# Meiosis – Definition, Types, Steps, Importance, Examples

> Learn what meiosis is, how meiosis I and II reduce chromosome number, how crossing over creates genetic variation, and how meiosis differs from mitosis.

Canonical URL: https://biologynotesonline.com/meiosis/
Author: Sourav Pan
Last updated: August 28, 2026

![Meiosis – Definition, Types, Steps, Importance, Examples](https://biologynotesonline.com/wp-content/uploads/2022/12/Meiosis.jpg)

Meiosis is a specialized form of cell division in sexually reproducing eukaryotes in which one round of DNA replication is followed by two chromosome-segregation divisions, reducing chromosome sets from diploid to haploid. It supports sexual reproduction while generating genetic variation through processes including crossing over and independent assortment.

## What Is Meiosis?

Meiosis is a specialized type of [cell division](https://biologynotesonline.com/cell-division-definition-types-mitosis-meiosis-cell-cycle/) in which the chromosome number is reduced by half and haploid cells are produced from a diploid parental cell. It is also known as reduction division. One round of DNA replication occurs before meiosis. Two divisions follow—meiosis I and meiosis II.

Meiosis I- homologous chromosomes are paired and separated into different daughter cells, while sister chromatids remain joined. Chromosome reduction takes place in this division. The diploid chromosome number becomes haploid.

Meiosis II- sister chromatids are separated. No additional round of DNA replication takes place between meiosis I and meiosis II, therefore both divisions occurs after the single replication event. Different chromosome behavior in each division.

Meiosis is closely associated with sexual reproduction in eukaryotes. In animals, meiosis takes place in germ cells during the formation of gametes, whereas in other eukaryotic life cycles the haploid products may develop differently rather than directly becoming gametes.

Homologous chromosome pairing also provides the condition for meiotic recombination. During this process, genetic material can be exchanged between homologs, which contributes to new combinations of inherited genetic information. Exchange between homologs- an important meiotic event.

From one diploid cell, meiosis commonly gives rise to four haploid products after completion of the two divisions. One DNA replication. Two chromosome-segregation divisions. Chromosome complement reduced by half.

## Where Meiosis Fits in Sexual Reproduction

Meiosis forms the chromosome-reduction part of sexual reproduction. During meiosis, diploid precursor cells are converted into haploid cells, which contain only one chromosome from each homologous pair. [Fertilization](https://biologynotesonline.com/fertilization/) does the opposite. Two haploid cells are joined and diploid chromosome condition is restored.

Meiosis and fertilization occur at different points of the sexual life cycle. Meiosis- chromosome number reduced by half. Fertilization- diploidy restored. This alternation prevents chromosome number from doubling in each successive generation.

In animals, meiosis takes place in germ line and haploid reproductive cells are produced—the sperm and egg. These [gametes](https://biologynotesonline.com/gamete-types-formation-functions-examples/) are then fused during fertilization, producing a diploid fertilized cell from which development of the new organism begins. Somatic cells, on the other hand, are generally increased by mitosis rather than meiosis.

The position of meiosis is not same in all sexually reproducing eukaryotes. In animals, the diploid stage is dominant and meiosis directly produces gametes. In many fungi and algae, haploid stages can form much of the life cycle, while a diploid zygote may undergo meiosis and haploid cells are produced.

Plants- both multicellular diploid and multicellular haploid stages are present. This is alternation of generations. In this life cycle, meiosis produces haploid cells that belong to the reproductive cycle rather than simply acting as final step of gamete production.

Sexual reproduction also combines chromosome reduction with genetic reshuffling. During meiosis, homologous chromosomes are paired and meiotic recombination can occur, while segregation distributes chromosome copies into the haploid products. Fertilization then brings genetic material from two haploid cells together. Two connected processes- different functions.

