The term meiosis was first introduced by Farmer and Moore. It can be defined as, "The reductional division occurring only in diploid cells for the formation of haploid cells in which the number of chromosomes and the nuclear DNA content are reduced to half and there is recombination of hereditary material."

The key features of meiosis are as follows:

(a) It involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II, but only a single cycle of DNA replication.

(b) Meiosis I can initiate only after the S phase (where parental chromosomes have replicated to produce identical sister chromatids)

(c) It involves pairing of homologous chromosomes and recombination between them.

(d) Four haploid cells are formed at the end of meiosis II.

(e) Meiotic events can be grouped under the following phases:

Meiosis-I Meiosis-II

Prophase-I Prophase-II

Metaphase-I Metaphase-II

Anaphase-I Anaphase-II

Telophase-I Telophase-II

Process of Meiosis

1. Meiosis I or Reductional or Heterotypic division : It starts after the interphase of the cell cycle where DNA duplication occurs in S phase. It results in the reduction of chromosome number to half.

Meiosis I consists of four stages, i.e., Prophase-I, Metaphase-I, Anaphase-I and Telophase-I.

(a) Prophase-I : It is very complex and of very long duration. It is divided into following substages on the basis of chromosome behaviour :

(i) Leptotene or leptonema :

The duplicated centrioles start moving apart. Aster formation occurs.

The chromatin fibres undergo progressive condensation, coiling, shortening and thickening to appear in the form of long, thin condensed filamentous chromosomes.

These possess darkly stained bead like structures called chromomeres along their entire length.

The chromosomes are replicated but the chromatids are not distinguishable due to the presence of nuclear protein between them.

The chromosomes form loops whose ends are attached to the nuclear membrane at attachment plate.

This specific arrangement of chromosomes is often called the "bouquet stage".

(ii) Zygotene or Zygonema or Synaptic stage:

The homologous chromosomes come to lie in pairs.

In each homologous pair of chromosomes, one chromosome comes from mother through ova and is called maternal chromosome and the other chromosome comes from father through sperm and is called paternal chromosome.

This pairing of homologous chromosomes is called synapsis and these paired chromosomes are called bivalents.

Homologous chromosomes in a pair are of the same length, carry the same genes in the same sequence.

Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex, tripartite, protienaceous structure called synaptonemal complex.

The number of bivalents in a cell is equal to the number of haploid chromosomes. The chromatids are still not visible.

(iii) Pachytene or Pachynema :

The bivalent chromosomes become more thickened, shortened and condensed.

Each chromosome of a bivalent consists of two chromatids and are called sister chromatids.

The two sister chromatids of one chromosome in a homologous pair with regard to the other chromosome of pair are called non-sister chromatids.

Four chromatids in a pair of homologous chromosomes constitute the tetrad.

This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes.

Crossing over is the exchange of genetic material between two homologous chromosomes and also an enzyme-mediated process and the enzyme involved is called recombinase.

The best theory to explain crossing over is Darlington's theory of breakage and union :

(a) The enzyme endonuclease will help in the development of breaks (nicking.)

(b) The formation of gaps in the nicks is done by exonuclease.

(c) The separation of chromatid segments in gaps is done by U-protein or unwindase.

(d) Re-annealing (rejoining) is done by R-protein or Re-annealing protein.

The newly constituted chromosomes, thus become different from the previous set of chromosomes.

This results in the formation of new characters (recombinants) and ultimately variations in the population. These variations form the basis of evolution.

(iv) Diplotene or Diplonema :

Longest phase of prophase-I. There is dissolution of synaptonemal complex, so the recombined homologous chromosomes will start separating i.e., desynapsis.

The separation of the homologous chromosomes is not completed.

They remain attached to one or more points where crossing over has occured.

These point of attachment are called as Chiasmata (X-shaped structures).

In oocytes of some vertebrates, the diplotene can last for months or years.

Suspended diplotene stage is called as Dictyotene stage.

At this stage the chromosomes decondense and are engaged in rapid synthesis of RNA. Terminalization of chiasmata starts.

(v) Diakinesis :

This stage is marked by complete terminalisation of chiasmata.

