Every cell in your body came from another cell. You started as one cell, and everything since has been the same trick repeated: divide the nucleus, then divide the cytoplasm. This page is about that second half — splitting the cell itself in two.
📘 What you need to know
New cells only come from pre-existing cells. A dividing cell is the parent; the two it produces are daughter cells.
Two kinds of cell division exist: one gives daughter cells that are genetically identical, the other gives cells that are genetically different.
The nucleus divides first (nuclear division), then the cytoplasm divides. Dividing the cytoplasm is cytokinesis.
In animal cells a cleavage furrow forms, pulled inwards by a contractile ring of actin and myosin.
In plant cells a cell plate forms at the equator from fusing vesicles, and new cell walls are built.
Cytokinesis is usually equal, but not always — oogenesis and yeast budding are the two examples to know.
Where all your cells came from
Cell theory says new cells are produced from cells that already exist. A whole multicellular organism is the result of one single cell going through cycle after cycle of division.
That first cell produces embryonic stem cells, which then specialise to build every tissue and organ in the body. All of them, from neurones to skin cells, trace back to one starting cell.
Two types of cell division exist, and the difference is what the daughter cells end up like:
One produces daughter cells that are genetically identical to each other and to the parent cell.
The other produces cells that are genetically different from each other and from the parent. This one is a major source of genetic variation in populations.
You will meet those two as mitosis and meiosis on the next page. For now, just hold on to the idea that cell division comes in an “identical copies” version and a “shuffled” version, and that they exist for completely different reasons.
Nuclear division, then cytokinesis
The order here is fixed and examiners test it. During cell division the nucleus divides first. Once there are two nuclei, the cytoplasm divides in two, with one nucleus moving into each new cell.
The order never changes
nuclear division → cytokinesis → two daughter cells
Do it the other way round and you would produce an anucleate cell — a cell with no nucleus, and no instructions.
How the cytoplasm splits
The process is slightly different in animal and plant cells, and the reason is the cell wall. An animal cell can be squeezed inwards; a plant cell cannot, so it has to build a new wall through the middle instead.
The cell wall is the whole reason for the difference. You cannot pinch a box in half, so a plant builds a partition instead.
In animal cells
Proteins called actin and myosin form a contractile ring just under the plasma membrane, at the equator of the cell.
As those proteins contract, the ring tightens and pulls the plasma membrane inwards.
The inward dent is the cleavage furrow. It deepens until the cell is separated into two daughter cells.
In plant cells
A cell plate forms at the equator. It is the precursor to the new cell wall.
The plate is built from vesicles carrying carbohydrates, lipids and proteins, which fuse together to create the two new plasma membranes.
More vesicles then deliver pectin and cellulose by exocytosis into the gap between those two membranes, and these become the new cell walls.
Once the plate reaches the existing cell walls of the parent cell, the daughter cells are fully separated.
Equal and unequal cytokinesis
Usually the cytoplasm splits roughly down the middle, giving two daughter cells of a similar size. That matters for more than tidiness:
Each daughter cell must receive at least one mitochondrion, or it cannot respire.
A plant daughter cell also needs at least one chloroplast, or it cannot photosynthesise.
These organelles can only be made by dividing an existing one, so a cell that ends up with none can never make more.
There are cases where the split is deliberately unequal. The two to know are oogenesis in humans and budding in yeast.
Oogenesis
Egg production starts before a girl is even born. Germinal epithelial cells in the ovaries of the female foetus divide to form immature ova called primary oocytes.
At puberty, a primary oocyte divides into a secondary oocyte and a much smaller polar body.
The secondary oocyte later divides again to give the ovum and a second polar body.
The polar bodies degenerate.
An ovum has to carry enough cytoplasm and organelles to support an early embryo. Splitting evenly would leave two cells that were both too small to do the job.
Worked examples
WORKED EXAMPLE
Explain the order of events
Explain why cytokinesis must take place after nuclear division rather than before it.
Step 1: What cytokinesis does
It divides the cytoplasm into two, producing two separate cells.
Step 2: What would go wrong
If the cytoplasm split first, there would be only one nucleus to share between two cells.
Step 3: Name the consequence
One daughter cell would be anucleate — it would have no genetic instructions and could not function.
Two nuclei must exist before the cell splitsThe word anucleate is worth using; it shows you know the specific term.
WORKED EXAMPLE
Compare animal and plant cytokinesis
State two differences between cytokinesis in an animal cell and in a plant cell.
Difference 1: The structure formed
Animal cells form a cleavage furrow; plant cells form a cell plate.
Difference 2: The direction of the process
In an animal cell the membrane is pulled inwards from the edges; in a plant cell the plate is built outwards from the centre towards the existing walls.
Cleavage furrow inwards; cell plate outwardsA third one if you need it: only plant cells build a new cell wall.
WORKED EXAMPLE
Explain an unequal division
During oogenesis the cytoplasm divides unequally. Suggest an advantage of this.
The observation
Almost all the cytoplasm goes into one cell; the polar bodies get very little.
The reason
The ovum needs a large store of cytoplasm, nutrients and organelles to support the zygote and early embryo after fertilisation.
The alternative
An equal split would give two cells, neither with enough cytoplasm to do that job.
One well-supplied ovum instead of two poor ones“Suggest” means apply your knowledge — a sensible, reasoned answer earns the mark.
💡 Exam tip
Cytokinesis means cytoplasm only. If a question asks about cytokinesis, do not describe chromosome movement.
Name the proteins: actin and myosin in the contractile ring. Vague answers about “proteins” lose the detail mark.
For plant cells, mention vesicles fusing and the deposition of pectin and cellulose.
Remember each daughter cell needs at least one mitochondrion — and a chloroplast too, in plants.
Learn the two unequal examples: oogenesis and budding in yeast.
Use the terms parent cell and daughter cells rather than “old cell” and “new cells”.
⚠ Common mix-up
Confusing cytokinesis with nuclear division. One splits the cytoplasm; the other splits the nucleus.
Saying plant cells form a cleavage furrow. The cell wall makes that impossible.
Thinking the cell plate is the cell wall. It is the precursor; the wall is built from what the vesicles deliver.
Assuming cytokinesis is always equal. Oogenesis is the classic exception.
Forgetting the organelles. Mitochondria and chloroplasts must be shared out, because new ones only come from existing ones.
Saying cells can be made from nothing. New cells always come from pre-existing cells.
Up next: Nuclear Division — the two ways a nucleus can divide, and what has to happen to the DNA before either of them can start.
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