Reproduction in Organisms – Class 12 Biology Notes

Reproduction:

It is the biological process by which new individual organisms – "offspring" – are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction.

The period between birth and natural death of an organism is called its lifespan. It can vary from less than a day to more than 400 yrs.

Lifespan of some organisms are given in the following table:

Living organism	Lifespan
Mayfly	1 day
Butterfly	1-2 weak
Wheat Plant	6 months
Crow	15 years
Dog	25 years
Banana Tree	25 years
Human	100 years
Parrot	140 years
Banyan Tree	200 years

 

Within the context of producing offspring, the reproductive system has four functions:

1. To produce egg and sperm cells
2. To transport and sustain these cells
3. To nurture the developing offspring
4. To produce hormones

Types of reproduction

Reproduction can divided in two types based on the participation of one or two organisms:

Asexual reproduction:

Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes.

The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent.

These are often referred to as clone. The unit of reproduction is commonly formed from the somatic cells of the parent.

Meiosis does not occur in asexual reproduction.

Asexual reproduction is common among single-celled organisms and in plants and animals with simple organisations.

Cell division in itself is a mode of reproduction in protists, e.g. bacteria and monerans, e.g. Amoeba (the parent cell divide into two to give rise to new individuals).

Types of asexual reproduction

1. Fission:

Fission occurs when a neutron slams into a larger atom, forcing it to excite and spilt into two smaller atoms—also known as fission products. Fission can be further classified as:

(i) Binary fission:

• The body of an individual divides into two equal halves. It can be following types:
  (a) Simple binary fission: When division occurs in any plane but it is always right angle to the elongated dividing nucleus, e.g. Amoeba.
  (b) Longitudinal binary fission: When division occurs along the longitudinal axis, e.g. Euglena, Vorticella.
  (c) Oblique binary fission: When division occurs at an angle to the transverse axis, e.g. Ceratium, Gonyaulax.
  (d) Transverse binary fission: When division occurs along the transverse axis of the individual, e.g. Paramecium, diatoms, bacteria, Planaria.

(ii) Multiple fission: The division of the parent body into many daughter organisms, e.g. Amoeba,
Plasmodium, Monocystis (all Protozoa).

Figure 1 : Binary fission in Amoeba

2. Budding:

Budding is an outgrowth or bud develops, grows, constricts at the base and separates from the parent body, e.g. yeast and Hydra. The complex budded condition is called torala stage.

3. Sporulation:

Sporulation occurs by tiny, single-celled, thin-walled spores that are extremely resistant to environmental extremes. The spores form new individuals. It commonly occurs in Monera, Protista, Fungi and Algae. Spores formed can be of following main types:

Pseudopodio spore (with fine pseudopodia)	Amoeba
Zoospores (motile with flagellated)	Chlamydomonas and Ulothrix
Conidia (non-motile)	Penicillium
Sporangiospores (non-motile)	Rhizopus
Gemmules (internal buds)	Sponges

4. Fragmentation:

Fragmentation occurs by breaking of the parent body into two or more parts, each of which grows to form an independent individual, e.g. algae like Spirogyra and bryophytes such as Marchantia, Riccia.

5. Regeneration:

Regeneration is a type of asexual reproduction in which the missing part of the organism is repaired by the proliferation of cells, e.g. Hydra, planaria and sponges.

6. Vegetative propagation:

Vegetative propagation is the formation of a new plants from vegetative parts like root, stem, leaf, etc., naturally, e.g. water hyacinth (scour age of the water bodies or Terror of Bengal) propagates very quickly by vegetative mode and drains out dissolved 02 from water bodies. Some artificial methods of vegetative propagation are also developed by the farmers like cutting, layering, grafting, etc. The units of vegetative propagation such as runner, rhizome, sucker, tuber, offset, bulb all have capability to give rise to new offsprings. These structures are known as vegetative propagules.

Sexual Reproduction

Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex.

These gametes fuse to form the zygote which develops to form the new organism. It is an elaborate, complex and slow process as compared to asexual reproduction.

Because of the fusion of male and female gametes, sexual reproduction results in offspring that are not identical to the parents or amongst themselves.

All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the juvenile phase. It is known as vegetative phase in plants. This phase is of variable durations in different organisms.

Events in sexual reproduction:

After attainment of maturity, all sexually reproducing organism exhibit events and processes that have remarkable fundamental similarity, even though the structures associated with sexual reproduction are indeed very different.

The events of sexual reproduction though elaborate and complex, follow a regular sequence. Sexual reproduction is characterised by the fusion (or fertilisation) of the male and female gametes, the formation of zygote and embryogenesis.

For convenience these sequential events may be grouped into three distinct stages namely, the pre-fertilisation, fertilisation and the post-fertilisation events.

1. Pre-fertilisation Events: These include all the events of sexual reproduction prior to the fusion of gametes. The two main pre-fertilisation events are gametogenesis and gamete transfer.
2. Fertilization: The most vital event of sexual reproduction is perhaps the fusion of gametes. This process called syngamy results in the formation of a diploid zygote. The term fertilisation is also often used for this process. The terms syngamy and fertilisation are frequently used though, interchangeably.
3. Post-fertilisation: Events in sexual reproduction after the formation of zygote are called post-fertilisation events.

Advantages of Sexual Reproduction:

1. Involvement of two parents results in the intermingling of genes resulting in the production of a new offspring
2. Genetically identical offsprings are produced
3. Variations in species increase the chances of survival hence in the evolutionary advancements

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