Each eukaryotic chromosome contains a single uninterrupted molecule of DNA tightly bound to a group of small, basic proteins called histones. Chromosomes consist of approximately half DNA and protein. This composite material is called chromatin. The primary structure of histones contains many amino acids with basic side chains. Through post-translational, reversible modifications such as acetylation, methylating, ribosylation, and phosphorylation, the properties of histones may be modulated which seems to play a roll in regulating the availability of DNA for transcription and replication. Nucleosomes are the repeating units of the chromatin fiber consisting of eight histones and about 200 base pairs. The nucleosomes are themselves further packed in higher order supercoils. During the resting phase of the cell cycle, the chromosomes are predominantly unwound. As cell division approaches, the chromosomes condense, assuming flexible rod-like shapes. This permits a more orderly apportionment of genetic material among the two daughter cells.