How do chromosomes change during cell division?

Chromosomes can be either single armed or double armed. When a cell is about to divide, a chromosome replicates its genetic material to become a double armed chromosome. A double armed chromosome has a centromere and two sister chromatids. During mitotic cell division these double armed chromosomes line up along the axis as the new cell forms. When the cell divides into two daughter cells, the centromeres of the chromosomes divide too. The sister chromatids are separated and two daughter cells with equal number of chromosomes, the same as the parent cell, are produced. It's important to remember that a chromosome can be either single or double armed, depending of which cell cycle stage its in, but it is still just one chromosome.