## Essential Chromosome Concepts for Understanding Meiosis

The chromosomes show different forms and behaviour during meiosis. Some terms related with chromosomes are important to understand before studying the meiotic division. These are as follows-

![Paired replicated homologous chromosomes showing sister and non-sister chromatids, centromeres, a tetrad, chiasma, and crossover between homologs.](https://biologynotesonline.com/wp-content/uploads/2024/04/Meiosis-Chromosome-Terminology-Homologs-Chromatids-Bivalent-and-Chiasma-1024x725.png)Key chromosome structures used in meiosis, including homologous chromosomes, sister chromatids, the bivalent or tetrad, and the chiasma formed during recombination.

Chromosome- [Chromosome](https://biologynotesonline.com/chromosome/) is a DNA-protein structure which contains the genetic material of a cell. The DNA is associated with different proteins and remains in the form of chromatin. During cell division, the chromatin becomes condensed and chromosomes can be clearly formed.

Homologous chromosomes- These are the chromosome pairs having same genes at corresponding positions. One chromosome of the pair is maternal and another is paternal in origin. Their genes are same in type but the alleles may be different.

Chromatid- A chromatid is one of the copies of a replicated chromosome. Before meiosis, DNA replication takes place and two chromatids are formed from each chromosome. Each chromatid contains one DNA molecule.

Sister chromatids- The two chromatids formed from replication of a same chromosome are called sister chromatids. These remain associated with each other after DNA replication. They are separated later during meiosis II.

Non-sister chromatids- These are the chromatids belonging to two homologous chromosomes. They are not the replicated copies of the same chromosome. During crossing over, exchange of chromosome segments takes place mainly between the non-sister chromatids.

Centromere- It is the specialized region of a chromosome where sister chromatids remain closely associated. Kinetochore is formed at this region. Spindle fibres become attached with the chromosome through kinetochore during meiotic division.

Homologous chromosome pairing- During prophase I, [homologous chromosomes](https://biologynotesonline.com/homologous-chromosomes-vs-sister-chromatids/) come near to each other and pair along their length. This process is called synapsis. A protein structure called synaptonemal complex is associated with this pairing in many organisms.

Bivalent- The paired homologous chromosomes during prophase I are called a bivalent. Each homolog has already replicated before meiosis. Thus, one bivalent contains two homologous chromosomes.

Tetrad- A bivalent has total four chromatids, two chromatids from each homologous chromosome. Due to the presence of four chromatids, it is also referred to as a tetrad.

Crossing over- [Crossing over](https://biologynotesonline.com/crossing-over-types-mechanism-significance-example/) is an exchange of chromosome segments between non-sister chromatids of homologous chromosomes. It takes place during meiotic prophase I. As a result, new combinations of maternal and paternal DNA are produced in the chromatids.

Chiasma- The point or region where homologous chromosomes remain physically associated following crossing over is seen as a chiasma. Chiasmata help to maintain the association of homologous chromosomes until their separation. They become clearly visible during later stages of prophase I.

Cohesion- Sister chromatids are held together by proteins called cohesins. During meiosis I, cohesion along the chromosome arms is removed but the centromeric cohesion remains protected. During meiosis II, this remaining cohesion is lost and sister chromatids can separate.

Diploid chromosome number- A diploid cell contains two sets of chromosomes and is represented as 2n. The two chromosomes of each homologous pair are present in this condition. The cell entering meiosis is generally diploid.

Haploid chromosome number- [Haploid cells](https://biologynotesonline.com/haploid/) contain one set of chromosomes and are represented as n. During meiosis I, the homologous chromosomes separate and chromosome number becomes reduced into half. The cells formed after meiosis therefore contain the haploid chromosome number.

Chromosome number after DNA replication- [DNA replication](https://biologynotesonline.com/dna-replication-steps/) does not double the chromosome number. It doubles the amount of DNA and every chromosome now consists of two sister chromatids. Thus, a replicated chromosome having two chromatids is still counted as one chromosome.

Meiosis I- During meiosis I, the homologous chromosomes of each pair are separated from each other. Sister chromatids remain joined at this stage. Therefore, meiosis I reduces the chromosome number and it is referred to as reductional division.

Meiosis II- In meiosis II, sister chromatids are separated. No another round of DNA replication takes place before this division. This division is generally referred to as equational division.

