The <yeast cross-species complementation experiment> showed that central regulatory functions could be conserved across large evolutionary distances. Human cancers can alter cyclin-CDK regulation, DNA-replication control and checkpoint responses, providing a mechanistic connection to uncontrolled proliferation and <genomic instability>. This does not mean that every yeast cdc gene is a human oncogene, nor that any observed arrest is necessarily a checkpoint response: direct loss of an essential activity can itself stop progression.
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