Chromosomes change form as a cell divides to ensure that each daughter cell gets a full, intact copy of the genome.
A two-cell stage embryo of the nematode worm (Caenorhabditis elegans) is caught in the act of dividing into four cells. The chromosomes at the center of each cell have already duplicated into two identical sets and are tightly packed, or condensed, into the familiar four-armed shapes that can be seen with a light microscope. The two cells are at two slightly different stages of cell division (or mitosis). The left cell’s paired chromosomes are perfectly lined up in the middle, ready to be pulled apart to opposite sides of the cell as it divides into two. The chromosomes of the right cell have yet to line up. Different proteins inside the cells make sure that the identical chromosome copies segregate perfectly. The protein condensin (colored red on the blue chromosomes) condenses and detangles chromosomes prior to mitosis. Another protein, tubulin (green), forms fibers that act like railroad tracks to direct each set of chromosomes to one half of the dividing cell. The embryo is around 50 micrometers in length—about half the width of human hair.
The two-cell nematode embryo was stained with different colors to see various components of the cells: condensin (red), microtubules (green), and chromosomes (blue). The stained embryos were viewed using a confocal microscope.
Image courtesy of HHMI investigator Barbara Meyer, PhD, University of California, Berkeley