Biology Notes for class 12 | Chapter - 1 : REPRODUCTION IN ORGANISMS

Biology Notes for class 12 | Chapter - 1: REPRODUCTION IN ORGANISMS - Here we are presenting Biology notes for class 12 chapter 1.

Reproduction In Organisms:

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

·         The period from birth to the natural death of an organism represents its life span.

·         Life Span can be as short as a few days or as long as a few thousand years.

·         life spans of organisms are not necessarily correlated with their sizes.

·         Whatever be the life span, death of every individual organism is a certainty, i.e., no individual is immortal, except single-celled organisms.

·         Reproduction is the process in the living organisms, that ensure the continuity of species generation after generation.

·         What is Reproduction - Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death.

·         Reproduction enables the continuity of the species, generation after generation.

·           Genetic variation is created and inherited during sexual reproduction.

·         Each organism has evolved its own mechanism to multiply and produce offspring.

·         The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces.

·         Based on whether there is participation of one organism or two in the process of reproduction, it is of two types.

·         When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual mode of reproduction.

·         When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual mode of reproduction.

ASEXUAL REPRODUCTION

·         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.

·         What is Clone? - A clone is morphologically and genetically similar individuals.

·         Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organizations. In Protists and Monerans, the organism or the parent cell divides into two, to give rise to new individuals. Thus, in these organisms cell division is itself a mode of reproduction.

·         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).

Binary Fission In Amoeba

·         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; called Budding.

Budding In Yeast

·         Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures.

·         The most common of these structures are: (a) Zoospores of chlamydomonas; (b) Conidia of Penicillium; (c) Buds in Hydra; (d) Gemmules in sponge.

·         Zoospores are microscopic motile structures.

·         In plants, instead of using asexual term, the term vegetative reproduction is frequently used.

·         In plants, the units of vegetative propagation (i.e vegetative propagules), are all capable of giving rise to new Offspring.

·         The formation of vegetative propagules does not require two parents, thus the process is asexual.

·         Vegetative propagules in angiosperms: (a) Eyes of potato; (b) Rhizome of ginger; (c) Bulbil of Agave; (d) Leaf buds of Bryophyllum; (e) Offset of water hyacinth.

Scourge of the water bodies or 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.

• ·    This plant was introduced in India because of its beautiful flowers and shape of leaves.

• ·    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 this plant.

How plants like potato, sugarcane, banana, ginger, dahlia are cultivated?

·         You have seen small plants emerging from the buds (called eyes) of the potato tuber, from the rhizomes of banana and ginger.

·           New plantlets arise from the nodes present in the modified stems of these plants. When these nodes come in contact with damp soil or water, they produce roots and new plants.

·           Similarly, adventitious buds arise from the notches present at margins of leaves of Bryophyllum.

·           This ability is fully exploited by gardeners and farmers for commercial propagation of such plants.

·         Note that asexual reproduction is the common method of reproduction in organisms that have a relatively simple organisation, like algae and fungi and that they shift to sexual method of reproduction just before the onset of adverse conditions.

·         Higher plants exhibits both asexual (vegetative) as well as sexual modes of reproduction.

·           While, Only sexual mode of reproduction is present in most of the animals.

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.

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

•   Plants, animals or fungi;  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.

Common Features of Sexual Reproduction

1.  Juvenile phase or Vegetative Phase: 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 the vegetative phase in plants. This phase is of variable durations in different organisms.

2.  Reproductive Phase: The end of juvenile/vegetative phase which marks the beginning of the reproductive phase. Reproductive phase can be seen easily in the higher plants when they come to flower.

3.  Senescence phase: The end of reproductive phase can be considered as one of the parameters of senescence or old age.

• There are some plants that flower throughout the year and some others that show seasonal flowering.

• There are three types of plants: Annual, Biennials and Perennials types.

·        Annual plants: These plants complete their life cycle in one growing season, i.e. one generation per year. Examples – broccoli, bean seed.

·        Biennial plants: The plants which their life cycle in two growing seasons. In first growing season they grow only foliage and in second growing season they form flowers and produce seeds. Example – foxgloves, Brussels.

·        Perennial plants: Perennial plants continue to grow and flower for more than two years. These plants life cycle vary widely from few years to many years. Example – Coneflower plant.

•   The annual and biennial plant types, show clear cut vegetative, reproductive and senescent phases, but in the perennial species it is very difficult to clearly define these phases.

