Class 12th Biology 2015 Set2 Delhi Board Paper Solution

The number of viable seeds produced by the brinjal plant through sexual reproduction = 360

(a) The number of ovules minimally involved in this process would be 360, as the number of viable seeds are 360. After fertilisation, the ovary turns into fruit and the ovules turn into seeds. Therefore, the number of ovules are corresponding to the number of seeds formed.

(b) During the process of gametogenesis, 360 megaspore mother cells are involved as only one megaspore of the tetrad becomes functional and develops further and the rest three megaspores get degenerated.

(c) The minimum number of pollen grains that must land on stigma for pollination are 360 because each pollen grain contains two male gametes. Out of theses two gametes, one fuses with polar nuclei and forms endosperm, while, the other male gamete fuses with the egg cell to form the zygote that eventually give rise to seeds. Therefore, in order to obtain 360 seeds, number of pollen grains needed would be 360.

(d) The number of male gametes involved in seed formation would be 360 as each male gamete will fuse with one egg nuclei to form zygote, which will further give rise to the seed.

(e) In the above case, 90 microspore mother cells must have undergone reduction division prior to dehiscence of anther, as each microspore mother cell would give rise to 4 microspores. Since 1 microspore mother cell would produce 4 microspores, therefore, to obtain 360 microspores, there must be 90 microspore mother cells.

 

OR

 

Menstrual cycle is the reproductive cycle in all primates and begins at puberty (menarche). In human females, menstruation occurs once in 28 to 29 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle. These changes are brought about by ovarian and pituitary hormones.

The changes occurring in the ovaries are as follows:

1) The cycle starts with the menstrual phase, where menstruation occurs. This is followed by the follicular phase. In this phase, the primary follicles mature into the Graffian follicles and the release of gonadotropins (LH and FSH) also increases. This causes follicular growth and the growing follicles produce oestrogen.

2) The LH and FSH are at their peak in the middle of the cycle (14th day), and cause the rupture of the Graffian follicles to release ovum. This phase is called the ovulatory phase.

3) The remains of the Graffian follicles get converted into the corpus luteum that secretes progesterone for the maintenance of the endometrium.

4) In the absence of fertilisation, the corpus luteum degenerates, thereby causing the disintegration of the endometrium and the start of a new cycle.

 

The changes occurring in the uterus are as follows:

1) In the absence of fertilisation, during the menstruation phase, the endometrium of the uterus breaks down. Blood vessels in liquid state are discharged.

2) During the follicular phase, the action of the gonadotrophins (LH and FSH) causes the regeneration of the endometrium to receive the fertilised egg.