1. Organisms reproduce and create new individuals of their kind. This is essential for perpetuation of species.
2. Information for inheritance of features from parents to next generation is present within chromosomes, in the form of DNA.
3. The DNA is information source for making proteins, which if changes will change the original body design.
4. Thus, for reproduction, each organism must first make copies of its DNA, so that the daughter cell produced can have its copy of DNA.
5. However, this DNA copying mechanism is not always fool-proof.
6. Any minor change in DNA, during copying, results in variation, which is the basis for evolution.
7. This variation is important for survival of species over time.
8. Different organisms use different methods of reproduction, depending on their body design.
9. Unicellular organisms may reproduce by fission. The parent cell may split into two daughter cells in any plane (e.g., Amoeba) or in specific plane (e.g., Leishmania). This is known as binary fission.
10. In some cases, as in Plasmodium, the parent cell may divide into many daughter cells - this is multiple fission.
11. Multicellular organisms with simple body organisation may break into small pieces, where each piece may grow into complete organism. This is observed in spirogyra and is known as fragmentation.
12. However, multicellular organisms with cell differentiation, when cut in small pieces, may regrow into complete organism with the help of specific cells, e.g., Planaria and Hydra. This process is known as regeneration.
13. Yeast and Hydra may grow a bud from their body, which can detach and mature into a complete organism. This is known as budding.
14. Many plants can give rise to new plants through their vegetative parts as roots, stem and leaves. This is known as vegetative propagation.
15. Tissue culture, is use of small tissues or cells from growing tip of plant to produce a new plant in artificial conditions.
16. In many fungi, like Rhizopus, spore formation takes place in spore sacs called sporangia. These spores, being thick walled, survive harsh conditions and can grow into a complete organism under favourable conditions.
17. All such methods of reproduction that do not involve fusion of male and female gametes are collectively termed as asexual modes of reproduction.
18. Sexual mode of reproduction involves combining of DNA from two different individuals thereby resulting in faster variations.
19. During sexual reproduction, two germ cells fuse to form zygote, giving rise to a new individual. Germ cells are produced as a result of special cell division called meiosis, which results in halving of chromosomes number. The chromosome number is restored after fusion.
20. Reproductive parts of angiosperms are located in the flower. Stamen is the male reproductive part and pistil is the female reproductive part.
21. Stamen consists of an anther and filament. Pollen grains are produced in the anther.
22. Pistil which is present in the centre of a flower is made of three parts. These parts are — swollen bottom part is the ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma.
23. Transfer of pollen grain from anther to stigma is termed as pollination. Pollination is of two types: self-pollination and cross-pollination. When the pollen grain is transferred to stigma of same flower or to any other flower growing on same plant is termed as self- pollination. Transfer of pollen grain from anther of one flower to stigma of another flower growing on another plant of same species is termed as cross-pollination.
24. On reaching the suitable stigma, pollen grain germinates, carrying the two male gametes in pollen tube.
25. The first male gamete fuses with the egg cell, present in the embryo sac resulting in the formation of zygote.
26. The second male gamete fuses with secondary nuclei and result in formation of primary endosperm nucleus. This is known as double fertilisation.
27. After fertilisation, the ovule develops and converts into a seed containing embryo. The embryo is the future plant and when it is provided with suitable conditions like water, air and optimum temperature, it develops into a seedling.