At the start of mitosis, sister chromatids are held together by a complex of proteins. Separase is an enzyme that cleaves the complex, enabling the chromatids to separate during mitosis. Separase is overexpressed in many cancer cells, and scientists hypothesized that they might be able to slow or stop the growth of cancer cells by blocking the activity of separase. The scientists found a compound they named Sepin‑1 that appears to effectively cleave and thus inactivate purified separase protein in vitro (in a test tube). To test whether Sepin‑1 inhibits the growth of cancer cells, the scientists added increasing concentrations of Sepin‑1 to many different types of cancer cell lines growing in culture. A representative sample of the data they obtained is shown in Figure 1. The scientists also proposed to examine whether there is a relationship between the sensitivity of different types of cancer cells to Sepin-1, as measured by the concentration of Sepin-1 that caused 50% of the cells to die, and the relative concentration of separase in the different cell lines. A representative sample of the data is shown in Table 1. Identify the point in mitosis at which separase cleaves the protein complex that holds sister chromatid pairs together. In normal cells, separase is kept in an inactive state until it is needed. Explain how the progression of cells past sequential cell cycle checkpoints and the activity of enzymes such as separase is controlled by interactions between two major groups of regulatory proteins.

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