Mitosis is the mechanism that allows the nuclei of cells to split and provide each daughter cell with a complete set of chromosomes during cellular division. In this part of the Photo Gallery, we illustrate the various steps in mitosis that occur in onion root tips, which are relatively easy to capture in all stages. In the future, we hope to obtain higher quality images. A normal resting cell exists in a state called interphase in which the chromatin is undifferentiated in the heavily-stained nucleus, as illustrated above. Before the cell enters the mitosis phase, it first undergoes a synthesis or S phase where each chromosome is duplicated and consists of two sister chromatids joined together by a specific DNA sequence known as a centromere .
Centromeres are crucial to segregation of the daughter chromatids during mitosis.
The first phase of mitosis is known as the prophase , where the nuclear chromatin starts to become organized and condenses into thick strands that eventually become chromosomes. During this period the chromosomes continue to condense and gradually shorten and thicken until they have completely formed the units that will undergo mitosis. The nucleolus also disappears during this period. The mitotic spindle microtubules are now free to enter the nuclear region, and formation of specialized protein complexes called kinetochores begins on each centromere. These complexes become attached to some of the spindle microtubules, which are then termed kinetochore microtubules .