Meiosis

Meiosis is a specialised form of cell division that occurs in the germ cells of sexually reproducing eukaryotic organisms. Its primary function is to produce haploid gametes—sperm or ova—each containing a single set of chromosomes. By halving the chromosome number, meiosis ensures that fertilisation restores the diploid state, thereby maintaining the stability of chromosome numbers across generations. The process also generates extensive genetic variation through chromosomal recombination and random segregation, contributing to evolutionary diversity.

Purpose and Biological Significance

Meiosis enables sexual reproduction by producing haploid cells that can fuse during fertilisation to form a diploid zygote. This alternation between haploid and diploid phases characterises the life cycles of animals, most plants, fungi, and many protists. In humans, meiosis produces gametes each carrying 23 chromosomes, which combine at fertilisation to give a zygote with 46 chromosomes.
A key outcome of meiosis is genetic variation, created through mechanisms such as chromosomal crossover and the random assortment of maternal and paternal chromosomes. These processes ensure that each gamete carries a unique combination of genetic material, providing the raw material on which natural selection acts.
Errors in meiosis, such as nondisjunction leading to aneuploidy, are the leading known causes of miscarriage and major contributors to genetic disorders.

Interphase and Preparation for Division

Before meiosis begins, the cell undergoes a preparatory interphase similar to that preceding mitosis. This includes:

• G₁ phase, during which the cell synthesises proteins and prepares for DNA replication.
• S phase, in which the DNA of each chromosome is replicated, producing two identical sister chromatids held together at the centromere. This replication does not alter ploidy.
• A prolonged prophase-like period sometimes compared to G₂, during which homologous chromosomes begin to pair.

This preparatory period includes essential events for meiotic recombination, making it distinct from the shorter interphase prior to mitosis.

Meiotic Recombination and Homologous Pairing

A defining feature of meiosis is homologous recombination, which occurs during early prophase I. Homologous chromosomes—one maternal and one paternal—pair and align with precision. DNA is deliberately cut and repaired in a controlled sequence of events that may produce chromosomal crossovers.
These crossovers form visible structures known as chiasmata, which physically link homologous chromosomes. The chiasmata play an important mechanical role in ensuring that each homologous pair segregates accurately during meiosis I. Recombinant chromatids emerging from this process carry new combinations of genetic information, contributing directly to the genetic distinctiveness of offspring.

Meiosis I: Reductional Division

Meiosis I separates homologous chromosomes and reduces the chromosome number from diploid to haploid. It includes the following stages:

• Prophase I: Homologs pair and recombine. Chromosomes condense, the nuclear envelope breaks down, and the spindle apparatus forms.
• Metaphase I: Paired homologous chromosomes (bivalents) align at the spindle equator.
• Anaphase I: Homologs are pulled to opposite poles, while sister chromatids remain attached.
• Telophase I and Cytokinesis I: Two daughter cells are formed, each haploid with duplicated sister chromatids.

In many species, a brief resting period, interkinesis, separates the two meiotic divisions.

Meiosis II: Equational Division

Meiosis II resembles mitosis. Sister chromatids separate and migrate to opposite poles. The stages mirror those of mitosis:

• Prophase II: Chromosomes re-condense, and the spindle forms.
• Metaphase II: Chromosomes align singly at the equator.
• Anaphase II: Sister chromatids separate to form daughter chromosomes.
• Telophase II and Cytokinesis II: Four haploid cells result from the original diploid parent.

In males, all four products typically develop into functional gametes. In female animals, however, three cells become polar bodies, leaving one large ovum.

Variations and Special Cases

While meiosis is universal among sexually reproducing eukaryotes, certain variations exist:

• In plants, fungi, and some protists, meiosis produces haploid spores rather than gametes. These spores may undergo vegetative division before participating in fertilisation.
• A few eukaryotic groups, such as bdelloid rotifers, have evolved complete dependence on parthenogenetic reproduction, lacking the ability to undergo meiosis.
• Meiosis does not occur in prokaryotes. Instead, bacteria and archaea may exchange genetic material through horizontal gene transfer, which includes mechanisms such as transformation, conjugation, and transduction.

Historical Discovery

The process of meiosis was first described in 1876 by Oscar Hertwig in sea urchin eggs and later in greater cytological detail by Edouard Van Beneden in 1883. Its fundamental role in heredity was clarified in 1890 by August Weismann, who recognised that a two-division sequence was necessary to maintain chromosome number across generations. Thomas Hunt Morgan’s work in 1911 demonstrated that recombination during meiosis underpins the chromosomal theory of inheritance.
The name “meiosis” derives from the Greek word for lessening, reflecting the reduction in chromosome number. It was introduced in 1905 by John Bretland Farmer and John Edmund Sharrock Moore, and the modern spelling was standardised shortly thereafter.

Phases and Chromosome Dynamics

Meiosis is traditionally divided into two major parts—meiosis I and meiosis II—each containing stages analogous to those in mitosis: prophase, metaphase, anaphase, and telophase. Meiosis I accomplishes the reductional division, while meiosis II is equational.
During interphase preceding meiosis, chromosome duplication produces pairs of sister chromatids that remain joined until their separation in meiosis II. This orchestrated sequence ensures the correct distribution of genetic material to each gamete.