Independent assortment- Homologous chromosome pairs arrange independently during metaphase I. Maternal and paternal chromosomes can therefore move into different combinations in the cells. It is one of the processes responsible for genetic variation during meiosis.

## Overview of Meiosis: One DNA Replication, Two Divisions

![Meiosis sequence showing one DNA replication followed by separation of homologous chromosomes in meiosis I and sister chromatids in meiosis II to form four haploid cells.](https://biologynotesonline.com/wp-content/uploads/2024/04/Overview-of-Meiosis-I-and-Meiosis-II-1024x576.webp)Meiosis combines one round of DNA replication with two divisions: homologs separate during meiosis I and sister chromatids separate during meiosis II.

Meiosis consists of one DNA replication and two successive divisions. DNA is copied only before the first meiotic division. After this, meiosis I and meiosis II take place one after another. A diploid cell finally gives haploid cells with reduced chromosome number.

Before meiosis- DNA replication takes place during premeiotic S phase. Each chromosome becomes duplicated and contains two sister chromatids. The amount of DNA increases. Chromosome number remains same.

Meiosis I- It is the first division and is also referred to as reductional division. Homologous chromosomes pair, and recombination can occur between their non-sister chromatids. Later, homologous chromosomes separate towards opposite poles but the sister chromatids remain together. The cells formed after meiosis I are haploid, although each chromosome is still made up of two chromatids.

Between meiosis I and meiosis II- A short interkinesis may be present. There is no another S phase and DNA is not replicated again.

Meiosis II- This division is more similar to mitotic division. Here the sister chromatids separate from one another. Chromosome number remains haploid. Thus, one meiotic cell generally produces four haploid products after completion of the two divisions.

## Stages of Meiosis I

Meiosis I is the first division of meiosis where the homologous chromosomes separate from each other. Sister chromatids do not separate in this division. DNA replication has already occurred before the beginning of meiosis I and each chromosome is therefore made up of two sister chromatids. The chromosome number becomes reduced to half. Thus, meiosis I is also referred to as reductional division.

The meiosis I occurs in following stages-

### 1. Prophase I

Prophase I is the longest and more complex stage of meiosis I. During this stage homologous chromosomes come together, pairing occurs and recombination also takes place. It is further divided into five stages, namely leptotene, zygotene, pachytene, diplotene and diakinesis.

![Leptotene, zygotene, pachytene, diplotene, and diakinesis showing chromosome condensation, synapsis, recombination, and visible chiasmata.](https://biologynotesonline.com/wp-content/uploads/2024/04/Five-Stages-of-Prophase-I-in-Meiosis-1024x439.png)During prophase I, homologous chromosomes pair, recombine, remain connected at chiasmata, and progressively condense before metaphase I.

Leptotene- Chromosomes start to condense and become visible as thin thread-like structures. The chromosomes are already duplicated, however the two sister chromatids cannot be easily distinguished at this stage. In many organisms, programmed double-strand breaks in DNA also begin during this period and these are involved in meiotic recombination.

Zygotene- In this stage, homologous chromosomes begin to pair with each other. This pairing is called synapsis. Synaptonemal complex starts developing between the homologous chromosomes and connects them more closely along their length.

Pachytene- Synapsis becomes complete. Each homologous pair now contains four chromatids. The paired chromosomes remain closely attached and recombination progresses between non-sister chromatids. Crossover products are established during this stage.

Diplotene- The synaptonemal complex starts disappearing and homologous chromosomes begin separating from one another. Complete separation does not occur. They remain connected at some points called chiasmata. These are associated with the crossing over which has occurred earlier and the chiasmata become clearly visible.

Diakinesis- It is the final stage of prophase I. Chromosomes are highly condensed and bivalents can be clearly seen. Chiasmata still hold the homologous chromosomes at certain regions. The nuclear envelope breaks down and the cell now proceeds towards metaphase I.

