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Q.
Recombination between homologous chromosomes is completed by the end of
NTA AbhyasNTA Abhyas 2020
Solution:
Prophase – I of meiosis is comparatively longer and more eventful than prophase of mitosis. It is divided into five subphases –
(a) Leptotene: Due to condensation and dehydration of chromatic material, chromosomes become gradually visible. Chromosomes are arranged in a specific bouquet stage.
(b) Zygotene: Pairing of chromosomes takes place; this process is called Synapsis. During this pairing, a synaptonemal complex is formed between two chromosomes and after pairing such chromosomes are called homologous chromosomes. Pair of synapsed chromosomes is also called Bivalent or Tetrad.
(c) Pachytene: Four chromatids of each bivalent are visible. Recombination nodules appear on chromatids of non-homologous chromosomes, at which crossing over takes place in the presence of recombinase enzyme.
(d) Diplotene: Dissolution of synaptonemal chromosomes then separation of chromosomes of homologous pair. Due to this, some X-shaped structures are visible called Chiasmata. In some vertebrates, during oogenesis, this diplotene stage remains continued for several months or years.
(e) Diakinesis: Characterised by terminalisation of chiasmata. Up to this phase, chromosomes become completely condensed. The nucleolus, nuclear membrane and cell organelles are disappeared, and formation of spindle fibre has been started.