Meiosis
Introduced| English |
|---|
| meiosis/meɪˈəʊsɪs/ |
| diploid/ˈdɪplɔɪd/ |
| haploid/ˈhæplɔɪd/ |
| gamete/ˈɡæmiːt/ |
Making sex cells
- Body cells divide by mitosis to make identical copies.
- But sex cells — sperm and eggs — need something different.
- When two of them join, the chromosome number must stay correct.
- The special division that makes them is called meiosis.
Diploid and haploid
- Most of your cells are diploid 二倍体 — two sets of chromosomes, one from each parent.
- A gamete 配子 (sperm or egg) must be haploid 单倍体 — just one set.
- That way, sperm and egg together rebuild the full double set.
- Meiosis is what halves the chromosome number to make gametes.
What does meiosis produce?
Meiosis makes gametes — sex cells with half the normal chromosome number.
Meiosis: divide by two, twice
- Meiosis 减数分裂 makes gametes by dividing twice.
- First the DNA is copied, once.
- Meiosis I separates the matching pairs of chromosomes.
- Meiosis II separates the sister chromatids — giving four haploid gametes.
How meiosis halves the chromosomes
Step through meiosis — one cell divides twice to make four gametes, each with half the chromosome number.
A haploid cell has…
A haploid cell has one set; a diploid body cell has two. Gametes are haploid.
Human body cells have 46 chromosomes. How many are in a human gamete?
Meiosis halves 46 to 23, so a sperm or egg carries 23 chromosomes.
Fertilization restores the set
- A haploid sperm joins a haploid egg at fertilization.
- One set plus one set makes a full diploid cell again.
- That first cell then grows by mitosis into a whole organism.
- Meiosis and fertilization keep the chromosome number stable across generations.
Meiosis involves two divisions, producing four cells from one.
Meiosis I and meiosis II together split one cell into four haploid gametes.
Select all true statements about meiosis.
Making two identical body cells is mitosis, not meiosis. The other three are correct.
Do not confuse meiosis with mitosis. Mitosis makes two identical, full (diploid) cells. Meiosis makes four gametes with half the chromosomes (haploid). Same start, very different results — remember: meiosis reduces.
Human chromosome maths:
- Your body cells each hold 46 chromosomes (diploid).
- Meiosis halves this, so each sperm or egg has 23 (haploid).
- At fertilization, $23 + 23 = 46$ — the full set is restored in the new cell.
Meiosis makes haploid gametes from a diploid cell by dividing twice, halving the chromosome number. At fertilization two gametes combine to restore the full diploid set, keeping the chromosome number stable from one generation to the next.