Final answer:
During metaphase, the sister chromatids align in the middle of the cell along the metaphase plate, positioned for subsequent separation.
Step-by-step explanation:
The phase of mitosis where the chromosomes line up in the middle of the cell is called metaphase. During this stage, the sister chromatids are maximally condensed and aligned along the metaphase plate, which is an imaginary plane located at the equator or middle of the cell. The sister chromatids, each connected to microtubule spindles from the centrosomes at opposing poles, are prepared for separation into two new cells.