### 2. Metaphase I

The homologous chromosome pairs or bivalents are arranged at the equatorial region of the spindle. One chromosome of each homologous pair remains directed towards one pole and its homolog towards the opposite pole.

The sister kinetochores act together in meiosis I. Therefore, both sister chromatids of a chromosome remain oriented towards the same pole instead of opposite poles.

Arrangement of each homologous pair is independent. Maternal and paternal homologs may therefore occur in different combinations in the daughter cells.

### 3. Anaphase I

In this stage, homologous chromosomes separate and move towards opposite poles. The sister chromatids remain joined together at their centromeric region.

Cohesion present along the chromosome arms is released. This allows the homologs connected through chiasmata to separate, while cohesion at the centromere is still protected. Each pole therefore receives one chromosome from each homologous pair. But the chromosome still contains two sister chromatids.

### 4. Telophase I

The separated chromosomes finally reach their respective poles. Nuclear envelope may develop around each set of chromosomes in many cells. In some organisms this stage is very short and the chromosomes directly proceed towards the second meiotic division.

Cytokinesis generally takes place and two haploid daughter cells are formed. The chromosome number has been reduced to half. However, every chromosome still consists of two sister chromatids.

There is no DNA replication between meiosis I and meiosis II. A short resting stage called interkinesis may be present before meiosis II.

## Stages of Meiosis II

Meiosis II takes place after the completion of meiosis I. There is no replication of DNA between meiosis I and meiosis II. It is similar to mitotic division because during this division the sister chromatids are separated. At the beginning of meiosis II, each chromosome is still made up of two chromatids.

The following are the stages of meiosis II-

### 1. Prophase II

During prophase II, the chromosomes become short and condensed. The spindle fibres are formed again in each of the two haploid cells.

If nuclear membrane was formed during telophase I, it starts to disappear. Nucleolus also disappears. The chromosomes now become ready for their movement during the next stages.

### 2. Metaphase II

In metaphase II, chromosomes are arranged at the equatorial region of the cell. Each chromosome consists of two sister chromatids joined at the centromere.

The spindle fibres become attached with the kinetochores of sister chromatids from the opposite poles. Here chromosomes are arranged singly and not as homologous chromosome pairs like in metaphase I.

### 3. Anaphase II

During anaphase II, the centromeric cohesion holding the sister chromatids is removed. The two sister chromatids now separate from each other.

After their separation, each chromatid behaves as an individual chromosome. These chromosomes move towards the opposite poles of the cell by the action of spindle fibres.

### 4. Telophase II

In telophase II, chromosomes reach at the opposite poles. The chromosomes now start to become less condensed.

Spindle fibres disappear and nuclear membrane is formed around each group of chromosomes. Thus, separate haploid nuclei are formed at the two poles.

### 5. Cytokinesis

Cytokinesis takes place after or along with telophase II. In this process, the cytoplasm is divided and separate daughter cells are formed.

At the end of meiosis II, four haploid cells are generally produced from one original diploid cell. Each chromosome of these haploid cells contains only one chromatid.

## How Does Meiosis Generate Genetic Variation?

![Crossing over between non-sister chromatids creates recombinant chromosomes, while independent orientation of homologous pairs produces different chromosome combinations.](https://biologynotesonline.com/wp-content/uploads/2024/04/Genetic-Variation-in-Meiosis-Crossing-Over-and-Independent-Assortment-1024x439.webp)Meiosis generates genetic variation through recombination between homologous chromosomes and independent assortment of homolog pairs during metaphase I.

Meiosis produces genetic variation by rearranging the genetic material present in homologous chromosomes. Two major processes are involved in this variation, crossing over and independent assortment.

- Crossing over- In prophase I, homologous chromosomes pair and crossing over occurs between their non-sister chromatids. Corresponding DNA segments are exchanged. This forms recombinant chromatids having new combinations of maternal and paternal alleles. The number and position of crossovers are not same in every meiosis, therefore different recombinant chromosomes can be formed.

