Meiosis is the cell division process organisms use to create reproductive cells, known as gametes. This process is necessary to ensure that when two gametes combine during fertilization, the resulting offspring has the correct number of chromosomes. Meiosis achieves this by involving two consecutive rounds of division, designated as Meiosis I and Meiosis II, following a single DNA replication event. The first division, Meiosis I, is the stage where the chromosome number is reduced by half, setting the foundation for sexual reproduction.
The Immediate Result of the First Division
Meiosis I concludes with the formation of two daughter cells from the original parent cell. Following Telophase I and the division of the cytoplasm, the two resulting cells are genetically distinct from each other and from the cell that began the process. These cells are immediately considered haploid, meaning they contain only one complete set of chromosomes, represented by the symbol \(n\).
The term haploid describes a cell that has half the number of chromosome sets found in a typical body cell, which is called diploid (\(2n\)). Even though the cells after Meiosis I are haploid in terms of chromosome number, each chromosome still consists of two joined sister chromatids. This means the genetic material has been halved in terms of sets, but the DNA content is still duplicated within each chromosome structure.
Why Chromosome Numbers Halve
The reduction in chromosome number during Meiosis I occurs because of the separation of homologous chromosomes. Before the division, every chromosome inherited from one parent has a matching, or homologous, chromosome inherited from the other parent. These homologous pairs align together in the cell’s center during Metaphase I.
During Anaphase I, the spindle fibers pull these homologous pairs apart, moving one entire replicated chromosome to one pole of the cell and the other replicated chromosome to the opposite pole. Because the homologous pairs separate, each new daughter cell receives only one chromosome from each pair, instantly reducing the chromosome count from diploid (\(2n\)) to haploid (\(n\)). This reduction is the defining event of Meiosis I, which is why it is often called a reductional division.
How the Second Division Differs
The two haploid cells produced by Meiosis I do not typically undergo another round of DNA replication before entering the second meiotic division, Meiosis II. Meiosis II is fundamentally different because it separates the sister chromatids that are still joined together in the chromosomes formed during Meiosis I. This second stage is mechanically similar to mitosis, a typical cell division, and is therefore called an equational division.
During Meiosis II, the two cells divide again, and the sister chromatids finally pull apart during Anaphase II. The end result of this second division is a total of four genetically unique haploid cells. Meiosis I achieves the reduction in chromosome number, while Meiosis II distributes the remaining duplicated DNA into four separate, functional gametes.