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Class 12th Biology 2013 Set3 Delhi Board Paper Solution

Question 29

(a) Draw a diagram of a mature embryo sac of an angiosperm and label the following parts in it:
(i) Filiform apparatus
(ii) Synergids
(iii) Central cell
(iv) Egg cell
(v) Polar nuclei
(vi) Antipodals
(b) Write the fate of egg cell and polar nuclei after fertilization. OR (a) Describe the events of Oogenesis with the help of schematic representation.
(b) Write two differences between Oogenesis and Spermatogenesis.


mature embryo sac of an angiosperm

(b) During fertilization, 1st male gamete fuses with the egg cell and forms the diploid zygote, while the second male gamete fuses with the polarnuclei forms the triploid endosperm.
(n)               +                     (n)  ------------------> 2n                   Diploid Zygote
          1st male                          Egg Cell
               (n)             +                    (2n) -------------------->  3n               Triploid endosperm
          2nd male                        Central Cell
            gamete                  having the polar nuclei

events of Oogenesis

The process of formation of a nature female gamete is called oogenesis, which is different from spermatogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary, no more oogonia are formed and added after birth. These cells start division and enter into prophase 1 of the meiotic division and get temporarily arrested of the stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle. The primary follicles get surrounded by more layers of granulose cells and a new theca and called secondary follicle which is characterized by a fluid filled cavity called antrum. Primary oocyte within the tertiary called antrum. Primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. The tertiary follicle further changes into the mature follicle or graffian follicle. Graffian follicle now ruptures to release the secondary
oocyte (ovum) from the ovary by the process called ovulation.