Meiosis is the specialized cell division process required for sexual reproduction, producing the gametes—sperm and egg cells. The purpose of meiosis is to halve the chromosome number so that when two gametes fuse during fertilization, the resulting offspring has the correct, full set of chromosomes. This necessary reduction is achieved through two sequential rounds of division, Meiosis I and Meiosis II, which perform distinct mechanical and genetic tasks. A cell begins the process with duplicated chromosomes, which are sorted and partitioned into four final cells, each containing a single, unique set of chromosomes.
Reductional Versus Equational Division
The fundamental difference between the two meiotic phases lies in how they affect the cell’s chromosome number, a distinction that gives them their technical names.
Meiosis I is designated the reductional division because it reduces the number of chromosomes by half. A starting diploid cell (2n) divides to produce two haploid cells (1n).
Meiosis II, by contrast, is known as the equational division because the chromosome number remains the same. The haploid cells entering Meiosis II divide to produce more haploid cells. This second division is similar to mitosis, where the goal is to separate the duplicated components of the chromosome without changing the overall chromosome number per cell.
Unique Processes of Meiosis I
Meiosis I is responsible for the most genetically significant events, including the mechanisms that generate genetic diversity.
During Prophase I, the homologous chromosomes, which are the pair of matching chromosomes inherited from each parent, physically align with each other in a process called synapsis. This pairing allows for crossing over, where non-sister chromatids exchange segments of genetic material, creating recombinant chromosomes.
The physical separation occurs in Anaphase I, when the spindle fibers pull the paired homologous chromosomes apart. Crucially, the sister chromatids remain attached and move as a unit to one pole of the cell.
This separation of homologous pairs, rather than sister chromatids, reduces the chromosome number from diploid to haploid. The random orientation of these homologous pairs at the metaphase plate before separation, known as independent assortment, contributes a second layer of genetic variation.
Unique Processes of Meiosis II
The second round of division is mechanically simpler and functions primarily to separate the remaining duplicated components of the chromosomes.
A notable feature of the transition between the two divisions is that there is no intervening period of DNA synthesis or replication. The cells proceed directly into Meiosis II with their chromosomes still consisting of two sister chromatids.
The defining event of Meiosis II occurs during Anaphase II, where the sister chromatids finally separate. The cohesion holding the identical copies together is released, and each chromatid is pulled to an opposite pole of the cell, officially becoming an individual, unduplicated chromosome.
This separation mirrors the process that occurs in mitosis, but it takes place in a haploid cell.
Final Products of Meiosis
The two-part process of meiosis transforms a single diploid cell into four genetically distinct haploid cells.
The end of Meiosis I results in two haploid cells, but each still contains duplicated chromosomes.
Meiosis II acts on those two cells, resulting in a total of four final daughter cells. Each of these four cells is haploid, containing a single, unduplicated set of chromosomes.
Due to recombination and independent assortment in Meiosis I and the final separation in Meiosis II, all four resulting gametes are genetically different from the original parent cell and from each other.