12th standard ncert cbse biology zoology chapter

REPRODUCTION IN ORGANISMS

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he earth is a beautiful place with a large diversity of living organisms. All living organisms grow and after a definite period of life on earth, they die. Before death an organism produces new individuals of its own. The biological process by which an organism produces its new offspring is called reproduction. It is an essential life process responsible for the continued existence and survival of a species. Apart from this, reproduction is also important for the maintenance of a sustainable ecosystem. Thus, it is clear that for life to continue, reproduction is vital.

The period from birth to the natural death of an organism represents its life span. Every organism has a specific average life span. The life span may range from one day to a few thousand

The biological process by which an organism produces its new offspring is called reproduction

years. The life spans of organisms are not necessarily correlated with their sizes; the sizes of crows and parrots are not very different yet their life spans show a wide difference. Similarly, a mango tree has a much shorter lifespan than a peepal tree.

All organisms except single-celled organisms are mortal. The single cell of unicellular organisms divides and produces its offspring, so there is no natural death in single-celled organisms.

ORGANISMS

LIFE SPAN

Elephant

60 – 90 Years

Rose

5 - 7 Years

Dog

20 - 25 Years

Banana Tree

25 Years

Cow

20 - 25 Years

Horse

60 Years

Fruit Fly

30 Days

Rice Plant

3- 4 Months

Banyan Tree

200 Years

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Each organism has its own mode of reproduction. It depends on several parameters such as habitat,

internal (physiological) factors, etc.

Reproduction is mainly classified into two, asexual and sexual reproduction.

ORGANISMS WITH LONGLIFE SPAN • The oldest known living individual tree in the world is a member of the Great Basin bristlecone pine (Pinus longaeva) species. It is 5066 years old.

• Some glass sponges found in the East China Sea and Southern Ocean have been discovered to be over 10,000 years old.

• The oldest mammals are Bowhead whales which may live up to 211 years.

ASEXUAL REPRODUCTION Asexual reproduction is the process of formation of new offspring from a single parent with or without the involvement of gamete formation. The offspring produced are morphologically and genetically identical with each other and also with their parents. These

Asexual reproduction is the process of formation of new offspring from a single parent with or without the involvement of gamete formation. identical organisms are clones. The cloning of an organism is a form of asexual reproduction.

Asexual reproduction is common among unicellular organisms, and in multicellular organisms with relatively simple organisations. Various modes

of asexual reproduction are found in organisms.

BINARY FISSION Binary fission is a type of asexual reproduction in which the fully grown parent cell splits into two halves, producing two new daughter cells. It is common among organisms such as Bacteria, Amoeba and Paramecium.

It takes place during favourable conditions. This method is simple and similar to mitosis.

Binary fission is a type of asexual reproduction in which the fully grown parent cell splits into two halves, producing two new daughter cells.

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Amoeba reproduces asexually by binary fission. At the time of binary fission the amoeba withdraws pseudopodia and becomes spherical in shape. The nucleus splits longitudinally and divides into two daughter nuclei (called as karyokinesis). These daughter nuclei move towards opposite poles. This is followed by the elongation of the parent cell. A fissure appears in the middle

from a single parent. One of them has the original contractile vacuole while the other develops a new one.

The entire process of binary fission is completed in less than half an hour; but this time varies with temperature. The daughter amoebae lead an independent life; become adults and the division process gets repeated. Thus, thousands of amoebae are produced within a few days from a single parent.

At the time of binary fission the amoeba withdraws pseudopodia and becomes spherical in shape. The nucleus splits longitudinally and divides into two daughter nuclei.

Binary fission is of three types based on the plane of division.

of the cytoplasm between the two daughter nuclei dividing the cell into two equal halves (cytokinesis). Thus, two daughter amoebae are produced

1. Simple Binary Fission: Fission occurs through any plane e.g., Amoeba.

2. Longitudinal Binary Fission: Fission occurs along the longitudinal axis of the organism e.g., Euglena.

3. Transverse Binary Fission: The plane of fission passes along the transverse axis of the organism e.g., Bacteria, Paramecium.

TAP

AMOEBA

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BUDDING

YEAST

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Yeast is a unicellular fungus which reproduces asexually by an unequal division process called budding. Budding Budding is a type of asexual reproduction in which a daughter individual is formed from a small projection, the bud, arising on the parent body.

is a type of asexual reproduction in which a daughter individual is formed from a small projection, the bud, arising on the parent body. In yeast, buds arise as small cytoplasmic outgrowths from the parent cell. This is followed by the division of the nucleus. The nucleus of the parent cell divides into two and one of the daughter nuclei migrates into the bud. The bud then enlarges and detaches from the parent cell by forming a constriction at the base. The daughter cell matures into a new yeast cell.

The daughter cell produced during the budding process is generally smaller than the mother cell. Unlike binary fission, in budding the parent cell remains intact.

Hydra reproduces both sexually as well as asexually. At the time of asexual reproduction small buds appear on the body wall. As the bud elongates, a mouth opens at its free anterior end followed by the development of tentacles at the base of the oral region. The newly developed hydra detaches from the parent body and fixes on a substratum.

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HYDRA 03

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