Mitosis, whereby a cell divides to give rise to two identical daughter cells, with exactly the same chromosomes and DNA as the mother cell, is one of the most elegant and intricately regulated processes in biology. Each stage of mitosis is strictly controlled in time. Several temporal checkpoints have been identified, that stop an event from occurring before an earlier event has been completed. In the latest issue of the journal Science, a research team led by Helder Maiato, at the Instituto de Biologia Molecular e Celular (IBMC), in Porto, describe a spatial checkpoint that ensures that the nuclei of the two new daughter cells form only once the chromosomes are properly separated, so that no chromosome gets left behind.
During mitosis, the cell’s chromosomes align along the midzone of the cell, in pairs. Each pair is then pulled apart, and chromosomes move along protein cables to opposite poles of the cell. Helder Maiato and his team found that a gradient of the protein Aurora acts as a sort of ruler in the cell, monitoring the positions of chromosomes from the cell midzone and preventing premature formation of new nuclei. This novel surveillance mechanism allows correction and re-integration of lagging chromosomes in the new nuclei before the membranes that enclose these nuclei are completely formed.
When the Aurora gradient is inactivated, either globally or at specific points, the control switch between chromosome separation and new nuclei forming is broken, giving rise to chromosomal abnormalities such as nuclei with extra copies of chromosomes, or micro-nuclei within a cell. Either situation could give rise to serious disease situations, namely cancer, so unraveling these surveillance mechanisms is a crucial step in being able to control them in health and disease.