One primary oocyte makes one egg, not four
- The idea
- Predict
- See it
- Why
- Check
The idea
“Egg formation works like sperm formation, so one cell makes four eggs.”
You learned meiosis as one cell making four, and in sperm formation all four become sperm. So four eggs from one cell seems like the obvious answer.
The short version
The mistake: Egg formation works like sperm formation, so one cell makes four eggs.
What's actually true: Both divisions of egg formation are uneven, so one primary oocyte makes one egg. The small leftover cells are polar bodies.
- Sperm and egg formation both use the two divisions of meiosis. In sperm formation the splits are even, so you get four sperm.
- In egg formation both splits are uneven. The same big cell keeps nearly all the cytoplasm each time and ends up as the egg. That cytoplasm supplies the early embryo.
- The small cells are polar bodies. They get their share of chromosomes but almost no cytoplasm. The first one sometimes divides again, so there are two or three.
On the exam: One primary oocyte, one egg. The egg always comes from the big cell.