Meiosis: Making Sex Cells with Half the Chromosomes
How one parent cell divides twice to yield four haploid sex cells, and how crossing over and independent assortment create variety.
🎯 By the end of this lesson
- Students should be able to explain why sex cells need half the usual number of chromosomes.
- Students should be able to describe the events of meiosis I and meiosis II.
- Students should be able to explain why one meiosis produces four haploid cells from one diploid cell.
- Students should be able to track chromosome and chromatid numbers through meiosis.
- Students should be able to describe crossing over and independent assortment and explain how each creates variation.
- Students should be able to calculate the number of chromosome combinations from independent assortment using 2 to the power n.
- Students should be able to compare mitosis and meiosis in a table.
- Students should be able to describe the three types of sexual life cycle.
1A problem of arithmetic
Mitosis makes identical copies, which is perfect for growth and repair. Now consider what would happen if it were also used to make eggs and sperm. A human body cell has 46 chromosomes. If an egg with 46 chromosomes were fertilized by a sperm with 46 chromosomes, the resulting zygote would have 92. The next generation would have 184, and then 368. Within a few generations the numbers would be impossible.
Sexual reproduction therefore needs a special kind of division that cuts the chromosome number in half when sex cells are made. That process is meiosis. According to the BC curriculum, meiosis is the process through which sex cells (eggs and sperm) are formed, in which a parent cell divides twice to yield four daughter cells.
Meiosis turns one diploid (2n) cell into four haploid (n) cells through two divisions. Fertilization then fuses two haploid gametes and restores the diploid number. Meiosis and fertilization together keep the chromosome number steady from one generation to the next.
2Meiosis at a glance
Meiosis begins like mitosis, with interphase. During the S phase, every chromosome is copied, so each one consists of two sister chromatids. After that, the cell goes through two rounds of division, with no further copying in between.
- Meiosis I separates the homologous chromosomes. Each new cell receives one chromosome from each pair, so the number of chromosome sets drops from two to one. Because the number is reduced, meiosis I is called a reductional division.
- Meiosis II separates the sister chromatids, in a way that closely resembles mitosis. The chromosome number stays the same, so meiosis II is called an equational division.
The end result is four haploid cells. Each has one set of chromosomes, and each is genetically different from the others and from the parent cell.
“Meiosis is just mitosis done twice.” If mitosis were repeated, the DNA would be copied before each division and the daughter cells would be identical. In meiosis, DNA is copied once, but the cell divides twice, and the first division separates homologous pairs rather than sister chromatids. That is why the products are haploid and genetically different.
3Meiosis I, step by step
Meiosis I contains the most important events for the creation of genetic variety. Its stages have the same names as those of mitosis, with the label “I” added.
- Prophase I. The chromosomes condense. Homologous chromosomes move toward each other and pair up tightly along their whole length, gene for gene, in a process called synapsis. A protein framework holds them together. While paired, crossing over takes place: non-sister chromatids exchange segments of DNA. Each pair needs at least one crossover, and there may be several. The X-shaped places where the exchanges happened are called chiasmata. A paired set of homologous chromosomes, now four chromatids, is called a tetrad. The nuclear envelope then breaks down and spindle fibres attach.
- Metaphase I. The tetrads line up along the middle of the cell. Each pair faces the poles in a random orientation: the maternal chromosome may face either pole, and this is independent of how the other pairs are facing. This is independent assortment.
- Anaphase I. The homologous chromosomes are pulled to opposite poles. The sister chromatids stay together at their centromeres. (In mitosis the sister chromatids separate here; in meiosis I they do not.)
- Telophase I and cytokinesis. The chromosomes arrive at the poles, and the cell divides into two. Each new cell is haploid, because it has only one chromosome from each homologous pair. However, each chromosome still consists of two sister chromatids. In some species there is a short pause (interkinesis) with no DNA copying.
