Meiosis and Why Gametes Are Haploid
Meiosis halves the chromosome number and makes four haploid cells that usually differ, so fertilisation puts the full set back. The contrast with mitosis is what explains why offspring vary and clones do not.
What a learner can do afterwards
- States how many cells meiosis makes and how their chromosome number compares with the parent cell.
- Explains what fertilisation restores, and why the number would otherwise double every generation.
- Names the points in meiosis that make the four cells genetically different from each other.
1 · Read
Remember the egg released each month. That egg is a gamete with one set of chromosomes, while your body cells carry two sets of chromosomes, one from each parent. Meiosis is the division that halves the number, making gametes with one set each. At fertilisation the two halves meet and the full double set is restored.
Without halving, the number would double every generation: two sets plus two sets makes four, then eight. Halving first keeps every generation at two sets. That is why gametes must be haploid.
One DNA doubling comes first, then two divisions follow. The first division splits partner chromosomes apart. The second splits sisters apart. Mitosis has no partner swaps. It copies sisters evenly.
Meiosis shuffles as it splits. Partners swap segments, then line up at random. That makes matching gametes unlikely, though never impossible. Those new combinations are the variety that natural selection acts on.
Mitosis keeps the starting set number. In our body-cell model it runs once and makes two identical diploid cells for growth and repair. Sperm-making follows the four-product model. Human egg-making keeps one working egg from the same steps.
Meiosis doubles once, splits twice and shuffles, and fertilisation restores the full set.
Original readings written for this topic. Video from Amoeba Sisters; terms unknown.
2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.