Every cell in your body came from another cell. You started as one, and everything since has been copying. But splitting a cell in two is a physical problem as well as a genetic one — and plants and animals solve it in completely different ways.
📘 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 types of cell division exist: one gives genetically identical daughter cells, the other gives genetically different ones.
The nucleus divides first (nuclear division), then the cytoplasm divides. Dividing the cytoplasm is cytokinesis.
In animal cells, a cleavage furrow forms: actin and myosin make a contractile ring at the equator that pulls the membrane inwards.
In plant cells, a cell plate forms at the equator from fusing vesicles, and new cell walls are built outwards.
Cytokinesis is usually equal, and each daughter cell must receive at least one mitochondrion (and a chloroplast in plants).
Unequal cytokinesis happens in oogenesis in humans and in budding in yeast.
Parent, daughters, and two kinds of division
A multicellular organism is the result of one cell dividing over and over again. That first cell forms embryonic stem cells, which then specialise into the tissues and organs of the body.
Two types of cell division exist, and the difference is what the daughter cells inherit:
One produces cells that are genetically identical to each other and to the parent. This is how organisms grow, repair and replace cells.
The other produces cells that are genetically different from each other and from the parent. This is a major source of genetic variation within populations.
Those two are mitosis and meiosis, and the next page deals with them properly. For now, just hold on to the difference: identical copies, or deliberately different ones.
Nucleus first, cytoplasm second
The order matters. During cell division the nucleus divides first — that is nuclear division. Once there are two nuclei, the cytoplasm splits so that one nucleus ends up in each new cell. That second step is cytokinesis.
Why this order? If the cytoplasm divided first, one daughter cell would be left with no nucleus at all — an anucleate cell, with no instructions and no future. Dividing the nucleus first guarantees each cell gets a complete set.
Cytokinesis in animal and plant cells
Both cell types end up with two daughter cells, but they get there in opposite directions: animals pinch inwards, plants build outwards.
Animal cells
A cleavage furrow forms and separates the daughter cells.
It appears because actin and myosin proteins form a contractile ring just under the plasma membrane, at the equator of the cell.
As those proteins contract, they pull the membrane in towards the centre until the cell is separated in two.
Plant cells
A cell plate — the precursor of a new cell wall — forms at the equator.
It is built from vesicles carrying carbohydrates, lipids and proteins, which fuse together to create the two new plasma membranes.
Other vesicles then deposit pectin and cellulose by exocytosis into the gap between those membranes, forming the new cell walls.
Once the cell plate reaches the parent cell’s walls, the daughter cells are separated.
A rigid cellulose wall cannot be squeezed inwards, so a plant cell has to construct a new wall across the middle instead.
Equal and unequal cytokinesis
Normally the cytoplasm divides roughly equally, giving two daughter cells of similar size. That is not just tidiness — each daughter cell needs at least one mitochondrion so it can respire, and a plant cell needs at least one chloroplast so it can photosynthesise. These organelles can only be made by dividing a pre-existing one, so a cell that receives none can never make them.
Sometimes, though, the division is deliberately uneven. Two examples you should know are budding in yeast and oogenesis in humans.
Oogenesis
Ova production begins in the ovaries of a female foetus, before birth. Germinal epithelial cells divide to form an immature ovum called a primary oocyte.
At puberty, the primary oocyte divides into a secondary oocyte and a much smaller polar body. The cytoplasm has been divided very unequally.
The secondary oocyte divides again to form an ovum and another polar body.
The polar bodies degenerate.
The ovum keeps the cytoplasm because it will have to supply everything a zygote needs in its first days. The polar bodies simply carry away spare chromosomes.
Worked examples
WORKED EXAMPLE
Why plants do it differently
Explain why a plant cell cannot divide its cytoplasm by forming a cleavage furrow.
Step 1: Identify the structural difference
A plant cell has a rigid cellulose cell wall outside the plasma membrane.
Step 2: Apply it to the mechanism
A contractile ring works by pulling the membrane inwards, but the wall cannot be pulled in.
Step 3: Give the alternative
Instead a cell plate forms at the equator and new membranes and walls are built outwards.
The rigid cell wall cannot be constricted, so a cell plate is built insteadName the wall material — “cellulose” often carries the mark.
WORKED EXAMPLE
Sharing out the organelles
Explain why it is important that each daughter cell receives at least one mitochondrion.
Step 1: State the organelle’s job
Mitochondria carry out aerobic respiration, supplying the cell with ATP.
Step 2: State how new ones are made
Mitochondria can only form by dividing a pre-existing mitochondrion.
Step 3: Put the two together
A daughter cell with none could never make any, so it could not release enough energy to survive.
No mitochondrion means no way to make one, and no ATP supplyThe same argument applies to chloroplasts in plant and algal cells.
WORKED EXAMPLE
The point of an unequal split
Suggest an advantage of the unequal division of cytoplasm during oogenesis.
Step 1: Say what the ovum receives
Almost all the cytoplasm and organelles end up in one cell.
Step 2: Link to what happens next
After fertilisation the zygote divides repeatedly before it can obtain nutrients from elsewhere.
Step 3: State the advantage
A large store of cytoplasm supplies those early divisions with materials and energy.
The ovum keeps the resources the early embryo will need“Suggest” means a sensible biological reason, not a memorised fact.
💡 Exam tip
Get the order right: nuclear division always comes before cytokinesis.
Name the proteins in the animal mechanism: actin and myosin forming a contractile ring.
For plants, name the structure (cell plate) and where the material comes from (vesicles).
Use the terms parent cell and daughter cells rather than “old” and “new” cells.
Remember the two unequal examples: oogenesis in humans and budding in yeast.
If asked why equal division matters, mention organelles, not just size.
⚠ Common mix-up
Using “cell division” and “cytokinesis” as synonyms. Cytokinesis is only the cytoplasm part.
Saying the cell plate is a cell wall. It is the precursor; the walls are deposited afterwards.
Thinking plant cells have a cleavage furrow too. They do not — the wall prevents it.
Assuming cytokinesis is always equal. Oogenesis and yeast budding are the exceptions to quote.
Forgetting the polar bodies degenerate. They are not functional gametes.
Saying organelles are built from scratch. Mitochondria and chloroplasts come from existing ones.
Up next: Nuclear Division — we have split the cytoplasm, so now for the harder half: how the nucleus divides, and why the DNA has to be copied before any of it can start.
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