Class 12th Biology 2015 Set3 Delhi Board Paper Solution

The number of viable seeds produced by the tomato plant through sexual reproduction = 200

(a) The number of ovules minimally involved in this process would be 200 as the number of viable seeds are 200. After fertilisation, the ovary turns into a fruit and the ovule turn into a seed. Thus, the number of ovules would correspond to number of seeds formed.

(b) In the above case, 50 microspore mother cells must have undergone reduction division prior to dehiscence of anther, as each microspore mother cell would give rise to 4 microspores. One microspore mother cell would produce 4 microspores; therefore, to obtain 200 microspores, there should be 50 microspore mother cells.

(c) The minimum number of pollen grains that must land on stigma for pollination are 200 because each pollen grain contains 2 male gametes. Out of theses 2 gametes, 1 fuses with polar nuclei and forms endosperm, while the other male gamete fuses with the egg cell to form the zygote, which eventually gives rise to seeds. Therefore, in order to obtain 200 seeds, the number of pollen grains needed would be 200.

(d) The number of male gametes involved in seed formation would be 200 as each male gamete will fuse with 1 egg nuclei to form zygote, which will further give rise to the seed.

(e) In the process of gametogenesis, 200 megaspore mother cells would be involved. Only one megaspore of the tetrad becomes functional and develops further and the rest three are degenerated.

 

OR

 

Oogenesis

The ovum (egg) is formed by the process of oogenesis. It starts during embryonic growth with millions of gamete mother cells (oogonia) forming in the foetal ovary. These cells undergo meiosis, but get temporarily arrested at the prophase and are called primary oocytes. Before reaching puberty, a large number of primary oocytes degenerate and the remaining ones get surrounded by layers of granulosa cells and new theca and are called secondary follicles. The secondary follicles are then converted into tertiary follicles that have a fluid-filled cavity called the antrum. At this stage, the primary oocyte present within the tertiary follicle completes meiosis, which results in the formation of haploid secondary oocyte and a tiny polar body. This tertiary follicle further changes into the Graafian follicle. The secondary oocyte is surrounded by the zona pellucida. Then the Graafian follicle ruptures to release the ovum by ovulation.