A human cell contains an enormous 1.8 metres of DNA partitioned into 46 chromosomes. These have to be copied and distributed equally into two daughter cells at every division. Condensation, the shortening of chromosomes, allows the cell to handle such huge amounts of genetic material during cell division and helps preventing fatal defects in chromosome separation. Now researchers from the European Molecular Biology Laboratory (EMBL) for the first time tracked chromosome condensation in mammalian cells over the entire course of cell division. In this weeks advanced online publication of Nature Cell Biology they report crucial new insights into timing, function and molecular basis of chromosome condensation.
What happens when chromosomes are not correctly separated and distributed during cell division we know very well; two daughter cells with either broken chromosomes or different numbers of chromosomes result and severe diseases including cancer can arise. But so far we know only little about condensation, a process crucial to the successful separation of chromosomes. Using powerful microscopes, researchers led by Jan Ellenberg at EMBL looked at living mammalian cells to find out how and when chromosomes shorten during cell division.
Condensation begins early, when the cell starts preparing for division, and the chromosomes become shorter and shorter until they are about to separate and migrate towards the poles of the cell.
It is at this stage that textbooks say chromosomes are shortest. Then, after separating they would expand again, says Ellenberg. But we found something very different. Shortly after they finish separating, chromosomes actually condense even further. This makes sense, because in this way they are shortest when the physical division of one cell body into two takes place. Like this, no long chromosome arms extend over the plane of division, because that could expose the DNA to serious mechanical damage.