A chromosome is a structure formed from DNA. Each chromosome has a different shape. We have 23 pairs in the nucleus of each of our cells, 22 of which are common to both sexes. The remaining two chromosomes are the sex chromosomes. In women these form a pair. They are known as the X chromosomes. In men they are different: one is an X chromosome and the other, far shorter, is called the Y chromosome.
Chromosomes can particularly be seen at the time of cell division and differ in number depending on the species. The number of chromosomes varies in some forms of sick cells, particularly in cancer cells, where generally the number of chromosome increases.