Meiosis is a specialized cell division fundamental to sexual reproduction, producing gametes (reproductive cells like sperm and egg cells). Its primary purpose is to halve the chromosome number in the parent cell. This ensures that when two gametes fuse during fertilization, the offspring inherits the correct number of chromosomes for its species, maintaining genetic stability across generations.
The Starting Point: Diploid Cells
Meiosis begins with a diploid cell, containing two complete sets of chromosomes—one inherited from each parent. In humans, a typical diploid cell (2n) contains 46 chromosomes, organized into 23 pairs. While most diploid cells undergo mitosis for growth and repair, specific diploid cells in reproductive organs are precursors to gametes, preparing for meiosis.
Meiosis I: Halving the Chromosome Number
Before Meiosis I, DNA replicates during interphase. Each of the 46 chromosomes then consists of two identical sister chromatids, joined at the centromere. Even though each chromosome now has two chromatids, it is still considered a single chromosome for counting purposes at this stage.
During prophase I, homologous chromosomes pair up, forming bivalents. This allows for crossing over, where genetic material is exchanged between non-sister chromatids, contributing to genetic diversity. In anaphase I, these homologous pairs separate and move to opposite poles.
This separation of homologous chromosomes is the defining event of Meiosis I, reducing the chromosome number by half. Each resulting daughter cell receives one chromosome from each homologous pair, still composed of two sister chromatids. After Meiosis I, human cells contain 23 chromosomes, each with two chromatids.
Meiosis II: Separating Sister Chromatids
Meiosis II follows Meiosis I without intervening DNA replication. The two cells from Meiosis I, each containing 23 chromosomes (with two chromatids per chromosome), proceed directly into a second division. This phase is similar in mechanism to mitosis, but it occurs in haploid cells.
During anaphase II, the sister chromatids of each chromosome finally separate from each other. These now-individual chromatids are considered full chromosomes and move to opposite poles of the cell. This separation results in four distinct cells at the end of Meiosis II.
Each of these four resulting cells is haploid, meaning it contains a single set of unduplicated chromosomes. In humans, this means each gamete produced at the end of Meiosis II contains 23 individual chromosomes. This final count represents half the number of chromosomes found in the original diploid parent cell.
The Final Count and Its Significance
The journey through meiosis begins with a single diploid cell containing 46 chromosomes, where each chromosome replicates to form two sister chromatids. After Meiosis I, two cells are formed, each with 23 chromosomes, but each of these chromosomes still consists of two chromatids. Meiosis II then separates these sister chromatids, yielding a final count of four haploid cells, each containing 23 single, unduplicated chromosomes.
This precise reduction in chromosome number is fundamental for maintaining the characteristic chromosome count of a species across generations. When a sperm cell (23 chromosomes) fertilizes an egg cell (23 chromosomes), the resulting zygote will have the correct diploid number of 46 chromosomes. Without meiosis, the chromosome number would double with each generation, leading to an unsustainable increase.
Meiosis contributes significantly to genetic diversity within a species. The processes of crossing over during prophase I and the independent assortment of homologous chromosomes during anaphase I ensure that each gamete produced is genetically unique. This genetic variation among offspring enhances a species’ ability to adapt to changing environments.