Cytokinesis and mitotic index

The big idea: Mitosis divides the nucleus into two identical nuclei. But the cell still has to split its cytoplasm (and everything in it) into two separate daughter cells.

That final step is called cytokinesis — the division of the cytoplasm.

It happens differently in animal cells and plant cells, because plant cells have a rigid cell wall and animal cells do not.

Cytokinesis

The division of a cell's cytoplasm to form two separate daughter cells, after the nucleus has divided by mitosis.

Cleavage furrow

A groove that forms in an animal cell's membrane and deepens until the cell is pinched in two.

Contractile ring

A ring of protein filaments just inside an animal cell's membrane that tightens to pull the membrane inwards.

Cell plate

A new wall built across the middle of a dividing plant cell, growing outwards until it separates the two daughter cells.

Daughter cells

The two cells produced when one parent cell divides.

Nucleus first, then cytoplasm: Mitosis sorts out the chromosomes; cytokinesis splits the cytoplasm.

They are two separate jobs: a cell could finish mitosis and still be one cell until cytokinesis physically pinches or walls it into two.

Both animal and plant cells must end up as two separate cells, but they get there in opposite ways.

An animal cell pinches inwards; a plant cell builds a new wall outwards.

Animal cells — a cleavage furrow pinches inwards: In an animal cell, a ring of protein filaments (the contractile ring) forms just inside the membrane around the middle of the cell.

This ring contracts and tightens, pulling the membrane inwards to make a deepening groove called a cleavage furrow.

The furrow gets deeper and deeper until the membrane meets in the middle and the cell is pinched into two separate daughter cells.

Plant cells — a cell plate builds outwards: A plant cell has a rigid cell wall, so it cannot pinch inwards.

Instead, small vesicles carrying wall material gather along the middle of the cell and fuse together to form a cell plate.

The cell plate grows outwards from the centre until it reaches the existing walls, forming a new wall and membrane that separate the two daughter cells.

Usually equal — but not always: In most divisions, the cytoplasm (and its organelles) is split roughly equally, so each daughter cell gets about the same amount.

The big exception is egg (gamete) formation. When an egg cell forms, the division is unequal: almost all the cytoplasm goes to one large egg cell, while the other products are tiny cells (polar bodies) with very little cytoplasm.

This unequal split makes sure the egg is packed with the nutrients a future embryo will need.

A memory hook: Animal = pinch in (furrow). Plant = wall out (plate).

And equal sharing is the rule — the egg is the famous unequal exception.

How this is tested: On Paper 1B a 2-mark Describe question can show a dividing animal cell (often a whitefish blastula) and ask how cytokinesis takes place — answer with the contractile ring and the cleavage furrow pinching the membrane inwards.

On Paper 1A a multiple-choice question may ask you to identify the exception to daughter cells receiving equal cytoplasm — the answer is egg / gamete formation.

On Paper 1B / Paper 3 a data question gives you cell counts from a root tip and asks you to calculate the mitotic index = cells in mitosis ÷ total cells.

✓ Why this scores full marks: It names the mechanism (contractile ring / membrane pinching inwards) and the result (cleavage furrow deepens until the cell splits).

A common slip is to describe a plant cell plate by mistake — the question said animal, so it must be a furrow, not a plate.

Now try the data skill — calculating a mitotic index from a count: