Lesson 3

• The female gamete formation and egg cell development
• Homework-crossword puzzle on gamete formation

Female gamete formation:
The embryo sac produces female gametes called the egg cell and two polar nuclei.

• The ovule is made up of diploid cells
• Meiosis occurs & this cell undergoes mitosis 3 times to form the embryo sac
• This contains the egg cell and 2 polar nuclei

• Each ovary contains one or more ovules
• An ovary has two walls called integuments
• Between the integuments is a small opening
(micropyle) through which a pollen tube can enter.
• The nucellus provides nutrients for growth in the ovule
Integuments micropyle nucellus

• After mitosis the embryo sac contains 8 haploid nuclei.
• 5 of these nuclei die
• With 3 nuclei left, 1 forms a haploid egg cell and the other 2 form haploid polar nuclei in the centre of the embryo sac.
• The egg cell and the polar nuclei are the female gametes.

• The two haploid nuclei in the centre of the mature embryo sac.
• These fuse with one of the male gamete nuclei to form the endosperm nucleus, which is triploid (3n)

• http://www.youtube.com/watch?v=a0stgD wBbN4 (4mins to 10mins)

• One cell in the ovule, the megaspore mother cell
(2n) divides by meiosis to form four haploid cells
• Three of these cells disintegrate
• The remaining cell is called the embryo sac.
Megaspore mother cell
(2n)
Meiosis
Embryo sac (n)

• The nucleus of the embryo sac (n) divides by mitosis three times forming eight haploid nuclei
• These are still contained within the embryo sac
• Five of these nuclei will later disintegrate
• The remaining three are the gametes
• Two of these form the polar nuclei in the embryo sac
• The remaining female gamete forms an egg cell
Embryo sac (n)
Mitosis (3 times)
Gametes Polar nuclei (n)
Gamete – Egg cell
(2n)

Meiosis
Megaspore mother cell (2n)
Embryo sac
(n)
Mitosis (3 times)
Polar nuclei
Egg cell

• http://www.edcodigital.ie/books/leavingcertificate-biology-revisededition/#ebookresources

• Embryo sac development involves:
• meiosis occurring in the ovule to produce a haploid embryo sac
• the embryo sac later producing eight haploid nuclei by mitosis:
• five of these nuclei degenerate
• two nuclei form the polar nuclei
• the remaining nucleus forms the egg cell
• the polar nuclei and egg cell are the gametes

• Female gamete formation:
• The embryo sac produces female gametes called the egg cell and two polar nuclei.