At metaphase, each chromosome has reached its maximum density. The homologous pairs and their sister chromatids also prepare for separation. They interact with spindle fibers which form from either side of the nuclear envelope of the cell. The spindle fibers are produced from a structure called a centrosome, composed of a pair of organelles called centrioles. The centrosome replicates and moves to opposite ends of the cell, which is referred to as poles. During metaphase, the chromosomes are lined by the spindle fibers at what is known as the metaphase plate. As in the cell picture, it looks like a line through the center of the cell.