Meiosis II, step by step
Meiosis II happens in both cells formed by meiosis I, and it looks very much like mitosis. The difference is that each cell has just one set of chromosomes to deal with.
- Prophase II. Chromosomes condense again, and new spindles form.
- Metaphase II. The chromosomes line up single file along the middle of the cell. Each sister chromatid is attached to a spindle fibre from opposite poles.
- Anaphase II. The sister chromatids separate and move to opposite poles.
- Telophase II and cytokinesis. Nuclei re-form, and both cells divide, producing a total of four haploid cells.
A species has a diploid number of 8 (2n = 8). Follow the chromosome numbers through meiosis.
- Before meiosis, a cell has 8 chromosomes. After S phase, there are still 8 chromosomes, but each has two sister chromatids (16 chromatids).
- After meiosis I, each of the two cells has 4 chromosomes (one from each of the 4 homologous pairs), each still with two chromatids (8 chromatids per cell).
- After meiosis II, each of the four cells has 4 chromosomes with a single chromatid.
So the gametes are haploid, with n = 4. If two gametes fuse, the zygote has 4 + 4 = 8 chromosomes, which restores 2n.
4Where genetic variety comes from
The four cells that come from one meiosis are not identical to each other. Meiosis produces variety in two main ways.
1. Crossing over
In prophase I, non-sister chromatids from the homologous chromosomes swap matching segments of DNA. The result is recombinant chromosomes, which carry a new combination of the alleles that came from the mother and the father. A chromosome passed on to a child may therefore be a patchwork of the parent's two inherited chromosomes.
2. Independent assortment
At metaphase I, the way each pair of homologous chromosomes faces the poles is random and independent of every other pair. With 2 pairs, there are 2 × 2 = 4 possible combinations of chromosomes in the gametes. With 3 pairs there are 2 × 2 × 2 = 8. In general, n pairs give 2n combinations.
Humans have 23 pairs of chromosomes. The number of different combinations of chromosomes that one person could produce through independent assortment alone is 223 = 8 388 608, which is more than eight million. This does not even count crossing over, which makes the possible number of different gametes vastly greater.
Since a child is formed from one egg and one sperm, each with their own huge range of possibilities, it is very unlikely that two children of the same parents (other than identical twins) would have the same combination of chromosomes.
The next lesson shows how fertilization adds a third layer of variety, because it is a matter of chance which sperm meets which egg.
Siblings from the same two parents can look quite different. One may have curly hair and another straight hair, one may be tall and another short. The reason is that each child received a different shuffled half of each parent's chromosomes. Meiosis does the shuffling.
Meiosis I and meiosis II side by side
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| Also called | Reductional division | Equational division |
| Starting cell | One diploid cell (2n) with copied chromosomes | Two haploid cells (n), chromosomes still doubled |
| What separates | Homologous chromosomes | Sister chromatids |
| Chromosome number change | Halved (2n to n) | No change (n stays n) |
| Special events | Synapsis, crossing over, independent assortment | None; resembles mitosis |
| Result | Two haploid cells | Four haploid cells |
Tracing genes through meiosis
Chromosome shuffling matters because chromosomes carry genes, and genes come in different versions called alleles. Homologous chromosomes carry the same genes in the same places, but they may carry different alleles, one inherited from each parent.
ABO blood type is controlled by a gene that has versions (alleles) called A, B and O. Each person has two alleles, one on each chromosome of a particular homologous pair. Suppose one parent has the alleles A and O, and the other parent has the alleles B and O.
Step 1. When the first parent's cells go through meiosis, the two homologous chromosomes separate in meiosis I. Every gamete receives one of them. About half of the gametes carry A and half carry O.
Step 2. The second parent's gametes similarly carry either B or O.
Step 3. Fertilization combines one gamete from each parent. The four possible allele combinations in a child are A with B, A with O, O with B, and O with O.
The same parents could therefore have children with different allele combinations, and each combination arises by chance. (See What are blood types? for how these combinations relate to the blood types.)
