How many meiosis and mitosis in megasporogenesis and microsporogenesis?
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The megaspore mother cell (MMC) is diploid and gets converted into megaspores (haploid) by the process of megasporogenesis. The MMC is large and contains a dense cytoplasm and a prominent nucleus. It undergoes meiosis to produce four megaspores.
Formation of embryosac:
The nucleus of the functional megaspore divides mitotically to form 2 nuclei, which move towards the opposite ends, forming a 2-nucleate embryo sac. Two more mitotic divisions ensue, leading to the formation of 4-nucleate and 8-nucleate embryo sacs.After the 8-nucleate stage, the cell walls are laid down and the typical female gametophyte (embryo sac) gets organised.Six of the 8-nuclei get surrounded by the cell wall and the remaining two, called polar nuclei, are situated below the egg apparatus in the large central cell.Three of the six cells are placed at the micropylar end and constitute the egg apparatus (2 synergids + 1 egg cell).The synergids have special thickenings at the micropylar end. These are together called the filiform apparatus. It helps in leading the pollen tubes into the synergids.Three cells are at the chalazal end, and are called antipodal cells.
So, there is one meiotic division and 3 mitotic divisions to form an embryo sac.
In order to form a microspore tetrad, 1 meiosis is required. Each cell of the tetrad undergoes 2 mitotic divisions to form 2 male gametes. Actually microsporogenesis means formation of microspores or pollen grains. So, only 1 meiotic division is involved for the formation of four microspores.
Formation of embryosac:
The nucleus of the functional megaspore divides mitotically to form 2 nuclei, which move towards the opposite ends, forming a 2-nucleate embryo sac. Two more mitotic divisions ensue, leading to the formation of 4-nucleate and 8-nucleate embryo sacs.After the 8-nucleate stage, the cell walls are laid down and the typical female gametophyte (embryo sac) gets organised.Six of the 8-nuclei get surrounded by the cell wall and the remaining two, called polar nuclei, are situated below the egg apparatus in the large central cell.Three of the six cells are placed at the micropylar end and constitute the egg apparatus (2 synergids + 1 egg cell).The synergids have special thickenings at the micropylar end. These are together called the filiform apparatus. It helps in leading the pollen tubes into the synergids.Three cells are at the chalazal end, and are called antipodal cells.
So, there is one meiotic division and 3 mitotic divisions to form an embryo sac.
In order to form a microspore tetrad, 1 meiosis is required. Each cell of the tetrad undergoes 2 mitotic divisions to form 2 male gametes. Actually microsporogenesis means formation of microspores or pollen grains. So, only 1 meiotic division is involved for the formation of four microspores.
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Answer:
one meiosis and 3 mitosis
Explanation:
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