Question

Arrangement of nuclei in normal dicot embryo sac is
(a) 3+3+2
(b) 2+4+2
(c) 3+2+3
(d) 2+3+3

Answer 1

Hint: In the egg apparatus, there is one egg cell and two synergids, two polar nuclei, and three antipodal cells. Two sequential mitotic divisions result in the formation of four- nucleate and ultimately eight-nucleate stages leading to the formation of the female gametophyte or embryo sac.

Complete answer:
Arrangement of nuclei in a normal dicot embryo sac is 3+2+3 that represents the three-celled egg apparatus viz. - Two synergids and an egg cell, two polar nuclei, and three antipodal cells. The functional megaspore divides mitotically forming two- nucleate embryo sacs. Synergids also are known as helper cells or co- operative cells. These cells facilitate the entry of the pollen tube inside the embryo cell. One synergid starts to degenerate just with pollination. They are characterized by the filiform apparatus.
The egg shows cytoplasmic polarity opposite to synergids. Antipodals, also known as vegetative cells. These are the vegetative cells of the embryo sacs. The central cell is the largest cell of the embryo sac that initially contains two polar nuclei and forms a secondary nucleus or definitive nucleus just before fertilization (2n).
- Megasporogenesis is the process of the formation of the female gametophyte from the Mother Megaspore cell (MMC).
- The pollen grain germinates on the stigma after pollination and the resulting pollen tube grows through the tissues of the stigma and style and reaches the ovule. The pollen tube enters the embryo sac through the micropyle end and discharges two male gametes.
- One of the male gametes fuses with the egg cell to form a zygote, this is known as syngamy. The other make gamete fuses with the diploid secondary nucleus to produce triploid primary endosperm nucleus (PEN).
- Thus, double fertilization occurs with triple fusion.
- After fertilization, the synergids and antipodal cells degenerate.
So, the correct answer is ‘ 3+2+3’.

Note:
- The egg cell is haploid i.e. n in nature whereas the central cell is diploid (2n).
- There are three antipodals at the chalazal end of the embryo sac which later degenerates.
- One synergid remains functional and the other degenerates after pollination.