The example shows the reason homologous chromosomes must separate in meiosis I. If they stayed together, every gamete would receive both alleles from a parent and the variation seen in a family would be impossible to explain.
Tracking numbers: a practice table
Questions about meiosis often ask for counts of chromosomes and chromatids at different stages. A reliable approach is to count the centromeres to find the number of chromosomes, and to count the chromatids as two per chromosome only if DNA has been copied and sister chromatids have not yet separated.
| Stage (for a cell with 2n = 6) | Chromosomes in each cell | Chromatids in each cell |
|---|---|---|
| Before S phase | 6 | 6 |
| After S phase, prophase I, metaphase I | 6 | 12 |
| After meiosis I (two cells) | 3 | 6 |
| Metaphase II | 3 | 6 |
| After meiosis II (four cells) | 3 | 3 |
“Cells after meiosis I have lost half of the DNA.” The cells after meiosis I have half the number of chromosomes, but each chromosome still has two chromatids. Counted by amount of DNA, each cell has the same as a diploid cell that has not yet replicated its DNA. The amount of DNA only falls to its final haploid level after meiosis II.
Why variation matters
The variety created by meiosis is the raw material for evolution by natural selection (see What is natural selection?). In a population in which individuals differ, some will happen to have traits that suit the environment better and will leave more descendants. The advantages and disadvantages of this strategy compared with asexual reproduction are explored in a later lesson.
Questions to think about: Why do identical twins, who arise from one fertilized egg, share the same genes, while ordinary siblings do not? Why would a species that reproduces only asexually show less variation among individuals? What would happen to the number of chromosomes in a species if one of its gametes were made by mitosis instead of meiosis?
5Meiosis compared with mitosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, replacement, asexual reproduction | Making sex cells (gametes) |
| DNA replication | Once, before division | Once, before meiosis I |
| Number of nuclear divisions | One | Two (I and II) |
| Daughter cells | Two | Four |
| Chromosome number of daughter cells | Same as the parent (diploid for a diploid parent) | Half the parent's (haploid) |
| Genetic makeup of daughter cells | Identical to the parent | Genetically unique |
| Pairing of homologous chromosomes (synapsis) | Does not occur | Occurs in prophase I |
| Crossing over | Does not occur | Occurs in prophase I |
| What lines up in the middle at metaphase | Individual chromosomes (each two sister chromatids) | Homologous pairs (metaphase I); single chromosomes (metaphase II) |
| What separates in anaphase | Sister chromatids | Homologous chromosomes (I), then sister chromatids (II) |
A good memory aid: mitosis = “same” (two identical diploid cells), meiosis = “half” (four different haploid cells).
6Where meiosis happens
In animals, including humans, meiosis takes place only in the reproductive organs (the gonads). The only haploid cells in the human body are the gametes, which are made by meiosis from cells in the testes and ovaries. The gametes do not divide again. After fertilization, the zygote grows into a new individual by repeated mitosis, so all the other body cells are diploid.
The processes differ a little between males and females. In the testes, one cell going through meiosis gives rise to four sperm cells. In the ovaries, the divisions are unequal, so that one cell makes one large egg and small cells called polar bodies that break down. These details are covered in the human reproduction lessons.
Other life cycles
Not all organisms follow the human pattern. Biologists describe three types of sexual life cycle.
| Life cycle type | Where meiosis fits | Examples |
|---|---|---|
| Diploid-dominant | Meiosis makes gametes; the only haploid cells are gametes; the zygote grows into a diploid body | Animals, including humans |
| Haploid-dominant | The main body is haploid. A diploid zygote forms and immediately undergoes meiosis to make haploid spores | Most fungi and some algae |
| Alternation of generations | Both haploid and diploid multicellular stages exist; meiosis in the diploid stage makes spores | Plants and some algae |
All three cycles share the same two key events: meiosis, which halves the chromosome number, and fertilization, which restores it.
