Meaning and mechanism
Homologous chromosomes separate in the first meiotic division, and sister chromatids separate in the second. Unlike mitosis, meiosis produces cells with one set of chromosomes.
Crossing over between homologous chromosomes changes combinations of genetic information. The way each homologous pair separates also contributes to gamete diversity.
Relationships and conditions
Ignoring crossing over, independent assortment alone gives 2ⁿ chromosome combinations from n homologous pairs. There is one round of DNA replication and two divisions.
Count chromosome number, DNA content, and the number of chromosome sets separately. The small chromosome numbers in the simulation illustrate the process; they are not the actual chromosome numbers of a species.
What to observe in the simulation
Homologous chromosomes separate in meiosis I, and sister chromatids in meiosis II. Independent assortment and crossing over create diversity.
As in the mitosis experiment, chromosomes are counted by centromeres and DNA content is relative to G1 DNA=2. From the end of meiosis I, the table reports one daughter cell. Per-nucleus values are undefined without a nuclear envelope. Crossing over occurs once between two selected nonsister chromatids of a homologous pair. Positions, colors, and timing are illustrative; genetic map distances and division errors are omitted.
Key relationships and symbols
1 DNA replication, 2 divisions → Gametes n=2, DNA=1
| Symbol | Meaning | Unit |
|---|---|---|
| n | Haploid chromosome number | items |
| 2ⁿ | Number of combinations excluding crossing over | outcomes |
Worked example
What is one gamete based on 2n=4 and G1 DNA=2?
DNA is copied once and then divided twice. In meiosis I, homologous chromosomes are separated, and in meiosis II, sister chromatids are separated. gamete n=2, DNA=1.
Common misconceptions
- After meiosis I, the number of chromosome sets is reduced, but each chromosome can still consist of two chromatids.