Meiosis is two divisions that turn one diploid cell into four haploid cells. The fastest way to sequence the stages is to ask what is separating.
This lesson builds on sequencing mitosis and belongs to cell division. Keep the mitosis table open while you read.
What separates in each division?
In the first division, homologous pairs separate. In the second, sister chromatids separate. That single difference explains the halving and the four cells.
| Stage | What happens |
|---|---|
| Interphase | DNA replicates once |
| Prophase I | Homologous chromosomes pair up, crossing over occurs |
| Metaphase I | Pairs line up at the equator |
| Anaphase I | Homologous chromosomes move to opposite poles |
| Telophase I | Two cells form, each with half the number |
| Meiosis II | Sister chromatids separate, as in mitosis |
| End | Four haploid cells |
Worked example: a cell with four chromosomes
Question. A cell has 4 chromosomes, made of 2 homologous pairs. State the chromosome number at the end of meiosis I and meiosis II, and the number of cells. (3 marks)
Step 1. Interphase replicates the DNA. The cell still has 4 chromosomes, each with two chromatids.
Step 2. In meiosis I, each homologous pair separates. Each of the two cells receives 2 chromosomes, still made of two chromatids.
Step 3. In meiosis II, the chromatids separate. Each of the four cells has 2 chromosomes.
Answer: after meiosis I, 2 chromosomes in each of 2 cells. After meiosis II, 2 chromosomes in each of 4 cells. The number halved once, in meiosis I.
The mistake that costs marks
A common slip is to say the number halves again in meiosis II and write 1 chromosome. It does not. In meiosis II, chromatids separate, and the number of chromosomes per cell stays at 2.
Another slip is to say homologous chromosomes separate in meiosis II. If the chromosomes at the poles are still double-stranded, the cell is in meiosis I.
Check yourself
A human body cell has 46 chromosomes. State the chromosome number in each cell at the end of meiosis I and at the end of meiosis II.
Answer
At the end of meiosis I, each cell has 23 chromosomes, each with two chromatids. At the end of meiosis II, each of the four cells has 23 chromosomes.
The halving happens only at meiosis I.
What to study next
Track numbers across both processes in comparing chromosome and DNA changes, then see how the stages create differences in explaining variation from meiosis. If you would like a teacher to draw the stages with you, see online one-to-one Biology tuition.