HSEB Notes on Meiosis Cell Division Prophase-I
CLASS : 11
Meiosis Cell Division
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.
Prophase-I
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- Different Sub-Phases of Prophase-I
- The nucleus increases in size
- Chromosomes become slender and separate and distinct threads
- Chromosomes contain bead like structures called chromomeres.
Zygotene
- The homologous chromosomes make pair
Human Chromosomes
The process of making pair by homologous chromosomes is called synapses.
- Each pair is called bivalent
- The process of synapses is initiated by a kind of attraction force between homologous chromosomes.
- The homologous chromosomes can be defined as morphologically and genetically similar one paternal and one maternal chromosome are called homologous chromosomes.
Pachytene
- The homologous pair coiled around each other, Chromosomes replicate,
- Chromosomes become shorter and thicker
- Each bivalent appears four stranded
Diplotene
- Uncoiling of chromosomes takes place from centromere due to a kind of repulsive force
- Some parts however remain still attached at certain points
- The attaching points are called chaismata.
- At chiasmata chromosomal segments are broken and rejoined cross wise i.e. Exchange of maternal and paternal chromosomal segments.
- This phenomenon is called crossing over
- Some auther mention the crossing over takes place in pachytene stage.
Diakinesis
- Chiasmata move towards the end of chromosome. This process is called terminalization
- The nuclear membrane starts to disappear
- Nucleolus also disappear
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Biology