Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. They are made up of DNA tightly coiled around proteins called histones, which provide structural support. Chromosomes help ensure that DNA is accurately copied during cell division, which is essential for the proper functioning of an organism. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Chromosomes are not visible in the cell’s nucleus when the cell is not dividing, but during cell division, the DNA molecules become more tightly packed and are then visible under a microscope. Humans have 23 pairs of chromosomes altogether, with 22 pairs of numbered chromosomes called autosomes and one pair of sex chromosomes (XX or XY). The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones, which allows DNA molecules to fit inside cells. Chromosomes also have a constriction point called the centromere, which divides the chromosome into two sections or "arms". 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.