7Summary
- Meiosis makes sex cells (eggs and sperm): one parent cell divides twice and yields four daughter cells.
- DNA is copied once. Meiosis I separates homologous chromosomes (reducing the number from 2n to n). Meiosis II separates sister chromatids.
- Crossing over (prophase I) and independent assortment (metaphase I) make each gamete genetically unique.
- Fertilization joins two haploid gametes to restore the diploid number.
- Mitosis makes identical diploid cells; meiosis makes unique haploid cells.
🔑Key terms
?Quick check
Try each question first, then reveal the answer.
1. Why must gametes be haploid?
Fertilization combines two gametes. If each had the full diploid number, the chromosome number would double every generation.
2. How many cells does one meiosis produce, and what is their chromosome number compared with the parent cell?
It produces four cells, each with half the parent cell's chromosome number (haploid).
3. What separates in anaphase I, and what separates in anaphase II?
Homologous chromosomes separate in anaphase I. Sister chromatids separate in anaphase II.
4. A cell has 2n = 10. State the number of chromosomes and chromatids in each cell after meiosis I and after meiosis II.
After meiosis I each cell has 5 chromosomes and 10 chromatids. After meiosis II each cell has 5 chromosomes and 5 chromatids.
5. Describe crossing over and say when it happens.
In prophase I, non-sister chromatids of paired homologous chromosomes swap matching segments of DNA. This creates chromosomes with new combinations of alleles.
6. A species has 4 pairs of chromosomes. How many different chromosome combinations can independent assortment produce, ignoring crossing over?
2 to the power 4 is 16 combinations.
7. Give three differences between mitosis and meiosis.
Mitosis has one division and makes two identical diploid cells, while meiosis has two divisions and makes four genetically different haploid cells. Homologous chromosomes pair and cross over only in meiosis. Mitosis is for growth and repair, meiosis for making gametes.
8. Explain why two children of the same parents are usually genetically different.
Each egg and sperm is a unique mix produced by crossing over and independent assortment, and which egg meets which sperm is a matter of chance, so each child receives a different combination.
BC curriculum content covered in this lesson
- Meiosis: the process through which sex cells (eggs and sperm) are formed, in which a parent cell divides twice to yield four daughter cells
- Haploid and diploid cells; sources of genetic variation in meiosis; comparison of mitosis and meiosis
Related articles
What Is DNA? The Molecule That Carries Your Instructions
DNA is the molecule that carries genetic information. A short guide to its structure and how its letters pair.
Survival of the Fittest? What Natural Selection Really Says
Natural selection is one of the basic mechanisms of evolution, and it is often misunderstood. Three sources explain what it needs, how it works and what it does not mean.
What Are Blood Types and Why Do They Matter?
Blood types come from inherited antigens on red cells. ABO and Rh groups decide which transfusions are safe and matter in pregnancy. How the groups were found and how common each is.
References
- BC Ministry of Education and Child Care. Science 9 curriculum (Big Ideas, Content, Curricular Competencies). Accessed October 7, 2026.
- OpenStax. Biology 2e, 11.1 The Process of Meiosis. Accessed October 7, 2026.
- OpenStax. Biology 2e, 11.2 Sexual Reproduction. Accessed October 7, 2026.
- OpenStax. Biology 2e, 10.1 Cell Division. Accessed October 7, 2026.
- OpenStax. Anatomy and Physiology 2e, 27.1 Anatomy and Physiology of the Testicular Reproductive System. Accessed October 7, 2026.
- OpenStax. Anatomy and Physiology 2e, 27.2 Anatomy and Physiology of the Ovarian Reproductive System. Accessed October 7, 2026.
These lessons follow the content areas listed in the British Columbia curriculum. They are study material written for this site and are not an official document. The official curriculum is the authority on what each course requires. Lessons are general education, not medical advice.