Investigating the hows and whys of DNA endoreduplication

Endoreduplication is a form of nuclear polyploidization that results in mul tiple, uniform copies of chromosomes. This process is common in plants and animals, especially in tissues with high metabolic activity, and it general ly occurs in cells that are terminally differentiated, In plants, endoredup lication is well documented in the endosperm and cotyledons of developing s eeds, but it also occurs in many tissues throughout the plant. It is though t that endoreduplication provides a mechanism to increase the level of gene expression, but the function of this process has not been thoroughly inves tigated. Numerous observations have been made of endoreduplication, or at l east extra cycles of S-phase, as a consequence of mutations in genes contro lling several aspects of cell cycle regulation. However, until recently the re were few studies directed at the molecular mechanisms responsible for th is specialized cell cycle. It is suggested that endoreduplication requires nothing more elaborate than a loss of M-phase cyclin-dependent kinase activ ity and oscillations in the activity of S-phase cyclin-dependent kinase.