Meiosis: The Process of Reductional Division

Introduction

All sexually reproducing organisms produce gametes by a mode of cell division, which reduces the chromosome number to half of the diploid state. This mode of cell division is called meiosis. The term meiosis was coined in 1905 from a word meaning “a lessening,” from meioun “to lessen,” from meion “less,” from PIE root *mei- “small.”

In sexually reproducing organisms, which are typically diploid with a pair of homologous chromosomes, one inherited from each parent, the number of chromosomes is halved during meiosis to form haploid gametes. Subsequently, the male and the female gamete fuse to form a diploid zygote where the paired state of chromosomes is restored. In animals, gametes (egg and sperm) are the only haploid stage. The meiotic division in the germ cells produces the eggs and the sperm. In plants, the gametophyte stage (free-living or dependent on sporophytes; short-lived or the dominant phase) represents the haploid stage. In most fungi, the haploid and the diploid stage alternate and can propagate by mitosis.

Significance of meiosis

Meiosis produces haploid gametes. Doing so introduces a change in the offspring’s genetic makeup relative to the parents. It introduces the change during the first round of meiotic division. During the first round of meiosis, recombination between the homologous chromosomes produces novel chromosomes by recombining the parental chromosomes. The random orientation of homologous on the spindle can produce a huge number (223) of paternal and maternal chromosomes. These processes (recombination and random segregation) produce gametes with the parents’ recombined set of genes.

It is important for maintaining the ploidy state of the organism. The first round of division, meiosis I, the unique segregational event, produces daughter cells with only one set of chromosomes instead of two. This phase of division is therefore called reductional division. The second round of division is similar to mitosis, also known as equational division. The result of meiosis is the production of haploid gametes. During fertilization, the gametes fuse and form a diploid zygote. Thus, the ploidy state is restored. Without meiosis, there would be a doubling of the number of chromosomes in the progeny at every generation.

Types of meiosis

Depending on the stage within which the meiosis takes place, it is of three types. These are:

  1. Gametic or terminal meiosis: It is linked to the formation of gametes. In male vertebrates, primary spermatocytes are committed to undergo meiosis and produce spermatids. In female vertebrates, oogonia become the primary oocyte which undergoes an extended phase of meiosis and produces oocytes. All multicellular animals and a few protists perform gametic meiosis.
  2. Zygotic or initial meiosis: Meiosis occurs just after fertilization to produce haploid spores. Protists and fungi undergo this type of meiosis.
  3. Sporic or intermediate meiosis: In plants, the zygote produces a multicellular organism (the sporophyte) by mitosis. The plants produce spores (sporogenesis) by meiosis. Spores germinate to form gametophyte, which produces gametes by mitotic division.

The stages of meiosis

Like mitosis, meiosis includes a DNA replication phase, the premeiotic S-phase. After DNA replication, it enters meiosis I, which produces two haploid cells. Meiosis I is divided into four substages. Prophase I, metaphase I, anaphase I, and telophase I. Prophase I is then subdivided into leptotene, zygotene, pachytene, diplotene, and diakinesis. The haploid cells produced by meiosis I undergo meiosis II, which is divided into Prophase II, metaphase II, anaphase II, and telophase II. After the completion of meiosis II, four haploid cells are produced. The sequence of stages during meiosis is summarized as follows.

The sequence of stages of meiosis

The stages of meiosis and the functions of each stage are given in the following table.

Stages of meiosis

Sub-stage

Sub-substage

Functions

Meiosis I

 

Prophase I

Leptotene

  • Chromosome compaction, however, chromosomes cannot be identified as paired chromatids;

 

Zygotene

  • Synapsis (pairing of homologous chromosomes). The complex formed is called a bivalent (as it contains two homologues) or a tetrad (as it has four chromatids). 
  • Synaptonemal complex (SC) of proteins formed between the homologues.

Pachytene

  • Synaptonemal complex (SC) is completely formed.

[Cover image: It is a schematic representation of SC formed between the homologous pair of chromosomes. The yellow lines represent the lateral and transverse protein elements of the SC.]

  • The homologues are held together along the length by SC.
  • Within the SC, recombination nodules are formed where crossing-over takes place. The enzyme which facilitates crossing-over is called recombinase. It takes place between non-sister chromatids of homologous chromosomes.