·        But a few plants exhibit unusual flowering phenomenon; such as-

      Bamboo species flower only once in their life time, generally after 50-100 years, produce large number of fruits and die.

   Strobilanthus kunthiana (neelakuranji), flowers once in 12 years. This plant flowered during September-October 2006. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue stretches.

•   In animals there occurs, morphological and physiological changes prior to active reproductive behaviour.

• The reproductive phase is also of variable duration in different organisms.

• The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase.

•  In non-primate mammals like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during reproduction are called oestrus cycle where as in primates (monkeys, apes, and humans) it is called menstrual cycle.

• Many mammals, especially those living in natural, wild conditions exhibit such cycles only during favourable seasons in their reproductive phase and are therefore called seasonal breeders.

•  Many other mammals are reproductively active throughout their reproductive phase and hence are called continuous breeders.

•   During Senescence phase, there are concomitant changes in the body (like slowing of metabolism, etc.)

•   Old age ultimately leads to death.

•   In both plants and animals, hormones are responsible for the transitions between all the three phases.

• Interaction between hormones and certain environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms.

Events in sexual reproduction

• After attainment of maturity, all sexually reproducing organisms exhibit events and processes that are fundamentally similar, but the structures associated with sexual reproduction may be different.

• The events of sexual reproduction though elaborate and complex, follow a regular sequence.

• Sexual reproduction is characterized by the fusion (or fertilisation) of the male and female gametes, the formation of zygote and embryogenesis.

•    These sequential events may be grouped into three distinct stages namely, the pre-fertilisation, fertilization and the post-fertilization events.

Pre-fertilisation Events

•  This include all the events of sexual reproduction prior to the fusion of gametes.

• The two main pre-fertilisation events are gametogenesis and gamete transfer.

Gametogenesis

•  It is the process of formation of the two types of gametes – male and female.

• Gametes are haploid cells.

• Homogametes (isogametes): In some algae the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes. Example - Isogametes of Cladophora (an alga).

• Heterogametes: Both the gametes produced are morphologically distinct types. In such organisms the male gamete is called the antherozoid or sperm and the female gamete is called the egg or ovum. Example – gametes of Fucus (an alga); & Human beings.

• Sexual reproduction in organisms generally involves the fusion of male and female gametes from two different individuals. But this is not always true. Sometimes male and female gametes are produced by a single body.

•  Homothallic or monoecious or Bisexual: Several plants and fungi, may have both male and female reproductive structures in the same individual. Example of monoecious plants are cucurbits and coconuts, Monoecious plant (Chara), Bisexual animal (Earthworm), Bisexual flower (sweet potato)

•  Heterothallic and Dioecious or Unisexual: Male and female reproductive structures are present in different individuals i.e. single individual body has only one sex, either male or female. Example - Dioecious plant (Marchantia), Unisexual animal (Cockroach).

•  In flowering plants, the unisexual male flower is staminate, i.e., bearing stamens, while the female is pistillate or bearing pistils. In some flowering plants, both male and female flowers may be present on the same individual (monoecious) or on separate individuals (dioecious).

•   Earthworms, sponge, tapeworm and leech, typical examples of bisexual animals that possess both male and female reproductive organs, are hermaphrodites.

•  Cockroach is an example of a unisexual animal species.

Cell division during  gamete formation

• ·Gametes are haploid though the parent plant body from which they arise may be either haploid or diploid.

• Several organisms belonging to monera, fungi, algae and bryophytes have haploid plant body, A haploid parent produces gametes by mitotic division.

• But the organisms belonging to pteridophytes, gymnosperms, angiosperms and most of the animals including human beings, the parental body is diploid. It is obvious that meiosis, the reduction division, has to occur if a diploid body has to produce haploid gametes.

• Specialized cells which undergo meiosis are called Meiocytes (Gamete mother cell). At the end of meiosis, only one set of chromosomes gets incorporated into each gamete.

Gamete Transfer

• After their formation, male and female gametes must be physically brought together to facilitate fusion (fertilisation).

• In most of the organisms, male gamete is motile and the female gamete is stationary.

• Exceptions are a few fungi and algae in which both types of gametes are motile.

Homogametes in Algae

• There is a need for a medium through which the male gametes move.

• In several simple plants like algae, bryophytes, and pteridophytes, water is the medium through which this male gamete move.

• A large number of the male gametes, however, fail to reach the female gametes.

