Meiosis- Definition, Stages and Diagram

Meiosis and Mitosis are the types of cell division. Do you know what type of cell division takes place in our germ cells? Is it mitosis or Meiosis? Humans have \(46\) chromosomes in each cell. How this number is maintained in every individual. Is it getting doubled, tripled….? Both mitosis and Meiosis take place after the interphase but are they similar or different.  In this article, let’s understand everything about Meiosis.

Cell Cycle

Cell cycle refers to the series of events that repeat several times and includes DNA synthesis or duplication, cell growth and cell division.

1. Cell cycle in eukaryotes is divided into following phases:
   a. Interphase– This phase includes:
   (i) G1 phase
   (ii) G0 phase
   (iii) S phase
   (iv) G2 phase
   b. ‘M’ phase– This phase includes both mitosis and meiosis. M phase is the dividing phase of the cell cycle.

Meiosis Definition

Meiosis involves cell division which results in the formation of four daughter cells, each with half the number of chromosomes compared to the single parent cell. Meiosis is also called REDUCTIONAL DIVISION.

Why do we need four daughter cells with half the number of chromosomes?

In sexual reproduction, two cells fuse which could make the zygote with a doubled chromosome number. In order to maintain the chromosome number in species, it is necessary that two fusing cells contain half the number of chromosomes, i.e. \(23\) chromosomes and not \(46\). So, to maintain the chromosome number in the zygote the chromosome number in the fusing cells needs to be halved. As two sequential cycles of nuclear and cell division are involved in meiosis, it gives us four-daughter cells with half the number of chromosomes.
Since chromosome number gets halved, it is also called reductional division.
1. Meiosis is a process that gives rise to a haploid cell from a diploid one. 
2. Haploid cell is the one that has a single set of chromosomes. While diploid cells have a double set of chromosomes.
3. Body cells are examples of diploid cells.
4. Reproductive cells are examples of haploid cells.
5. Meiosis is discovered by Van Benden.
6. The Meiosis term is coined by Farmer and Moore.
7. Cells in which meiosis occurs are called meiocytes.
8. It occurs in higher organisms.

Meiosis Stages

In meiosis two divisions take place which results in the formation of four haploid daughter cells. The two stages of meiosis are-

1. Meiosis I- Homologous pairs of chromosomes separate during meiosis I.

2. Meiosis II- Sister chromatids are formed due to replication in ‘S’ phase are separated during Meiosis-II.

Meiosis Diagram

Figure showing stages of meiosis.

Meiosis I

When a cell undergoes meiosis I, it has already undergone Interphase, where DNA replication and centriole duplication has taken place. Meiosis I is divided into the following stages:-

1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I

Prophase I is further divided into the following stages:

a. Leptotene- ‘Leptos’ means thin, and ‘tene’ means thread.
1. In this stage, a thin thread of chromatin fibre appears.
2. Here the nuclear membrane remains intact. Centriole remains at one pole.
3. Chromatin fibres start to condense. But it occurs at intervals which results in a beaded structure.
4. Thread remains attached to the nuclear membrane from their one end and so this stage appears like a bouquet and is so-called a bouquet stage.
5. Other membranous structures also remain intact.

Figure showing different stages of Prophase I

b. Zygotene

1. Nuclear membrane still intact.
2. Pairing of homologous chromosomes takes place and this pairing is called synapsis.
3. Further condensation also takes place.
4. Paired chromosomes are called dyad. These chromatids undergo vertical splitting and form tetrad.
5. To hold this pair together, a synaptonemal complex is formed.
6. Sister chromatids refer to the arms of one chromosome whereas non-sister chromatids refer to the arms of two homologous chromosomes. 
7. Crossing over takes place between non-sister chromatids which do not take place at this stage but two non-sister chromatids have started to come closer but have not touched each other.

Fig:- Homologous chromosomes

c. Pachytene

1. Nuclear membranes remain intact. Centrioles are still in the same position.
2. Crossing Over non-sister chromatids of homologous chromosomes takes place.
3. Crossing results in the formation of Chiasma.
4. Recombinase enzymes are responsible for the exchange of genetic material called recombination. This results in variation.

d. Diplotene-

1. Everything is still intact. Like nuclear membrane, etc.
2. The non-sister chromatids still remain attached but the synaptonemal complex disintegrates.
3. Separation of homologous chromosomes is called disjunction.
4. Terminalisation starts. This helps in separation of non-sister chromatids like a zipper.

e. Diakinesis

1. Nuclear membrane disappears.
2. All changes that take place in the prophase of meiosis will take place at this stage.
3.  Centrioles move apart.
4. Spindle fibres become visible which attaches to the chromosome.
5.  All membranes disappear and disassociate.
6. Chromosomes with exchanged genetic material are visible.

Metaphase I

Homologous chromosomes align themselves along the equator. This alignment occurs randomly and this is called independent assortment. This results in genetic diversity.

Anaphase I

1. One chromosome of the homologous pair gets separated from each other and is retracted towards the pole as spindle fibre retracts.
2. This helps in equal division of DNA between two cells.

Telophase I

1. Nuclear membrane reappears.
2. Spindle fibres disintegrate.
3. Telophase is followed by cytokinesis to form two daughter cells.

Cytokinesis

Cytoplasm gets divided into two parts by the process of cytokinesis. It results in the formation of two daughter cells with a haploid set of chromosomes.

Fig: Meiosis I

Meiosis II

Meiosis II includes the following stages:

Prophase II
During this stage, the nuclear membrane disintegrates and centrioles move apart from each other to the opposite poles. This happens in each of the haploid daughter cells resulting from meiosis I.
Metaphase II
Sister chromatids are lined by the spindle fibres on the equator to form a metaphase plate.
Anaphase II 
When spindle fibres retract, sister chromatids separate and move towards opposite poles.
Telophase II 
This is the last stage of meiosis II and there is a breakdown of spindle fibres and reformation of the nuclear membrane around each nucleus resulting in four haploid daughter cells followed by cytokinesis.

Summary

Meiosis results in the formation of four daughter cells with half the number of chromosomes. This process takes place in gametes formation which are involved in sexual reproduction in higher organisms.

FAQs

Q.1. What is mitosis and meiosis?
Ans: Mitosis refers to the cell division which results in equational division and takes place in lower organisms. Meiosis refers to cell division which results in reductional division and takes place in higher organisms.

Q.2. Explain why meiosis and gametogenesis are always interlinked?
Ans: Gametogenesis refers to the formation of gametes and meiosis results in the formation of four haploid cells by reductional division. Gametes are formed by this meiosis process.

Q.3. Genetic recombination occurs in which phase of meiosis \(1\)?
Ans: Genetic recombination occurs in the pachytene phase of prophase I of meiosis I.

Q.4. What is the significance of meiosis?
Ans: Meiosis is a reductional division and helps in the production of haploid gametes in higher organisms which after fertilization form zygote.

Q.5 What is meiosis and its function?
Ans: Meiosis is the type of cell division that takes place after the interphase in higher organisms. It is also called reductional division. It helps in the formation of haploid cells like germ cells which fuse to form a zygote.

Q.6. What is the difference between mitosis and meiosis?
Ans:

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