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Meiosis Is a Reduction Division: One Diploid Cell Becomes Four Haploid Cells
Definition
Meiosis
Meiosis is a type of cell division that reduces the chromosome number by half, producing four genetically distinct haploid cells. This process occurs in gamete-producing cells and is essential for sexual reproduction.
- A diploid cell contains two full sets of chromosomes, written as 2n.
- A haploid cell contains one full set of chromosomes, written as n.
- Meiosis halves the chromosome number, taking one diploid nucleus to four haploid nuclei.
- It does this through two divisions: meiosis I and meiosis II.
- The halving happens in meiosis I, which is why meiosis is called a reduction division.
Analogy
- Think of a diploid cell as holding a matched pair of gloves, one left and one right, for every chromosome.
- Meiosis I hands out one glove of each pair to each new cell.
- That is why the set is halved, not just split.
The Big Picture: Two Divisions Take 2n to n
- The cell copies its DNA once before meiosis begins.
- Two divisions then follow with no further DNA copying in between.
- One copy step plus two divisions is what produces four cells, each with half the starting chromosome number.
Meiosis I: Homologous Chromosomes Separate, Halving the Number
Meiosis I is the reduction step: it separates homologous chromosomes so each new cell gets only one chromosome from each pair.
- Homologous chromosomes are pairs carrying the same genes in the same order, one inherited from each parent.
- During meiosis I the two members of each pair are pulled to opposite ends of the cell.
- Each of the two new cells is now haploid because it holds only one chromosome from every pair.
Common Mistake
- Meiosis I separates homologous chromosomes, not sister chromatids.
- Sister chromatids stay joined all the way through meiosis I.
- They are only separated later, in meiosis II.
