What meiosis is for
In sexual reproduction, two gametes (a sperm and an egg in animals) fuse at fertilisation. If each carried a full set of chromosomes, the number would double every generation. Meiosis prevents this by halving the chromosome number, making haploid gametes (n) from a diploid cell (2n). Fertilisation then restores the diploid number: n + n = 2n. Meiosis also shuffles the parents’ genes, so every gamete is genetically unique.
Chromosomes, homologues and chromatids
A diploid cell has two of each kind of chromosome: homologous chromosomes, one inherited from each parent. Homologues have the same size and carry the same genes, but may carry different versions of them (alleles). Before meiosis the DNA is copied, so each chromosome consists of two identicalsister chromatids. Keep these apart: homologues come from different parents; sister chromatids are copies of each other. Humans have 46 chromosomes in 23 pairs, and gametes have 23.
Meiosis I
In prophase I, chromosomes condense and homologues pair up along their length (synapsis), forming a bivalent of four chromatids, and crossing over takes place. Inmetaphase I the pairs line up across the middle of the cell. In anaphase I the homologues separate to opposite poles while sister chromatids stay joined. After telophase I and cytokinesis there are two haploid cells, each with one chromosome from every pair. Because the chromosome number is halved, meiosis I is called the reduction division.
Meiosis II
Meiosis II follows without another round of DNA replication and works like mitosis. In each cell the chromosomes line up singly in metaphase II, the sister chromatids separate in anaphase II, and aftertelophase II and cytokinesis there are four haploid cells, each with one chromatid of every chromosome.
Crossing over
While homologues are paired, non-sister chromatids can break and rejoin at the same point, exchanging matching segments. The crossing points are visible as chiasmata. The result is recombinantchromatids carrying a new mix of the mother’s and father’s alleles. Human chromosomes usually have one to three crossovers each, at positions that vary from cell to cell.
Independent assortment
At metaphase I, each homologous pair lines up in a random orientation, independently of every other pair. The maternal chromosome of pair 1 might face the same pole as the paternal chromosome of pair 2, or not. Each gamete therefore receives a random mixture of maternal and paternal chromosomes.
How much variation?
With n pairs, independent assortment alone gives 2ⁿ possible gametes. For humans that is 2²³, about8.4 million. Crossing over multiplies this enormously, and random fertilisation, any sperm meeting any egg, squares it again: about 70 trillion combinations from one couple even without crossing over. This variation is the raw material for natural selection.
Meiosis vs mitosis
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | One | Two |
| Homologues pair and cross over | No | Yes, in prophase I |
| What separates | Sister chromatids | Homologues (I), then sister chromatids (II) |
| Result | 2 identical diploid cells | 4 different haploid cells |
Key terms
- Haploid (n) / diploid (2n)
- One set of chromosomes / two sets, one from each parent.
- Bivalent (tetrad)
- A pair of homologous chromosomes joined during prophase I, made of four chromatids.
- Chiasma
- The point where non-sister chromatids have crossed over.
- Non-disjunction
- Failure of chromosomes to separate properly, giving gametes with an extra or missing chromosome; it causes conditions such as Down syndrome.
Common misconceptions
- “Homologous chromosomes are identical.” They carry the same genes but often different alleles.
- “DNA is copied again before meiosis II.” It is copied only once, before meiosis I.
- “Each gamete gets all of one parent’s chromosomes.” Independent assortment mixes them.
Key takeaways
- Meiosis makes four haploid, genetically different cells from one diploid cell.
- Meiosis I separates homologues (reduction); meiosis II separates sister chromatids.
- Crossing over and independent assortment create new combinations of alleles.
- Fertilisation restores the diploid number and adds still more variation.
Model note: the simulator uses 2 or 3 chromosome pairs with one crossover per pair between two adjacent non-sister chromatids (the lab picks any non-sister pair). Real cells have more chromosomes, can have several crossovers per pair, and in many species meiosis is unequal: in human females only one of the four cells becomes an egg.