Final answer:
During metaphase of mitosis, the chromosomes line up along the equator of the cell, also known as the metaphase plate.
Step-by-step explanation:
During metaphase of mitosis, what lines up along the equator of the cell, also known as the metaphase plate, are the chromosomes. Specifically, these are the sister chromatids that are aligned in preparation for separation. To break down the process, spindle fibers attach to the centromere of each pair of sister chromatids, and then the sister chromatids organize themselves at the center of the cell, forming the metaphase plate. The spindle fibers play an essential role as they will pull the sister chromatids apart into different daughter cells during cell division.