Embryo Sac Formation: Free Nuclear Divisions — NEET Biology
✅ Asked in NEET 2025Let us study about the formation of the embryo sac in detail.. The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. It is of interest to note that these mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation. After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac. Observe the distribution of cells inside the embryo sac. Six of the eight nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell.
NTA tests the sequential mitotic divisions in embryo sac development: 2-nucleate → 4-nucleate → 8-nucleate stages, with a key concept that these are FREE NUCLEAR divisions (no cell walls form immediately). Students commonly confuse this with regular mitosis where cytokinesis follows each nuclear division. The critical trap: forgetting that cell walls appear ONLY after the 8-nucleate stage, not during earlier divisions. Remember: the first three mitotic divisions are free nuclear (nuclei only, no cells), then cell organization occurs, resulting in egg apparatus (3 cells: 1 egg + 2 synergids) and polar nuclei in the central cell. This distinguishes plant gametophyte from animal development.
This paragraph was tested 2 times in NEET.
How many meiotic and mitotic divisions need to occur for the development of a mature female gametophyte from the megaspore mother cell in an angiosperm plant?
In angiosperms, the correct sequence of events leading to the formation of female gametophyte in the ovule is: (NEET 2023) A. 3 successive free nuclear divisions in functional megaspore. B. Degeneration of 3 megaspores. C. Meiotic division in megaspore mother cell. D. Migration of 3 nuclei towards each pole. E. Formation of a wall resulting in seven-celled embryo sac.
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