Diplotene

  • SC dissolves. It leaves homologues attached at specific points called chiasmata. These are sites where crossing-over had occurred.
  • Homologues tend to move away from each other while remaining attached at chiasma.
  • Chromosome decompaction takes place in many species.
  • Vertebrate females can have an extended diplotene stage.

Diakinesis

  • The assemblage of meiotic spindle.
  • Chromosomes recompacted.
  • Dissolution of recombination nodules (chiasma); in most eukaryotes, it remains till the homologues separate during anaphase.
  • The nucleus and nucleolus disappear.
  • Tetrads begin to move at the metaphase plate.

Metaphase I

  • The homologues align at the metaphase plate.
  • The sister chromatids attach to the spindle fibers from the same pole and the homologues from opposite poles.
  • The maternal and paternal homologues orient randomly towards either pole.

Anaphase I

  • Homologues separate and start to move toward opposite poles.

Telophase I

  • Chromosomes disperse to some extent.
  • The nuclear envelope may or may not reform.

 

 

 

 

 

Interkinesis

  • It is the stage between two meiotic divisions.
  • It is generally short-lived.
  • In animals, cells at this stage are referred to as secondary spermatocytes or secondary oocytes.
  • These cells are haploid, as they contain only one member of each homologous pair. The chromosomes are represented by a pair of chromatids attached to the centromere.
  • As each chromosome has two chromatids, the amount of DNA is twice that of the haploid cell. 

 

 

Meiosis II

Prophase II

  • The nuclear envelope dissolves (if reformed at telophase I).
  • Chromosomes recompact.

Metaphase II

  • Spindle fibers from opposite poles attach to the kinetochores of the sister chromatids and align at the metaphase plate.
  • The progression of vertebrate oocytes halts at this stage and remains arrested till fertilization by sperm.  

Anaphase II

  • The fertilized egg completes metaphase II and enters anaphase II.
  • The sister chromatids split at the centromere and move towards opposite poles.

Telophase II

  • Chromatids reach the opposite poles.
  • Nuclear membrane forms around each cluster of chromosomes.

 

Cytokinesis

  • Four haploid cells are formed.
  • Each cell contains one of the homologous chromosomes from each pair.
  • The amount of DNA in each cell is half that of a diploid cell.
Chromosomal behaviour at the different stages of meiosis

The Result of Meiosis

In male animals, the meiotic division is equal. One primary spermatocyte produces four sperms. Thus, in males, a large number of gametes are produced. In female animals, the meiotic division is unequal. In meiosis I, a primary oocyte produces large secondary oocytes and a smaller first polar body. The secondary oocytes undergo meiosis II, producing a large ovum and a smaller second polar body. The unequal meiosis in female animals helps them retain almost all the cytoplasm in the ovum and discard half of the chromosomes in the polar body. The formation of male gametes is called spermatogenesis, which takes place in the testis, and the development of female gamete is called oogenesis, which takes place in the ovary.

In plants, the male gametes are called microspores, formed by microsporogenesis. It takes place in the anther where cells of sporogenous tissue undergo meiosis, and each sporogenous cell, called the pollen mother cell (PMC), produces four microspores (pollen grains). The female gamete is called a megaspore produced by the process called megasporogenesis. Within the ovule, a megaspore mother cell (MMC) undergoes meiosis and produces four megaspore cells. In most plants, only one of the four megaspores develops into a mature embryo sac, and the rest degenerate.

Meiosis generates a recombined set of genes in the gametes, which fuses to form a diploid individual with a unique gene set. Thus, meiosis introduces variation among offspring. It is essential for the evolution of new species.   

Summary

Meiosis is a special type of cell division that produces haploid gametes. The production of haploid gametes is essential for maintaining the ploidy state of an organism. Without meiosis, chromosome numbers would double in each generation. It takes place in all sexually reproducing organisms. With a little difference, the process of meiosis is identical in males and females, plants and animals. The time taken to complete the meiosis may vary from a few days to many days to years.

Read mitosis here

References:

  • Karp, G., Iwasa, J., & Marshall, W. (2015). Karp’s Cell and Molecular Biology: Concepts and Experiments (8th ed.). Wiley.
  • Md, T. P. D., Earnshaw, W. C., PhD, L. J., & Cmi, J. G. M. P. (2016). Cell Biology (3rd ed.). Elsevier.
  • Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., & Walter, P. (2014). Molecular Biology of the Cell (6th ed.). Garland Science.
  • meiosis | Search Online Etymology Dictionary (etymonline.com)

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