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12th Biology 2012 Set1 Delhi Board Paper Solution

Question 29

The following is the illustration of the sequence of ovarian events (a - i) in a human female.

sequence of ovarian events (a - i) in a human  female

(i) Identify the figure that illustrates ovulation and mention the stage of oogenesis it represents.
(ii) Name the ovarian hormone and the pituitary hormone that have caused the above mentioned event.
(iii) Explain the changes that occur in the uterus simultaneously in anticipation.
(iv) Write the difference between ‘c’ and ‘h’.
(v) Draw a labeled sketch of the structure of a human ovum prior to fertilization.
OR
How does the megaspore mother cell develop into 7-celled, 8 nucleate embryo sac in an angiosperm? Draw a labeled diagram of a mature embryo sac.

Answer

Figure 'f' illustrates ovulation. It represents the ovulatory stage of oogenesis. Progesterone is the ovarian hormone released during ovulation. Follicle stimulating hormone (FSH) and Luteinizing hormone (LH) are the pituitary hormone released during ovulation. In anticipation of receiving the fertilised egg, the endometrium of the uterus gets thickened and also the blood supply to the endometrium increases. In the figure, (c) stage represents the secondary follicle and the (h) stage represents the          degenerating corpus luteum.

                                                    

Secondary follicle   Corpus luteum
It is Surrounded by layers of granulosa cells   Layers of granulosa cells absent
Presence of theca layer    No theca layer is present


   5.  Labeled diagram of the human ovum prior to fertilization.

human ovum prior to fertilization

                                                                         OR

The megaspore mother cell undergoes mitosis to form two nuclei which migrate to opposite poles, forming a 2-nucleate embryo sac. Further mitotic divisions lead to the formation of 4- nucleate followed by 8-nucleate stages of the embryo sac. In these mitotic divisions, nuclear division is not followed by cell division.

After the 8-nucleate stage, cell walls are laid down and a typical female gametophyte or embryo sac is formed. Among the eight nuclei, six are enclosed by cell walls and organised into cells, while the emaining two nuclei (called polar nuclei) are situated above the egg apparatus in a large central cell. Out of the six cells, three are grouped at the micropylar end, and constitute the egg apparatus made up of two synergids and one egg cell. The other three cells are located at  the chalazal end, and are called antipodals. Thus, a typical angiosperm embryo sac after maturity is 8-nucleated and 7-celled. 

angiosperm embryo