Final answer:
Sister chromatids are aligned at the center of the cell during metaphase, which is characterized by the chromosomes lining up on the metaphase plate. This stage is marked by maximally condensed chromosomes and is crucial for the following separation of chromatids in anaphase.
Step-by-step explanation:
When sister chromatids are aligned at the middle center of a cell, they are in the metaphase stage of cell division. During metaphase, the chromosomes line up on a plane called the metaphase plate, or the equatorial plane, which is situated roughly halfway between the cell's two poles. The sister chromatids are closely connected by cohesin proteins and are highly condensed, making them visible under a light microscope at this stage. Metaphase is a critical phase of both mitosis and meiosis, where the spindle fibers attach to kinetochores on the sister chromatids. This arrangement sets up the necessary conditions for the following stage, anaphase, when these connections will be broken, and sister chromatids will be pulled to opposite ends of the cell.