The answer is anaphase.
Ron is obviously observing cell division - mitosis. Mitosis consists of four phases - prophase, metaphase, anaphase, telophase and cytokinesis.
1. Prophase: Chromatin in the nucleus condenses and chromosomes pair up
2. Metaphase: Chromosomes line up at the center of the cell. The sister chromatids are joined together.
3. Anaphase: The sister chromatids separate from each other to the opposite sides of the cells.
4. Telophase and Cytokinesis: Membrane forms around each set of chromosomes on two opposite sides of the cells and cell divides into two identical daughter cells.
Therefore, sister chromatids being are pulled to the opposite ends of the cell in anaphase.