At the Institut Curie, the CNRS team of Genevieve Almouzni has just discovered how the protein Asf1 ensures the correct (re)organization of duplicated DNA. During DNA replication, all the information in the mother cell must be transmitted to the daughter cells.
The DNA must be faithfully copied, of course, but also properly organized within the cell. DNA is wrapped around proteins called histones, to form chromatin. This complex structure contains so-called epigenetic information, which governs gene expression and gives each cell its specific identity. The histone chaperone, Asf1, coordinates the removal of histones from the chromatin to allow the replication machinery to move along the DNA, with the supply of new histones to reform the chromatin once the replication machinery has passed. This discovery sheds new light on the transmission of epigenetic information in cells, and was published in the 21 December 2007 issue of Science.
DNA inherited from both parents is copied during each cell division and transmitted to all cells. Each of our cells therefore contains the same genetic information. So, what is the difference between a neuron and a white blood cell? The difference lies in the fact that although every cell in our body has the same number of genes, only some of these genes are active in any given cell. Depending on cell type, certain genes are “locked” to prevent their expression. Information on the locking and unlocking of genes is essential for cell function, and is not carried in the genes themselves but by epigenetic factors. These can be chemical modifications(2) or the organization of the DNA within the cell. The DNA double helix (diameter 2 nanometers) is wrapped around histones, proteins that facilitate its compaction, to form nucleosomes, which are strung along the DNA like beads on a string. This bead necklace then folds on itself to form a fiber—chromatin.