cytokinesis
/ sy-toh-kih-NEE-sis /
Sorting the contents of a shared room into two halves is only half the job — you still have to build a wall down the middle so each half becomes its own separate space. A cell faces the same final task after it has separated its chromosomes: it has two complete sets of DNA, but they are still sitting in one big cell. Cytokinesis is the final physical act of dividing one cell's body into two, each with its own copy of the genome.
Cytokinesis means 'cell movement/division' and usually overlaps with the end of mitosis. How it happens differs between cell types. In animal cells, a ring of the protein actin and myosin assembles just under the membrane around the cell's equator and contracts like a drawstring, pinching the cell inward and forming a cleavage furrow that deepens until the cell splits in two. Plant cells cannot pinch, because they are boxed in by a rigid cell wall; instead, vesicles carrying wall material gather at the center and fuse to build a new partition, the cell plate, that grows outward until it joins the existing wall.
Cytokinesis matters because it completes the division and ensures each daughter receives not just DNA but a fair share of cytoplasm, organelles, and membrane. It must be carefully timed and positioned: dividing in the wrong place would leave one daughter without a nucleus. A common misconception is that cytokinesis and mitosis are the same thing — they normally happen together, but they are separate, separately controlled events, and some cells deliberately do mitosis without cytokinesis, ending up with multiple nuclei in one cell.
Watch a fertilized frog egg divide and you can see the cleavage furrow with the naked eye: a groove forms across the surface and tightens like a tied shoelace, slicing the egg into two cells, then four, then eight.
The cleavage furrow tightens like a drawstring to split an animal cell.
The deep difference between animal and plant cytokinesis comes from the cell wall: animals pinch inward with a contractile ring, while plants build a new wall outward with a cell plate because their rigid wall cannot be squeezed.