Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. Chromosome analysis is undertaken to study the chromosomes. The chromosomes only become visible under the microscope during cell division, when the DNA becomes more tightly packed. Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or "arms." The short arm of the chromosome is labeled the "p arm." The long arm of the chromosome is labeled the "q arm." The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes. One pair of chromosomes, the sex chromosomes, determines the sex of an individual. These are XX in women and XY in men. The remaining 22 pairs of non-sex chromosomes (numbered 1 to 22) are known as the autosomes. Genes are arranged in a specific sequence on the chromosomes.