During mitosis of eucaryotic cells, the duplication and separation of the centrosome is crucial for proper chromosome segregation. I propose to characterize rosario, a Drosophila mitotic mutant identified in Pr. D. Glover's laboratory. Embryos laid by homozygous female for rosario fail to develop and show bipolar mitotic spindles nucleated by clusters of centrosomes. rosario mutants show centrosome separation defects without duplication failure, as well as chromatid separation defects at anaphase. I will determine whether the primary mitotic defect is in chromosome segregation or in centrosome separation, using real time observations. Furthermore, I will map the transcription units within an interval of 25kb of genomic DNA in which rosario lies and functionally identify rosario, by testing the ability of genomic fragments from this region to rescue the mutation following gerrn line transformation. Characterization of the rosario gene and its associated phenotype will help to understand relationships of the centrosome replication cycle to the process of chromosomes segregation.