Cytokinesis and the significance of mitosis

After the nucleus divides, the cytoplasm must divide too: cytokinesis.

• In animal cells, a groove (furrow) appears in the plasma membrane and deepens until it meets in the centre, pinching the cell into two.
• In plant cells, the stiff cell wall cannot pinch in. Instead a cell plate forms in the centre of the cell and grows outwards until it meets the side walls. The cell plate becomes the middle lamella between the two new cells.

Organelles like mitochondria and plastids are shared out between the two daughter cells.

Sometimes the nucleus divides but the cytoplasm does not. The result is one big cell with many nuclei, a syncytium. The liquid endosperm (the water) of a tender coconut is an example.

Why mitosis matters:

• It makes two diploid daughter cells with identical genetic material.
• It lets many-celled organisms grow.
• It restores the right balance between the size of the nucleus and the cytoplasm (the nucleo-cytoplasmic ratio), which gets upset as a cell grows.
• It repairs and replaces cells: the outer layer of the skin, the lining of the gut, and blood cells are constantly renewed.
• In plants, mitosis in the meristems (at the tips, and the lateral cambium) lets them grow throughout life.

In some lower plants and in social insects (like male honey bees), even haploid cells divide by mitosis.