Key ideas
- Diploid (2n) cells have two sets of chromosomes, one from each parent. Human body cells are diploid: 2n = 46.
- Haploid (n) cells have one set. Human eggs and sperm are haploid: n = 23. Fertilization joins two haploid cells to make a diploid zygote.
- Homologous chromosomes are a matching pair, one from each parent, carrying the same genes.
- Sister chromatids are the two identical copies of one chromosome made when DNA is copied in S phase. They stay joined at the centromere until they are pulled apart.
| Mitosis | Meiosis | |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Making gametes (eggs and sperm) |
| Number of divisions | 1 | 2 (meiosis I and meiosis II) |
| Cells produced | 2 | 4 |
| Chromosome number | Stays the same (2n to 2n) | Halved (2n to n) |
| Daughter cells | Identical to the parent and each other | Genetically different from each other |
| Homologous pairs | Do not pair up | Pair up in prophase I and separate in anaphase I |
| Crossing over | No | Yes, in prophase I |
| Where in humans | Almost all body cells | Only in the ovaries and testes |
The phases
Before either division, the cell copies its DNA during S phase of interphase. Mitosis then runs through prophase, prometaphase, metaphase, anaphase and telophase, and cytokinesis splits the cytoplasm. Many classes group prometaphase with prophase and remember the order as PMAT. Meiosis runs through those phases twice: in meiosis I the homologous pairs separate, and in meiosis II the sister chromatids separate, like a mitosis.
Worked examples
Example 1: chromosome counts in a cat
Problem A cat's body cells have 38 chromosomes. How many chromosomes are in a cat skin cell made by mitosis, and in a cat egg made by meiosis?
- Mitosis keeps the number the same: 38.
- Meiosis halves it: .
- Check: an egg (19) plus a sperm (19) gives 38 again at fertilization.
Answer 38 in the skin cell and 19 in the egg
Example 2: chromosomes vs chromatids through mitosis
Problem A human cell is about to divide. How many chromosomes and chromatids does it have in G1, after S phase, and in each daughter cell?
- G1: 46 chromosomes, each a single chromatid, so 46 chromatids.
- After S phase: DNA is copied, but the copies stay joined, so still 46 chromosomes, now with chromatids.
- Anaphase pulls sister chromatids apart, so each daughter cell gets 46 single-chromatid chromosomes.
Answer G1: 46 chromosomes, 46 chromatids. After S: 46 chromosomes, 92 chromatids. Each daughter: 46 chromosomes
Example 3: through meiosis
Problem A fruit fly has 2n = 8. How many chromosomes are in each cell after meiosis I, and after meiosis II?
- Meiosis I separates homologous pairs, so each of the 2 cells gets one of each pair: 4 chromosomes, each still made of 2 sister chromatids.
- Meiosis II separates sister chromatids, so each of the 4 cells keeps 4 chromosomes, now single.
Answer 4 chromosomes per cell after meiosis I and 4 per cell after meiosis II (4 cells in total)
Common mistakes and how to fix them
- Thinking meiosis II halves the number again. The halving happens in meiosis I, when homologs separate. Meiosis II separates sister chromatids, so the count stays at n.
- Counting chromatids as chromosomes. Fix: count centromeres. Each centromere is one chromosome, whether it holds one chromatid or two.
- Saying mitosis makes gametes in humans. Human gametes come from meiosis. (Some plants and fungi do make gametes by mitosis, but not humans.)
- Mixing up homologous chromosomes and sister chromatids. Homologs come from different parents and can carry different alleles. Sister chromatids are copies of each other.
Practice questions
How many cells does meiosis produce from one starting cell?
- 1
- 2
- 4
- 8
Show answer
Answer: 4
Two divisions: one cell becomes 2 after meiosis I, then 4 after meiosis II.
A dog's body cells have 78 chromosomes. How many are in a dog sperm cell?
Show answer
Answer: 39 chromosomes
Sperm are made by meiosis, which halves the number: .
In which phase does crossing over happen?
- Prophase of mitosis
- Prophase I of meiosis
- Anaphase II of meiosis
- S phase
Show answer
Answer: Prophase I of meiosis
Homologous chromosomes pair up only in meiosis I, and crossing over happens while they are paired in prophase I.
A cell with 2n = 12 copies its DNA in S phase. How many chromatids does it have right after S phase?
Show answer
Answer: 24 chromatids
It still has 12 chromosomes, but each is now two sister chromatids: .
Which process makes cells that are genetically identical to the parent?
- Mitosis
- Meiosis
- Both
- Neither
Show answer
Answer: Mitosis
Mitosis gives each daughter an exact copy of the parent's chromosomes. Meiosis shuffles them with crossing over and independent assortment.
Frequently asked questions
What is the easiest way to remember the difference?
Learn three numbers for each. Mitosis: 1 division, 2 identical cells, same chromosome number, used for growth and repair. Meiosis: 2 divisions, 4 different cells, half the chromosome number, used to make eggs and sperm. Those numbers answer most comparison questions.
What happens if meiosis goes wrong?
If chromosomes fail to separate, called nondisjunction, a gamete can end up with an extra or a missing chromosome. If it is fertilized, the embryo has the wrong number. Down syndrome, with three copies of chromosome 21, is the best known example.
Is cytokinesis part of mitosis?
Strictly, no. Mitosis is the division of the nucleus, and cytokinesis is the division of the cytoplasm that usually follows. Many teachers include it when they list the stages, so check how your class counts them. OpenStax calls the two together the mitotic phase.
Sources
- OpenStax Biology 2e, 10.2 The Cell Cycle, accessed October 1, 2026
- OpenStax Biology 2e, 11.1 The Process of Meiosis, accessed October 1, 2026
- OpenStax Biology 2e, 11.2 Sexual Reproduction, accessed October 1, 2026
