Reproduction In Organisms Handwritten Notes

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 · Reproduction In Organisms Class 12  Notes Biology

 · Class 12 Biology Revision Notes Chapter 01

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• REPRODUCTION IN ORGANISMS

• Life Span

         • Period from birth till natural death. Every organism lives only for certain period of time Eg Elephant 60 -90 years, Fruit fly 4-5 weeks. 

· Every organism live only for a certain period of time.

• REPRODUCTION

Producing young-ones of their kind, generation after generation.

• TYPES OF REPRODUCTION

      • ASEXUAL REPRODUCTION :- Single parent capable of producing offsprings .Somatogenic reproduction

       • SEXUAL REPRODUCTION :- Two parents are invovled in producing offspring

1.1 ASEXUAL REPRODUCTION

In this method, a single individual (parent) is capable of producing offspring. As a result, the offspring that are produced are not only identical to one another but are also exact copies of their parent. Are these offspring likely to be genetically identical or different? The term clone is used to describe such morphologically and genetically similar individuals.

Many single-celled organisms reproduce by binary fission, where a cell divides into two halves and each rapidly grows into an adult (e.g., Amoeba, Paramecium). In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which, eventually gets separated and mature into new yeast organisms (cells). Under unfavourable condition the Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed as encystation. When favourable conditions return, the encysted Amoeba divides by multiple fission and produces many minute amoeba or pseudopodiospores; the cyst wall bursts out, and the spores are liberated in the surrounding medium to grow up into many amoebae. This phenomenon is known as sporulation.

Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures (Figure 1.3). The most common of these structures are zoospores that usually are microscopic motile structures. Other common asexual reproductive structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).

While in animals and other simple organisms the term asexual is used unambiguously, in plants, the term vegetative reproduction is frequently used. In plants, the units of vegetative propagation such as runner, rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new offspring (Figure1.4). These structures are called vegetative propagules.

Obviously, since the formation of these structures does not involve two parents, the process involved is asexual. In some organisms, if the body breaks into distinct pieces (fragments) each fragment grows into an adult capable of producing offspring (e.g., Hydra). This is also a mode of asexual reproduction called fragmentation.

You must have heard about the scourge of the water bodies or about the ‘terror of Bengal’. This is nothing but the aquatic plant ‘water hyacinth’ which is one of the most invasive weeds found growing wherever there is standing water. It drains oxygen from the water, which leads to death of fishes. You will learn more about it in Chapters 13 and 14. You may find it interesting to know that this plant was introduced in India because of its beautiful flowers and shape of leaves. Since it can propagate vegetatively at a phenomenal rate and spread all over the water body in a short period of time, it is very difficult to get rid off them.

1.2 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.

A study of diverse organisms–plants, animals or fungi–show that though they differ so greatly in external morphology, internal structure and physiology, when it comes to sexual mode of reproduction, surprisingly, they share a similar pattern. Let us first discuss what features are common to these diverse organisms.

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 organisms 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.

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