The nucleoli and the nucleolar membrane disintegrate and disappear.

The spindle fibres extend from one pole of paired centrioles to the other pole of paired centrioles.

Astral rays and asters become fully developed.

Centrioles are absent in plant cells and thus, no asters are formed.

This marks the end of prophase-I and transition to metaphase-I.

(b) Metaphase-I :

The bivalents arrange themselves on the equator of the bipolar spindle.

Since, there are two centromeres in each bivalent, the centromeres of all the bivalents produce a double equatorial or metaphasic plate.

Each plate will have half the number of diploid chromosomes.

The microtubule from the opposite poles of the spindle attach to the pair of homologous chromosomes.

(c) Anaphase-I :

One chromosome of each homologous pair moves to the opposite poles with recombined characters of both paternal and maternal chromosomes.

The movement of chromosomes occurs along the path of their tractile fibres (chromosome fibres).

Each chromosome consists of two chromatid threads joined by a centromere.

There is no division of centromere.

At the end of anaphase-I half of the chromosomes reach one pole and other half reach on to the other pole.

There occurs true reduction in the number of chromosomes at this stage.

(d) Telophase-I :

The haploid number of chromosomes which has reached at each pole, undergo elongation, uncoiling and decondensation and changes into chromatin network.

Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus.

The nuclear envelope develops from the elements of ER around the chromatin fibres. New nucleolus is formed.

The astral rays and spindle fibres disintegrate and disappear.

The two daughter nuclei, each containing haploid number of chromosomes are formed.

The end of telophase-I marks the end of karyokinesis-I also.

Karyokinesis-I may be followed by cytokinesis-I.

By a cleavage furrow in an animal cell and a cell plate in a plant cell, the diploid parental cell is divided into two haploid daughter cells each having half the number of chromosomes, but double content of DNA.

Interkinesis or intrameiotic interphase :

Each daughter cell formed from meiosis-I sometimes undergoes interkinesis, where there is no replication phase of chromosomes and no duplication of genes.

2. Meiosis II or Homotypic Division : It resembles mitotic division.

Meiosis-II is necessary because each daughter cell formed from meiosis-I contains chromosomes, each having two chromatids, each chromatid with 2C amount of DNA. This reduction in DNA from 2C to 1 C DNA occurs only in Meiosis-II.

It consists of following four phases :

(a) Prophase-II: One pair each of the centrioles migrate to the opposite poles. The nuclear membrane and the nucleolus disintegrate and disappear. The chromatin fibres undergo compaction to appear in the form of distinct chromosomes. Each chromosome consists of two chromatid threads, joined by a centromere.

Different Stage of Meiosis

(b) Metaphase II : Chromosomes come to lie on the equator of the cell and thus, form a Single equatorial or metaphasic plate. The spindle fibres become attached to both kinetochores of the centromere of each chromosome.

(c) Anaphase-II: Begins with simultaneous splitting of the centromere of each chromosome that allows them to move towards opposite poles of the cell.

(d) Telophase-II: The daughter chromosomes on the opposite poles become decondensed to form chromatin fibres. The nuclear envelope develops from ER. New nucleolus is reorganised. The spindle fibres and astral rays disintegrate and disappear. It marks the end of the telophase-II and karyokinesis-II . It is followed by cytokinesis (similar in mitosis) which divides each cell into two daughter cells, resulting in formation of tetrad of cells.

Concept Builder

(i) Meiosis is commonly studied using onion buds.

(ii) Meiosis was first demonstrated by Van Benden and described by Winiwarter.

(iii) Gametic meiosis is also called as terminal meiosis.

(iv) Zygotic meiosis is also called as initial meiosis.

(v) Sporogenic meiosis is also called as intermediate meiosis.

(vi) Cytokinesis: Cytokinesis can be of two types, successive and simultaneous. In successive type, cytokinesis occurs after every nuclear division. The four cells formed by successive cytokinesis can be arranged either in a linear or isobilateral tetrad.

In the simultaneous type, cytokinesis takes place only at the end of both the divisions. The nuclei are generally arranged in the form of a tetrahedron.

