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Animation of human chromosomes. Chromosomes are structures containing an organism's DNA genetic material, which form during cell division. DNA usually exists as a long strand in the cell's nucleus, and it is replicated in this form. During replication, another copy of the DNA is produced. In preparation for mitotic cell division, the DNA condenses into chromosomes, a tightly-packed form of DNA shaped like an X. The X is formed of the two identical copies of the genetic material left after replication. These are linked by a central region called the centromere. During mitosis, the two halves of the chromosome (chromatids) separate at the centromere, and each half is pulled to opposite sides of the cell. This then divides, giving rise to two identical daughter cells. Humans have 23 pairs of chromosomes, half of each pair from the mother and father. This animation is also available on a black background (K005 5517) and a transparent background (K005 5521).