[Structure of chromatin and cell cycle. 3]

During the cell cycle, chromatin undergoes structural changes associated with cellular functions such as DNA replication during the S phase or the transcriptional inactivation during metaphase. All levels of chromatin organization from the nucleosome to the loops attached to the scaffold are concerned by these changes. Biochemical or biophysical studies from 70's, as well as more recent studies using nondestructive in situ methods, that allow to obtain information at the individual cell level, tend to demonstrate a correlation between the condensation state of the chromatin and the position of the cell in the cycle. The majority of these studies confirm the existence of one single cycle of condensation-decondensation for the chromatin. These studies are discussed in the first part of this chapter. In the second part, the role of the cdc (cell division cycle) gene's products in the control of the cell cycle and in the regulation of the chromatin structure are analyzed.