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Question
An ovule generally has a single embryo sac formed from a megaspore through
An ovule generally has a single embryo sac formed from a megaspore through
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Solution
The correct option is C Mitotic divisions
The embryo sac
The ovule is the female megasporangium which bears the embryo sac or the female gametophyte.
Development of embryo sac
- The megaspore mother cell of the ovule divides by meiosis to produce 4 haploid megaspores.
- Usually, the megaspore cell of the chalazal side remains functional while the other three degenerate.
- The haploid nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles to form a 2-nucleate embryo sac.
- Two more mitotic nuclear divisions result in the formation of an 8-nucleate embryo sac.
- 4 out of 8 nuclei occur at the micropylar end while the other 4 remain at the chalazal end. Three out of 4 nuclei at the micropylar end form the egg apparatus consisting of the egg cell and two synergids. The fourth one moves to the centre of the embryo sac and forms one of the polar nuclei.
- 3 nuclei at the chalazal end form the antipodal cells and the fourth one migrates to the centre to form the second polar nuclei.
- This is followed by cell wall formation forming 7 cells.
- Thus, the embryo sac becomes a 7-celled structure with 8 nuclei.
Development of embryo sac
Final answer
b. Mitotic divisions
The embryo sac
The ovule is the female megasporangium which bears the embryo sac or the female gametophyte.
Development of embryo sac
- The megaspore mother cell of the ovule divides by meiosis to produce 4 haploid megaspores.
- Usually, the megaspore cell of the chalazal side remains functional while the other three degenerate.
- The haploid nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles to form a 2-nucleate embryo sac.
- Two more mitotic nuclear divisions result in the formation of an 8-nucleate embryo sac.
- 4 out of 8 nuclei occur at the micropylar end while the other 4 remain at the chalazal end. Three out of 4 nuclei at the micropylar end form the egg apparatus consisting of the egg cell and two synergids. The fourth one moves to the centre of the embryo sac and forms one of the polar nuclei.
- 3 nuclei at the chalazal end form the antipodal cells and the fourth one migrates to the centre to form the second polar nuclei.
- This is followed by cell wall formation forming 7 cells.
- Thus, the embryo sac becomes a 7-celled structure with 8 nuclei.
Final answer
b. Mitotic divisions
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