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Cells expand, repair, and propagate through two primary division processes: mitosis and meiosis. While mitosis yields two diploid daughters, meiosis produces four haploid cells—gametes—each containing half the parental chromosome set.
Meiosis is a specialized form of cell division that reduces the chromosome number by half, creating genetically distinct gametes such as eggs, sperm, and certain plant spores. The process comprises two sequential divisions (meiosis I and meiosis II) preceded by an interphase that mirrors mitotic interphase but sets the stage for the unique pairing and recombination events that follow.
Meiotic interphase is subdivided into three phases—G1, S, and G2—mirroring mitotic timing. Each phase performs distinct preparatory tasks that collectively ensure a successful reductional division.
During G1, the cell enlarges, synthesizes proteins, and engages in intercellular communication. Chromosomes remain fully condensed and sequestered within a nuclear envelope, maintaining genomic integrity while the cell prepares for DNA replication.
The S phase is where the cell duplicates its entire genome. Each chromosome is copied into an identical sister chromatid, linked at the centromere. Although the nuclear envelope persists, the chromatid pair remains indistinguishable until the onset of meiosis I. In plant cells, a spindle apparatus begins to form during this stage, setting the groundwork for subsequent chromosomal segregation.
G2 continues growth and functional maintenance while the duplicated chromosomes remain enclosed within the nucleus. In animal cells, centrosomes duplicate and the centrioles become well‑defined, forming the spindle microtubules that will later separate the sister chromatids during meiosis I.
Unlike mitosis, which culminates in a single division, meiosis involves two consecutive divisions without an intervening interphase. The first division (meiosis I) mirrors mitosis, producing two diploid cells that have already doubled their chromosomes. The second division (meiosis II) then halves the chromosome number, resulting in four haploid cells. Because no new DNA synthesis occurs between divisions, the sister chromatids that were duplicated during S phase remain intact and are segregated, ensuring each gamete receives a single copy of each chromosome.
When two gametes fuse during fertilization, their haploid genomes combine to restore the diploid state, initiating the development of a new organism.
