Chromosomes line up in the center of a cell during the metaphase portion of mitosis. Metaphase is the second subphase of mitosis, or the events occurring during the M phase of the cell cycle.
Mitosis is the process used by most of the body's cells in order to divide or reproduce. It consists of four, distinct subphases which are, in order, prophase, metaphase, anaphase and telephase. During each of these phases, as well as during its inactive, non-reproducing time, or interphase, the cell's chromosomes behave in a specific way to allow for proper reproduction and function throughout the body.
As a cell moves from the inactive interphase into prophase, its chromosomes condense and become visible within the nucleus. However, at this time, the chromosomes are in constant motion without a clear orientation.
As the cell moves to metaphase, two specific structures, called centrioles, take position on opposite sides or poles of the nucleus. Then, during metaphase, all the condensed chromosomes follow the centrioles' lead and begin to line up across a geometric plane known as the metaphase plate. This plane is essentially a line down the center of the cell which designates where it will divide in the ensuing phases of mitosis.
Once established, chromosomes take their place on opposing sides of the metaphase plate (anaphase). They eventually reach opposite poles of the nucleus where new membranes form around them (telephase). The result is two identical cells, and the process of mitosis is complete.