Concept:

• The cell is the basic unit of life. Life arises from pre-existing cells. Cells grow and multiply to form a diversity of life forms, this process of growth and multiplication of cells is called Cell Division.
• Cell division is of three types:

1. Mitosis – Equational division, occurs in somatic (non-sex) cells
2. Meiosis – Reducttional division, occurs in sex cells
3. Amitosis – Direct type of division, occurs in prokaryotes

• Meiosis can be further divided into two stages – Meiosis I and Meiosis II

Explanation:

• Prophase I of Meiosis I has 5 sub-stages
• Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
• The Zygotene stage is characterized by the pairing of homologous chromosomes called the ”Synapsis”
• The pairs of homologous chromosomes are called Bivalents.
• There develops a structure between the homologous chromosomes called the synaptonemal complex. It is a tripartite structure i.e. it is made up of 3 thick lines of DNA and protein.

Additional Information

• Leptotene: During leptotene, the chromatin condenses to form the chromosomes. Chromosomes are the longest and thinnest in this stage.
• Pachytene: This stage is characterized by the occurrence of crossing over. Non-sister chromatids of the homologous chromosomes exchange their genetic parts.
• Metaphase I: The first metaphase of meiosis characterized by the alignment of paired chromosomes along the center (metaphase plate) of a cell, which ensures that two complete copies of chromosomes are present in the resulting two daughter cells of meiosis I.