The structure of the sleeping genome: implications of sperm DNA organization for somatic cells

The tertiary structure of the DNA that makes up the eukaryotic genome is remarkably plastic, taking many different forms in response to the different needs of the cell. During the cell cycle of one cell, the DNA is replicated, reorganized into mitotic chromosomes, and decondensed into interphase chromatin. Within one cell at any given point in time, the chromatin is divided into hetero- and euchromatin reflecting active and inactive states of the DNA. This organization varies within one organism since different parts of the genome are active in different cell types. This article focuses on the most dramatic cell-type-specific DNA organization, that found in spermatozoa, in which the entire genome is reorganized into an inactive state that is more highly condensed than mitotic chromosomes. This unique example of eukaryotic DNA organization offers some interesting clues to the still unanswered questions about the role that the three-dimensional packaging of DNA plays in its function.