Meiosis is a specialized cell division fundamental to sexual reproduction. Its primary purpose is creating reproductive cells, known as gametes, which possess half the number of chromosomes found in typical body cells. This reduction ensures that when two gametes fuse during fertilization, the offspring maintains the correct chromosome count for its species across generations.
Meiosis I: Halving the Chromosome Number
Meiosis I, the first major stage, is a reductional division because it halves the chromosome number of the parent cell. This division begins after DNA replication, resulting in chromosomes that each consist of two identical sister chromatids. The events within Meiosis I set the stage for genetic variation.
Prophase I
In Prophase I, homologous chromosomes, one inherited from each parent, pair up, forming a close association called synapsis. This forms bivalents or tetrads, consisting of four chromatids. During this pairing, crossing over occurs, where non-sister chromatids exchange segments of genetic material. This exchange creates new combinations of alleles, increasing genetic diversity within the gametes. The points where these exchanges happen are visible as chiasmata, which also help hold the homologous chromosomes together.
Metaphase I
In Metaphase I, paired homologous chromosomes, still connected by chiasmata, align along the central plane of the cell. Each homologous pair positions itself independently, a phenomenon known as independent assortment. This random orientation sorts maternal and paternal chromosomes into daughter cells in numerous combinations, contributing to genetic diversity. Spindle fibers attach to the centromeres of these homologous pairs, preparing them for separation.
Anaphase I
In Anaphase I, homologous chromosomes separate and are pulled towards opposite poles of the cell. During this separation, sister chromatids within each chromosome remain attached at their centromeres. This differs from mitosis, where sister chromatids would separate at this point. Each forming daughter cell receives one chromosome from each original pair, though each chromosome still consists of two chromatids.
Telophase I & Cytokinesis
As homologous chromosomes reach the opposite poles, Telophase I begins, and a nuclear envelope may reform around each set of chromosomes. The chromosomes may decondense slightly before the next stage. Cytokinesis, the division of the cytoplasm, then follows, physically separating the single parent cell into two daughter cells. Each resulting cell is haploid in chromosome number, but each chromosome still comprises two sister chromatids.
Meiosis II: Separating Sister Chromatids
Meiosis II follows Meiosis I, often after a brief resting period called interkinesis where no DNA replication occurs. This second division is similar to mitosis, as its primary role is to separate the sister chromatids that remained attached after Meiosis I. Each of the two haploid cells from Meiosis I proceeds through Meiosis II, ultimately yielding four haploid cells.
Prophase II
In Prophase II, chromosomes within each haploid cell condense. The nuclear envelope, if it reformed during Telophase I, breaks down, and a new spindle apparatus begins to form in each cell. This spindle guides the movement of chromosomes during the subsequent stages.
Metaphase II
In Metaphase II, chromosomes, each still composed of two sister chromatids, align individually along the metaphase plate in each cell. Unlike Metaphase I, where homologous pairs aligned, here individual chromosomes line up. Spindle fibers attach to the kinetochores, specialized protein structures at the centromeres of each sister chromatid.
Anaphase II
Anaphase II marks the separation of sister chromatids. The centromeres holding them together divide, allowing individual chromatids, now considered separate chromosomes, to be pulled to opposite poles of the cell. This separation ensures that each pole receives a complete set of single, unduplicated chromosomes.
Telophase II & Cytokinesis
Upon reaching the poles, newly separated chromosomes begin to decondense during Telophase II, and nuclear envelopes reform around each set. Concurrently, cytokinesis occurs, dividing the cytoplasm of each cell. This results in four genetically distinct haploid cells from the original single diploid cell. Each contains half the number of chromosomes of the original parent cell, and each chromosome consists of a single chromatid.
The Biological Significance of Meiosis
Meiosis serves two main biological purposes for sexually reproducing organisms. Firstly, it ensures the reduction of the chromosome number by half, a necessary step before fertilization. By producing haploid gametes, meiosis guarantees that when two gametes combine, the resulting zygote will restore the species-specific diploid chromosome number.
Secondly, meiosis is a powerful generator of genetic diversity. Crossing over during Prophase I shuffles genetic material between homologous chromosomes, creating recombinant chromosomes with novel combinations of alleles. Independent assortment of homologous chromosomes in Metaphase I randomly distributes maternal and paternal chromosomes into the daughter cells. These two events ensure that the four haploid cells produced are genetically unique, providing the raw material for variation within a population.