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Fig. 2. Cells lacking Ark1 function have defects in spindle formation, chromosome segregation, chromosome resolution during anaphase and cell cycle progression. Spores from diploid strain IH1877 were germinated in minimal medium supplemented with all amino acids except leucine in order to select for those spores containing the ark1.{Delta}1 allele. 24 hours after germination cells were stained to reveal microtubules and chromatin. (A-C) The first panel in each series bears the label and shows chromatin, the next microtubules, the third a combined chromatin and DIC image. (A) Chromosome resolution is defective in ark1.{Delta}1 cells. Chromatin is stretched out along the spindle. (B) ark1.{Delta}1 cells have abnormal mitotic figures in which condensed chromosomes associate with a normal spindle. In each case the chromosomes have not split into two sister chromatids. In the left and middle cells the chromosomes are clustered at one end of the spindle. In the cell on the right, one chromosome is at either end and one is in the middle of the spindle. As the spindle on the right is elongating the chromosomes should already have split into two sister chromatids and moved to the spindle poles. The fact that a chromosome is lagging on this spindle and that one is apparently unattached to the spindle (arrow) indicates that kinetochore function is compromised when Ark1 function is compromised. (C) Ark1 is required for spindle formation. Instead of forming a bipolar spindle, microtubules extend from a single focus around which the chromosomes cluster. These figures became prominent at later time points when the germination was at 30°C, suggesting that spindle formation does not require as much Ark1 activity as chromosome resolution in anaphase B, the first defect to be seen. (D) ark1.{Delta} 1 cells exhibit a novel cdc– phenotype. From left to right; DAPI, microtubules, DAPI/DIC. An extensive microtubule network extends between the cell tips while a small compact nucleus is present in the middle of the cell. Note that this panel is printed to 80% of the scale of the others in the figure.