- [Independent assortment](https://biologynotesonline.com/mendels-law-of-independent-assortment/)- Each homologous chromosome pair can take either orientation on the spindle during metaphase I. One pair is arranged independently of other chromosome pairs. When homologs separate at anaphase I, different mixtures of maternal and paternal chromosomes pass into the daughter cells. For n homologous pairs, independent assortment alone can give 2^n chromosome combinations, without including the variation produced by crossing over.

## Chromosome Number and DNA Content During Meiosis

Chromosome number and DNA content change differently during meiosis. Here n shows chromosome sets and C represents amount of DNA. DNA content first becomes doubled, but chromosome number remains same.

![Timeline showing 2n 2C before replication, 2n 4C after S phase, n 2C after meiosis I, and n 1C after meiosis II.](https://biologynotesonline.com/wp-content/uploads/2024/04/Chromosome-Number-and-DNA-Content-During-Meiosis-1024x512.png)DNA replication doubles DNA content without changing chromosome number; meiosis I reduces chromosome sets, and meiosis II separates sister chromatids.

- Before DNA replication- The cell is diploid with 2n chromosomes and 2C DNA. Each chromosome has one chromatid.

- After S phase- DNA has been replicated. So, DNA content becomes 4C, whereas chromosome number is still 2n. The chromosomes now contain two sister chromatids. This condition remains during prophase I and metaphase I.

- After meiosis I- Homologous chromosomes are separated into two cells. Each cell now has n chromosomes and 2C DNA. The chromosomes still remain duplicated, with two sister chromatids in each chromosome.

- Meiosis II- It starts with n, 2C condition. No DNA replication takes place between meiosis I and meiosis II. Sister chromatids separate during anaphase II.

- After meiosis II- The chromosome condition is n and DNA content becomes 1C. Each chromosome now consists of a single chromatid.

## Where Does Meiosis Occur and What Does It Produce?

Meiosis occurs in cells involved in sexual reproduction. Its site is not same in all organisms, also the products differ. In animals, meiosis is associated with germ cells. In plants the products are spores, not gametes directly.

- In animals- Germ cells undergo meiosis in the reproductive organs or gonads. The male germ cells are present in testes. They form haploid cells, which later develop into sperm. In females, meiosis occurs in germ cells of ovaries and the haploid egg lineage is produced. The chromosome number by these divisions is reduced from diploid to haploid.

- In plants- Meiosis takes place in diploid cells of the sporophyte. Haploid spores are formed. Not gametes directly. In flowering plants, the microspore mother cells undergo meiosis in anther and microspores are produced. Megaspore formation occurs inside the ovule. These spores later undergo mitotic divisions. Gametophytes are developed from them, from which the gametes are formed.

- In some unicellular eukaryotes- Organisms such as yeast also undergo meiosis. Under specific environmental conditions the diploid cells divide meiotically, and haploid spores are produced.

## Importance of Meiosis

The importance of meiosis are as follows-

- Maintenance of chromosome number- Meiosis reduces the chromosome number from diploid to haploid in the reproductive cells. Half number is produced. After fertilization, two haploid sets combine and the diploid chromosome number is restored again.

- Genetic variation- Crossing over and independent assortment occur during meiosis and different combinations of genetic material are formed. The products are therefore genetically different. Maternal and paternal chromosomes are assorted in different combinations, while crossing over exchanges DNA between homologous chromosomes.

- Sexual reproduction- Meiosis forms the haploid cells required in sexual reproduction. These cells carry one chromosome set, which allows diploid condition to be formed again after fertilization.

- Proper chromosome segregation- Homologous chromosomes are separated in meiosis I and sister chromatids in meiosis II. Hence, one member of each homologous pair passes into the haploid meiotic products. Crossing over also helps proper segregation of homologous chromosomes in many eukaryotes.

- Variation for evolution- The genetic differences produced through meiosis provide variation among offspring. This variation forms the material on which natural selection can act.

## What Happens When Meiosis Goes Wrong?

Meiosis needs proper pairing of chromosomes, recombination and their separation. Errors in these events may give unequal chromosome distribution. In some germ cells the meiotic process stops itself, so normal gametes are not formed.

