Science [2 Mark Questions]

1. It serves a barrier to prevent arrival of sperm to egg and thus avoids unwanted pregnancy.
2. It also protects a person from sexually transmitted diseases.

Every organism faces a lot of competition for survival. Barring a very few, most of the organisms end up being food for some other organism. Competition and predation result in drastic reduction in population of a species. But reproduction ensures that population of a species is maintained at optimum level. So, it can be said that reproduction is linked to stability of population of a species.

Endometrium thickens, becomes glandular and highly vascular. The contact region between embryo and uterine wall grows into placenta. Placenta helps the embryo in obtaining nourishment and oxygen from blood sinuses of the uterus. It also helps in transfer of wastes from embryo to the blood of the mother.

The parents are diploid (2n) as each of them has two sets of chromosomes (DNAs). They form haploid (In) male and female gametes through the process of meiosis. The haploid gametes have one set of chromosomes (DNAs). During fertilization, one male gamete fuses with one female gamete. It restores the diploid (2n) chromosome/ DNA number in the offspring that is formed from fusion product or zygote (2n).

The open flowers are generally cross pollinated. Removal of stamens of a bisexual flower will not affect pollination of its intact pistil and formation of fruit.

General growth is the growth of an individual in size, height, shape, weight, etc. Sexual maturation is a set of changes in body of an individual that enable one to take part in reproduction, e.g., maturation of gonads, genitalia and other accessory structures. In human beings, beginning of sexual maturation is indicated by puberty changes like cracking of voice, new hair pattern, development of breast in female, etc.

Variations appear in the progeny of sexually reproducing organisms due to:

1. Random separation and coming together of chromosomes during gamete formation and gamete fusion.
2. Crossing over and mutations.
3. Coming together of chromosomes of the parents.

Male and female gametes of a species have same number of chromosomes. Hence, number of chromosome in the female gamete of tobacco plants is 24. Zygote is formed by fusion of male and female gamete, it will have 48 chromosomes.

The ratio of chromosome number between egg and its zygote is 1 : 2. Egg is product of meiosis and hence contains half the chromosome number of its parent cell. Zygote is product of fertilization of male and female gamete and therefore, contains double the chromosome number of egg cell.Sperms and eggs differ from each other in type of sex chromosome. The sperm contain either X or Y-chromosome whereas an egg will always have an X-chromosome.

If fertilization does not occur, thickened endometrium (inner lining of uterus) is disintegrated and sloughed off during menstruation phase (day 1-5). Endometrium is needed to support implantation and nutrition of embryo only under pregnancy and is otherwise disintegrated as it does not have any function in non pregnant woman.

Pollination is the process by which pollen grains are transferred to the sigma. Once pollen grains land on stigma, they germinate to produce pollen tubes and subsequently to facilitate fertilization. Hence, fertilization cannot take place in flowers if pollination does not occur.