The big idea: Cytokinesis usually shares the cytoplasm equally, but not always. Whatever the split, each daughter cell needs at least one mitochondrion.
The points to remember
- Equal cytokinesis: two daughter cells of about the same size. This is the usual case.
- Unequal cytokinesis: one daughter cell gets much more cytoplasm than the other.
- Human oogenesis (egg formation): the egg keeps most of the cytoplasm; the polar body gets little.
- Budding in yeast: a small bud grows out of the parent cell and separates.
- Every daughter cell must get at least one mitochondrion, because new mitochondria only come from old ones dividing.
Remember it as: Fair shares usually; the egg and the yeast bud are the exceptions.
Real example: the human egg. A human egg is about 0.1 mm across, one of the largest cells in the body and big enough to see without a microscope. The polar bodies made alongside it are tiny and soon break down.
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Organelles that cannot be made from scratch: A cell cannot build a mitochondrion (or, in plants, a chloroplast) from nothing. New ones form only when existing ones grow and divide, so each daughter cell must inherit at least one.
| Example | What happens | Why it is unequal |
|---|---|---|
| Oogenesis in humans | Meiosis makes one large egg and small polar bodies | The egg stores the cytoplasm and organelles an early embryo needs |
| Budding in yeast | A small bud grows from the parent cell and pinches off | The bud starts small and grows after it separates |
Unequal is not unfair: Even the tiny polar body and the yeast bud each get a nucleus and at least one mitochondrion.
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How it is asked: Usually a 1A item: which cell comes from unequal cytokinesis (an egg), or which example is the exception to equal sharing (yeast budding). Sometimes a short written answer on yeast reproduction.
IB-style questionOutline[2 marks]
Outline why cytokinesis is unequal when a human egg cell is formed.
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See the answer broken into clear points, with guidance on how each point earns marks.