Final answer:
During metaphase, chromosomes line up in the middle of the cell and begin to be pulled apart by spindle fibers.
Step-by-step explanation:
The phase of mitosis during which chromosomes line up in the middle of the cell and begin to be pulled apart by spindle fibers is metaphase. During metaphase, the sister chromatids align at the equator of the cell, forming the metaphase plate. The spindle fibers attach to the centromere of each pair of sister chromatids, ensuring their separation during cell division.