• To compensate for this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced.

In Seed Plants

• Pollen grains are the carriers of male gametes and Pollen grains produced in anthers

• Ovule have the egg (female gamete)

• Pollen grains produced in anthers therefore, have to be transferred to the stigma before it can lead to fertilisation.

• In bisexual, self-fertilizing plants, e.g., peas, transfer of pollen grains to the stigma is relatively easy as anthers and stigma are located close to each other.

• But in cross-pollinating plants (including dioecious plants), a specialized event called pollination facilitates the transfer of pollen grains to the stigma.

• Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.

In Dioecious Animals

• Because male and female gametes are formed in different individuals, the organism must evolve a special mechanism for gamete transfer.

A successful transfer will lead to the most critical event in sexual reproduction, the fertilization i.e. (coming together of gametes).

Fertilization

• This is the most vital event of sexual reproduction.

• Fertilization i.e. fusion of gametes also called as Syngamy

• Syngamy results in the formation of a diploid zygote.

What is parthenogenesis?

In some organisms like rotifers, honeybees and even some lizards and birds (turkey), the female gamete undergoes development to form new organisms without fertilization.

i.e. development of new organism without fertilization. 

Where does syngamy occur?

• In most aquatic organisms, such as a majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water),

 i.e., outside the body of the organism. This type of gametic fusion is called external fertilisation.

• Organisms having external fertilisation show great synchrony between the sexes and release a large number of gametes into the surrounding medium (water) in order to enhance the chances of syngamy.

Example: bony fishes and frogs where a large number of offspring are

produced.

Disadvantage of External Fertilisation:

• A major disadvantage is that the offspring are extremely vulnerable to predators threatening their survival up to adulthood.

Internal Fertilisation

• In this case, Syngamy occurs inside the body of the organism, hence the process is called internal fertilisation.

Example: many terrestrial organisms, belonging to fungi, higher animals such as reptiles birds, mammals and in a majority of plants (bryophytes, pteridophytes, gymnosperms, and angiosperms)

• The egg is formed inside the female body where they fuse with the male gamete.

• In organisms exhibiting internal fertilisation, the male gamete is motile and has to reach the egg in order to fuse with it.

• The number of sperms produced is very large, there is a significant reduction in the number of eggs produced.

In seed plants, however, the non-motile male gametes are carried to female gamete by pollen tubes.

Post-fertilisation Events

All the Events occuring in sexual reproduction, after the formation of zygote are called 

post-fertilisation events.

The Zygote

• Formation of the diploid zygote is universal in all sexually reproducing organisms.

• In organisms with external fertilisation, zygote is formed in the external medium (usually water).

• In the organisms with internal fertilisation, zygote is formed inside the body of the organism.

• Development of the zygote depends on the type of life cycle the organism has and the environment it is exposed to.

• In organisms belonging to fungi and algae, zygote develops a thick wall that is resistant to desiccation and damage. It undergoes a period of rest before germination.

• In organisms with a haplontic life cycle, zygote divides by meiosis to form haploid spores that grow into haploid individuals.

• A zygote is a vital link that ensures continuity of species between organisms of one generation and the next.

• Every sexually reproducing organism, including human beings begin life as a single cell– the zygote.

Embryogenesis

• embryogenesis is process of development of embryo from the zygote.

• During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation.

• While cell divisions increase the number of cells in the developing embryo; cell differentiation helps groups of cells to undergo certain modifications to form specialised tissues and organs to form an organism.

Oviparous Animals:

• Where the development of the zygote takes place outside the body of the female parent i.e. they lay fertilised/unfertilised eggs.

• In oviparous animals like reptiles and birds,the fertilised eggs covered by hard calcareous shell are laid in a safe place in the environment; after a period of incubation young ones hatch out.

Viviparous Animals:

• Where the development of the zygote takes place inside the body of the female parent, i.e. give birth to young ones.

• in viviparous animals (the majority of mammals including human beings), the zygote develops into a young one inside the body of the female organism. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism.

Because of proper embryonic care and protection, the chances of survival of young ones is greater in viviparous organisms.

In flowering plants:

• the zygote is formed inside the ovule.

• After fertilisation the sepals, petals and stamens of the flower wither and fall off.

• The pistil however, remains attached to the plant.

• The zygote develops into the embryo and the ovules develop into the seed.

• The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function.

• After dispersal, seeds germinate under favorable conditions to produce new plants.

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