Meiotic Cell Division

Meiotic Cell Division is the process in which a mother cell divides to form four daughter cells having the number of chromosomes of the mother cell.

The process in which a mother cell divides to form four daughter cells having the number of chromosomes half of that of the mother cell is called a meiotic cell division. It occurs in reproductive cells.

Homologous Pair: The identical male and female chromosome which makes a pair is called a homologous pair of the chromosome.

Synopsis: The process of the pairing of homologous chromosomes is called synapsis.

Crossing Over: The process of exchanging the hereditary materials between two non-sister chromatids of homologous pairs is called crossing over. It brings variation in the organisms.

Stages of Meiotic Cell Division

Meiosis completes into two successive divisions, they are:

1. Meiosis - I
2. Meiosis - II

1. Meiosis - I

It completes in four stages, they are:

a. Prophase-I

b. Metaphase-I

c. Anaphase-I

d. Telophase-I

a. Prophase-I

It is the initial and the longest stage of cell division. This stage is divided into five sub stage, they are:

i. Leptotene

ii. Zygotene

iii. Pachytene

iv. Diplotene

v. Diakinesis

i. Leptotene

Characteristics

• The site of the cell and the nucleus increases at this stage.
• Bead-like chromosomes are seen in the middle of the chromosome when observed under a microscope.
• The chromosomes pair up but look like a single one.
• The nuclear membrane and nucleolus become distinct.

ii. Zygotene

Characteristics

• Identical male and female chromosomes make a pair.
• Chromosomes are shorter and thicker.
• In each pair, one chromosome comes from paternal and other from maternal.

iii. Pachytene

Characteristics

• It begins after the pairing of homologous chromosomes which are much shorter and thicker.
• At the end of this stage, homologous chromosomes split and a tetrad is formed.
• The exchange of hereditary materials or segments of chromosomes takes place.

iv. Diplotene

Characteristics

• The nuclear membrane begins to disappear.
• The chromosome becomes shortened and more coiled.
• Chromosomes begin to split by repulsive force and make contact at certain points called chiasmata.
• Chiasmata move away from centromere with the increase in its breadth.

Chiasmata: It is the point of contact between two non-sister chromatids of homologous pairs.

v. Diakinesis

Characteristics

• Chromosomes are shorter and thicker.
• The chiasma moves towards the end of the chromosome. The shifting of chiasmata towards the end of the chromosome is called terminalization.
• The nuclear membrane and nucleus tend to disappear.
• Centrioles reach towards the opposite poles and spindle fibers begin to form.

b. Metaphase-I

Characteristics

• The nuclear membrane disappears.
• Centriole reaches the equatorial plane.
• Spindle fibers attach with the centromere of the chromosome.
• Pairs of homologous chromosomes lie in the middle.

c. Anaphase-I

Characteristics

• Homologous pair separate from each other and move towards the opposite poles of the cell.
• The real reduction takes place at this stage because the chromosome number of the daughter nuclei formed is reduced to half than in the mother cell.

d. Telophase-I

Characteristics

• The nucleous are formed in each pole which are haploid.
• Two daughter cells are formed by dividing the cell membrane or cell plate in cytokinesis.
• Meiosis-II begins after this stage.

2. Meiosis - II

It completes in four stages, they are:

a. Prophase-II

b. Metaphase-II

c. Anaphase-II

d. Telophase-II

a. Prophase-II

Characteristics

• While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II.
• The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole.
• The two pairs of centrioles separate into two centrosomes.
• The nuclear envelope breaks down, and the spindle apparatus forms.

b. Metaphase-II

Characteristics

• Each of the daughter cells completes the formation of a spindle apparatus.
• Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis.
• For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore-microtubule coming from that pole.

c. Anaphase-II

Characteristics

• The centromeres divide and the spindle fibers pull the chromatids to opposite poles.
• The separated chromatids are now called chromosomes in their own right.
• Four groups of chromosomes are formed at the end of Anaphase- II.

d. Telophase-II