Significance of Meiosis

1. Meiosis forms gametes that are essential for sexual reproduction.

2. Meiosis maintains the fixed number of chromosomes generation after generation in sexually reproducing organisms. It is essential since the chromosome number becomes double after fertilization.

3. Meiosis is the main cause of production of variations, which are very important for evolutionary process.

Some Major Differences between Mitosis and Meiosis


It is the method of asexual reproduction, which occurs, in acellular organisms like bacteria, protozoans, diseased cells, old cells, mammalian cartilage cells and in foetal membranes. 

it was first discovered by Remak. It is also called direct cell division. 


During amitosis, the nucleus of the cell elongates. 

Then, a constriction appears in the nucleus which gradually deepens and divides the nucleus into two daughter nuclei. 

Finally, a constriction appears in the cytoplasm which divides the cytoplasm and the nuclei into two daughter cells, each with a nucleus. 

In this division, no spindle formation and no distinct chromosome formation occurs. Nuclear envelope remains intact. 

The daughter cells are approximately the two equal halves of a parental cell. 

 Concept Builder
(i)    Meiosis is commonly studied using onion buds.

(ii)    Meiosis was first demonstrated by Van Benden and described by Winiwarter.

(iii)    Gametic meiosis is also called as terminal meiosis.

(iv)    Zygotic meiosis is also called as Initial meiosis.

(v)    Sporogenic meiosis is also called as Intermediate meiosis.

(vi)    If we have to calculate the number of mitotic divisions for the formation of n number of cell, it will be n-1 i.e. for getting 100 cells 99 mitotic divisions are required.

(vii)    Number of generations of mitosis required can be calculated (Like mitosis generations required for producing 128 cells are 7) using 2n, n is number of generation. 

(viii)    In animal cell, mitosis is called as Amphlastral (Spindle is associated with 2 asters). 

(ix)    In plant cells, the mitosis is called as Anastral (no aster, no centriole).

(x)    If mitosis is extranuelear, it is Eumitosls. 

(xi)    If mitosis is intranuclear, it is called as Premitosis. If centrioles are present then it is called as centric. 

(xii)    Cytokinesis: Cytokinesis can be of two types, successive and simultaneous. In successive type, cytokinesis occurs after every nuclear division. The four cells formed by successive cytokinesis can be arranged either in a linear or isobilateral tetrad. 
In the simultaneous type, cytokinesis takes place only at the end of both the divisions. The nuclei are generally arranged in the form of a tetrahedron.

Formulae Chart :

1.    Number of mitotic division for the formation of n number of cells.  
        Example : For getting 100 cells 99 mitotic division are required.

2.    Number of generations (n) of mitosis for producing 'x' cells.          

3.    Number of meiosis for the formation of 'n' seeds/grains/fruits.             


1.    According to the cell theory, cells arise from pre-existing cells. The process by which this occurs is called cell division.

2.    Any sexually reproducing organism starts its life cycle from a single-celled zygote.

3.    Cell division does not stop with the formation of the mature organism but continues throughout its life cycle.

4.    The stages through which a cell passes from one division to the next is called the cell cycle.

5.    Cell cycle is divided into two phases called (i) Interphase -a period of preparation for cell division, and (ii) Mitosis (M phase) -the actual period of cell division.

6.    Interphase is further subdivided into G1, Sand G2. G1-phase is the period when the cell grows and carries out normal metabolism. Most of the organelle duplication also occurs during this phase.

7.    S-phase marks the phase of DNA replication and chromosome duplication.

8.    G2-phase is the period of cytoplasmic growth.

9.    Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase.

10.    Chromosome condensation occurs during prophase.

11.    Simultaneously, the centrioles move to the opposite poles.

12.    The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing.

13.    Metaphase is marked by the alignment of chromosomes at the equatorial plate.

14.    During anaphase, the centromeres divide and the chromatids start moving towards the two opposite poles.

15.    Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase.

16.    Nuclear division is then followed by the cytoplasmic division and is called cytokinesis.

17.    Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell.

18.    In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes.

19.    In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent.

20.    Meiosis is divided into two phases -meiosis-I and meiosis-II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over.

