Biological process that results in the generation of new young ones or offspring from an organism is called reproduction. The young ones produced attain similar characteristics to the individual giving birth to its offspring. Thus, reproduction is a part of the cycle where birth, growth and death take place. It enables for the continuity of the species, one generation after another. As all organisms have limited lifespan and are bound to die and reproduction ensures the survival of species in all generations. All organisms undergo reproduction be it microorganisms, plants or animals.
This article is about Reproduction in organisms class 12 Unit VI in NCERT books which also covers sexual reproduction in flowering plants, human reproduction and reproductive health in addition to this. Not only in higher classes, chapter- Reproduction in animals class 8 and also Reproduction class 10 chapter also makes students well aware about the topic. So, this write-up is helpful for students from class 8 onwards.
Different methods of reproduction in plants and animals are known, here, we can find majorly two divisions:
Commonly seen in Protists, Monerans and Fungi.
For example: Amoeba reproduces by binary fission where the parent cell divides into two halves and each half further develops into a daughter cell. Likewise, bacteria also reproduces by binary fission. Yeast reproduces through budding and other members of Fungi kingdom such as Penicillium reproduces through conidia.
Further, organisms in Anamalia kingdom like Hydra reproduce through reproductive structures called bud and sponge produces internal buds called gemmule that help in asexual reproduction.
Apart from these, Asexual Reproduction is seen in simple plants as well as higher plants such as algae and potato. Algae (Chlamydomonas) produce zoospores that mature later as plants. During unfavourable conditions algae undergoes sexual reproduction.
Vegetative propagation is the asexual reproduction method in plants. It is done by the generation of vegetative propagules in higher plants. For example, specialized structures in potatoes, the stem of the plant, develop buds known as ‘eyes’ that later germinate as new plants. In ginger, rhizomes are the specialized modified stems having buds and nodes undergo vegetative propagation. A bryophyllum leaf has notches on the leaf margins called adventitious buds that later fall off to generate new plants. Runners, offset, bulb and suckers are other examples of vegetative propagules in plants. Farmers and gardeners make full use of this process for commercial production of strawberry farms, potato cultivation and ginger fields. However, excess growth of water hyacinth chokes the Bay of Bengal and kills millions of fish. It is their problematic side.
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Organisms that engage in sexual reproduction vary in their external and internal structures; the pattern of reproduction remains similar.
Juvenile phase, reproductive phase and senescence phase are the three phases of life. Juvenile phase is characterized by the increase in height in all human beings and animals; in plants, emergence of new leaves can be seen in the juvenile phase or vegetative phase that varies in different plants. Juvenile period lasts for 13-16 years in humans and mango trees remain in this phase for around 3-5 years.
The second is the reproductive phase, flowers start appearing in plants and in Mango, a perennial plant, it is difficult to separate the phases as it keeps shedding the leaves throughout the year which is a sign of senescence. Bamboos flower once in 100 years and neelakurinji flowers once in 12 years. Plants develop specialized structures during the reproductive phase, similarly in humans, the reproductive phase starts with puberty and leads to development of sexual characteristics such as facial hair in males and breast in females. There are cyclic changes happening in ovaries and ducts as well as hormones namely menstrual cycle in primates and oestrous or estrous cycles in non-primates. The mammals exhibit the cyclic changes and are seasonal breeders, eg- sheep. Human beings are continuous breeders as they can breed all the time during their reproductive phase.
Senescence phase is characterised by senility, non-existence of menstrual cycle and clowning of metabolism, ultimately leading to death. The phase is controlled by hormones and also environmental factors. In most organisms, the reproductive phase plays an important role as it ensures continuity of life.
The sequential events of sexual reproductive phase are divided into three, namely:
Pre-Fertilization Events: Gametogenesis and gamete transfer are the two main pre-fertilization events. These are explained in detail in NCERT books of class 12 Biology books.
Fertilization events: It is an important event where the fusion of gametes occurs called syngamy that results in zygote formation. Also, external fertilization and internal fertilization are the two terms which should be well understood.
Post-Fertilization Events: It includes the formation of diploid zygote and the development of embryo from the zygote called embryogenesis during which cell division and cell differentiation occurs.
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1. What are Oviparous and Viviparous Animals?
Ans: The categorization of animals is done based on whether the zygote development takes place outside or inside the female parent. If they lay fertilized or unfertilized eggs, animals are termed as oviparous animals such as reptiles and birds, and if they give birth to young ones, they are termed viviparous animals.
2. How do Prokaryotes Reproduce?
Ans: Prokaryotes reproduce asexually by cell division or binary fission of the parent plants. Specialized structures are developed in plants such as runners, suckers, tubers, rhizomes and offsets that give rise to new offspring. It is also referred to as vegetative propagation.
3. What is Sexual Reproduction?
Ans: It involves the formation of gametes which later fuse to form a zygote (in case of humans). It is a complex as well as slower process than asexual reproduction and most of the higher animals entirely reproduce through this method. It is divided into pre-fertilisation, fertilisation and post-fertilisation stages.