Meiosis Cell Division - Process and Significant | Biology

Notes | Zoology
Complete Notes on Meiosis Cell Division
For: Science Class 11

The process of cell division which takes place in a diploid cell and produces four haploid cell is meiosis cell division i.e. Each cell having half the number of chromosomes as that of parent cell.

Meiosis I

The meiosis I the division is called reductional division or heterotypic division because the cell divides into two daughter cells having chromosome number half in two daughter cells as that present in the mother cell.

Karyokiesis

Prophase I

It is very complex and ling phase. It is further divided into five successive sub phases

Leptotene

1. The nucleus increases in size

2. Chromosomes become slender and separate and distinct threads

3. Chromosomes contain bead like structures called chromomeres.

Zygotene 

1. The homologous chromosomes make pair 

2. The process of making pair by homologous chromosomes is called synapses. 

3. Each pair is called bivalent 

4. The process of synapses is initiated by a kind of attraction force between homologous chromosomes. 

5. The homologous chromosomes can be defined as morphologically and genetically similar one paternal and one maternal chromosome are called homologous chromosomes. 

Pachytene 

1. The homologous pair coiled around each other, Chromosomes replicate, 

2. Chromosomes become shorter and thicker 

3. Each bivalent appears four stranded 

Diplotene 

1. Uncoiling of chromosomes takes place from centromere due to a kind of repulsive force 

2. Some parts however remain still attached at certain points 

3. The attaching points are called chaismata. 

4. At chiasmata chromosomal segments are broken and rejoined cross wise i.e. Exchange of maternal and paternal chromosomal segments. 

5. This phenomenon is called crossing over 

6. Some auther mention the crossing over takes place in pachytene stage. 

Diakinesis 

1. Chiasmata move towards the end of chromosome. This process is called terminalization 

2. The nuclear membrane starts to disappear 

3. Nucleolus also disappear

Metaphase I 

1. Spindle fibers appears 

2. Chromosomes arrange at the equator centromere of each bivalent face towards opposite pole. 

3. One centromere of each bivalent becomes attached with spindle fibers of one pole and another centromere of each bivalent gets attached with spindle fibers of another pole. 

Anaphase I 

1. Each bivalent separate together 

2. Separated bivalent move towards opposite poles due to contraction of spindle fiber 

3. Two groups of chromosomes are formed at each pole having haploid number of chromosomes 

Telophase I 

1. Chromosomes arrange into two groups one at each pole 

2. Nuclear membrane and nucleolus reappears 

3. Spindle fibers disappear 

4. Chromosomes become long elongated to form chromatin 

Cytokinesis: It may or may not occurs 

Meiosis II 

It is called equational division or homotypical division because the cells produced after meiosis I undergo further division to produce four cells where chromosome number remains same as that of parent cells, similar to mitosis. 

Karyokinesis

Prophase II 

The Chromosome become shorter and thicker, Nuclear membrane and nucleolus disappear. Spindle fibers start to appear 

Metaphase II 

1. Chromosomes arranged at the equator 

2. The centromere of each chromosome divides into two centromeres in longitudinal way. 

3. One centromere of each chromosome is connected with spindle fiber of one pole and another centromere is connected with spindle fiber of another pole.

Anaphase II 

1. A kind of repulsive force is developed between two daughter centromeres 

2. One of the two centromeres having its own chromatids move towards one pole and another centromere move towards another pole 

3. Two groups of chromosomes are formed at each pole due to contraction of spindle fibers. 

Telophase II 

1. Chromosomes organize into nucleus at each pole 

2. Chromosomes become long, elongated forming chromatin 

3. Nuclear membrane and nucleolus reappears 

4.  Spindle fibers disappears 

Cytokinesis 

Successive method 

The first cytokinesis takes place after meiosis first to form two daughter cells and second cytokinesis takes place after meiosis II to form four daughter cells. The process is called successive Method. 

Simultaneous Method 

In this method, cytokinesis takes place only after Meiosis II to produce four daughter cells. 

Significance of Meiosis cell division 

1. Gametes and spores are formed by meiosis divisions that are essential for sexual reproduction 

2. Meiosis helps to maintain fix number of chromosomes in organisms 

3. Crossing over causes the genetic variation among the species 

4. It helps in alternation of generations of haploid and diploid generations of plants and animals 

5. The segregation of paired chromosome resulting in different combination of character daughter cells.


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