This soccer player's knee will be as good as new in a few weeks, thanks to mitosis, a type of cell division that generates new cells for growth and repair. Let's move into a cell to witness the events of mitosis. Before a cell can divide, it must first duplicate the chromosomes stored in its nucleus. During chromosome duplication, several "bubbles" open up along the chromosome. Each bubble grows until it merges with an adjacent bubble. Each chromosome now consists of two identical copies called sister chromatids. Getting closer, we see that each sister chromatid consists of DNA wound around small proteins called histones. The sister chromatids begin to coil into tight helical fibers. Outside the nucleus, centrosomes that duplicated earlier move away from each other to opposite sides of the cell. Microtubules extend from the centrosomes, forming the mitotic spindle. Back in the nucleus, the DNA forms loops, becoming more compacted. These structures fold back on themselves, eventually condensing into a shorter and thicker chromosome consisting of two sister chromatids. As the chromosomes continue to condense, the nuclear envelope breaks up. The array of spindle microtubules is now extensive and the chromosomes are fully condensed. Spindle fibers from each pole attach to protein structures located at the centromere of each sister chromatid. As the chromosomes are bound by spindle fibers from opposite poles, they move first one way and then another. The counteracting forces of the spindle eventually cause all the chromosomes to end up at the center of the cell, as if arranged on an imaginary plate. The sister chromatids are released from each other, each becoming a full-fledged chromosome. They are moved toward opposite poles of the cell, pulled along the spindle fibers attached to them. At the same time, overlapping spindle fibers that are not attached to chromosomes continue to lengthen, pushing the poles farther apart. Once the chromosomes arrive at their destination, they become less condensed. Two new nuclear envelopes form, completing mitosis, the division of one nucleus into two genetically identical daughter nuclei. The cytoplasm divides by the process of cytokinesis, forming two separate daughter cells. In your body, millions of cells divide every second, providing new cells for growth and for repair of damaged cells. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
