Polo-like kinase essential for mitotic cell cycle; controls targeting, activation of Rho1p at cell division site via Rho1p guanine nucleotide exchange factors; regulates shape of nucleus, expansion of nuclear envelope during mitosis; implicated in cell divisions in telomerase-negative strains through adaptation to DNA damage, which allows bypass of cell cycle arrest and contributes senescence-specific genome instability; human homologs PLK1, PLK3 can each complement cdc5 thermosensitive mutants