![Comparison of meiosis I nondisjunction producing two n+1 and two n−1 products with meiosis II nondisjunction producing two normal, one n+1, and one n−1 product.](https://biologynotesonline.com/wp-content/uploads/2024/04/Nondisjunction-in-Meiosis-I-and-Meiosis-II-1024x683.webp)Nondisjunction has different outcomes depending on whether homologous chromosomes fail to separate in meiosis I or sister chromatids fail to separate in meiosis II.

Nondisjunction- Failure of chromosomes to separate properly during meiosis is called [nondisjunction](https://biologynotesonline.com/nondisjunction-types-causes-consequences-examples/). The products have abnormal chromosome number. One chromosome extra or one missing. This condition is referred to as aneuploidy.

- Meiosis I nondisjunction- Here the homologous chromosomes fail to separate at anaphase I and both pass towards the same pole. Meiosis II then produces two products having n+1 chromosomes. Other two have n−1.

- Meiosis II nondisjunction- The first meiotic division occurs normally. In one cell, however, sister chromatids do not separate during anaphase II. Two products remain normal, while one receives n+1 and the other n−1 chromosome.

After fertilization- An aneuploid gamete may unite with a normal gamete. Chromosome imbalance is then present in the zygote. Extra chromosome gives trisomy. Loss of one chromosome results in monosomy. Many such chromosome imbalances do not survive development and pregnancy loss can occur, although some are viable. Down syndrome, Turner syndrome and Klinefelter syndrome are among the conditions resulting from chromosome-number abnormalities.

Errors in chromosome pairing and recombination- Abnormal synapsis or recombination can disturb chromosome segregation. Meiosis becomes arrested in some germ cells. Others may form aneuploid gametes. Such errors are also associated with infertility.

In human oocytes, chromosome segregation errors are more frequent with increasing maternal age. Maternally derived aneuploidies are an important cause of miscarriage and age-related infertility, and most cases of trisomy 21 also arise from them.

## Meiosis vs. Mitosis

The major differences between meiosis and [mitosis](https://biologynotesonline.com/mitosis/) are as follows-

CharacteristicsMeiosisMitosisType of divisionIt is a reductional type of cell division.It is generally an equational cell division.Number of divisionsTwo successive divisions occur, meiosis I and meiosis II.Only one division takes place.DNA replicationDNA replicates once before meiosis I. No DNA replication occurs between meiosis I and II.DNA replication occurs once before the mitotic division.Number of daughter cellsUsually four haploid cells are produced from one diploid cell.Two daughter cells are formed from one parent cell.Chromosome numberChromosome number is reduced to half. Diploid becomes haploid.Chromosome number generally remains same as the parent cell.Pairing of homologous chromosomesHomologous chromosomes pair during prophase I. This pairing is called synapsis.Homologous chromosome pairing does not normally occur.Crossing overCrossing over takes place between non-sister chromatids of homologous chromosomes during prophase I.Crossing over is normally absent.Metaphase arrangementHomologous chromosome pairs are arranged at the equatorial region in metaphase I.Individual duplicated chromosomes are arranged at the metaphase plate.First separationHomologous chromosomes separate during anaphase I. Sister chromatids remain joined.Sister chromatids separate during anaphase.Genetic nature of productsThe meiotic products are genetically different from one another due to recombination and independent assortment.Daughter cells are usually genetically very similar to the parent cell and to each other, except for mutations.OccurrenceIt occurs in cells involved with sexual reproduction. In animals it occurs in germ cells, while in plants it forms haploid spores.It occurs mainly in somatic cells and also in cells undergoing growth or asexual proliferation.Major functionIt forms haploid products and helps maintain chromosome number between generations during sexual reproduction. Genetic variation is also produced.It is used for growth, cell replacement and asexual reproduction in many organisms.

![Meiosis Overview Poster](https://biologynotesonline.com/wp-content/uploads/2024/04/What-Is-Meiosis-725x1024.webp)Meiosis Overview Poster

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