When a cell is about to divide, the chromosomes have coiled and thickened and are visible. At other times in the cell's life cycle, they are not able to be visualized. A cell can be arrested during cell division by applying colchichine. The chromosomes are stained, usually with Giemsa stain. When preparing a karyotype, a cell's 23 pairs of chromosomes are microphotographed and a chart is made with the chromosomes arranged by size and position of centromeres for pairs 1 through 22. The 23rd pair is the sex chromosomes which are either XX or XY.