21.    Meiosis I has a long prophase, which is divided further into five phases. These are leptotene, zygotene, pachytene, diplotene and diakinesis.

22.    During metaphase-I , the bivalents arrange on the equatorial plate.

23.    In anaphase-I, homologous chromosomes move to the opposite poles with both their chromatids. Each pole receives half the chromosome number of the parent cell.

24.    In telophase-I, the nuclear membrane and nucleolus reappear.

25.    Meiosis-II is similar to mitosis.

26.    During anaphase-II, the sister chromatids separate. Thus at the end of meiosis, four haploid cells are formed.

Meiosis and its significance

Meiosis is a type of cell division that results in the production of four gamete cells and a 50% reduction in the number of chromosomes in the parent cell. To develop egg and sperm cells for sexual reproduction, this process is necessary. There are four haploid daughter cells formed during meiosis (containing half as many chromosomes as the parent cell).Following are the main characteristics of meiosis:

  • Meiosis requires just one cycle of DNA replication but two successive cycles of nuclear and cell division called meiosis I and meiosis II.
  • After the paternal chromosomes have duplicated to form identical sister chromatids at the S phase, meiosis I begins.
  • Meiosis involves the pairing and recombination of homologous chromosomes.
  • At the end of meiosis II, four haploid cells are produced.

Events of Meiosis are classified as:


Meiosis 1

Prophase I: When compared to the prophase of mitosis, the prophase of the first meiotic division is often longer and more complex. Based on chromosomal behaviour, it has been further split into the following five phases: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

Leptotene: The chromosomes begin to condense at this stage and are connected to the nuclear membrane via their telomeres.

 Zygotene:  A synaptonemal complex forms between homologous chromosomes at the start of synapsis.

Pachytene - During this stage, genetic material is transferred between chromatids that are not sisters. This is called crossing over.

Diplotene - At this stage, homologous pairs are still connected at the chiasmata after synapsis has ended and the synaptonemal complex has vanished.

Diakinesis: Before metaphase 1, the nuclear membrane finally breaks down and the chromosomes are entirely condensed.

Figure 3: Sub Stages of Prophase 1, Meiosis 1

Metaphase I: The bivalent chromosomes line up on the equatorial plate during metaphase I. The spindle's opposing poles' microtubules connect to the pair of homologous chromosomes.

Anaphase I: Sister chromatids are still connected at their centromeres during anaphase I, but homologous chromosomes split.

Figure 4: Meiosis 1

Telophase I: Cytokinesis occurs after the nuclear membrane and nucleolus re-emerge, and this is known as the "diad of cells." Even though the chromosomes do experience some dispersion in many instances, they rarely reach the interphase nucleus's very stretched state. Interkinesis, which occurs between the two meiotic divisions, is typically a transient stage. Prophase II, which is substantially less complex than prophase I, comes after interkinesis.

Meiosis 2:

Prophase II: Following cytokinesis, meiosis II begins right away, typically before the chromosomes have fully expanded. Meiosis II resembles a typical mitosis in contrast to meiosis I. By the end of prophase II, the nuclear membrane is gone. Chromosomes once more become condensed. 

Metaphase II: Chromosomes align at the equator during metaphase II, and sister chromatids' kinetochores get attachments of microtubules from the spindle's opposing poles.

AnaphaseII: Beginning with the simultaneous division of each chromosome's centromere (which had been binding the sister chromatids together), anaphase II allows the chromosomes to migrate toward their respective poles of the cell.

Telophase II: Telophase II marks the completion of meiosis, during which the two chromosomal groups are once more encased in a nuclear membrane. Cytokinesis then takes place, culminating in the production of a tetrad of cells, or four haploid daughter cells.

Figure 5: Stages of Meiosis 2

Significance of Meiosis:

Even though the process itself paradoxically reduces the number of chromosomes by half, meiosis is the mechanism that allows sexually reproducing animals to maintain the particular chromosomal number of each species throughout generations. From one generation to the next, it also makes the population of organisms more genetically variable. For the process of evolution, variations are crucial.