Chromosomes are found in the cell nucleus. Each one of them contains a single extremely long DNA molecule that is packaged by various proteins into a compact domain. In a human cell nucleus there are 46 chromosomes, or 23 pairs; in every cell except the mature egg and sperm which have a set of only 23 chromosomes (haploid), half the number that is found in ordinary cells.
In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny.