Reproduction in Organism-Notes
(i) One of the characteristic features of all living organisms is the ability to reproduce.
(ii) After all, for a species to continue there has to be the addition of young individuals. Otherwise, it will simply end there.
(iii) Reproduction is a process through which the cycle of life is maintained.
(iv) A species gives rise to its offspring, which in turn gives rise to its offspring, continuing this life cycle. Thus, reproduction enables continuity of generations.
(v) There is a huge diversity of the species that live on the Earth.
(vi) So, even the way they reproduce is different.
(vii) When a new individual is produced by a single parent, it is called asexual reproduction.
(viii) If two parents are involved, then it is called sexual reproduction.
(i) When new individuals form from the cells of a single parent, then this mode of reproduction is called asexual reproduction.
(ii) Here the new individuals that are produced, are identical and are copies of the parent.
(iii) These similar looking individuals, both genetically and morphologically are called clones.
(iv) Asexual reproduction is quite commonly observed in different living organisms.
(v) It is seen in both plants and animals.
(vi) It is common in the case of single-celled organisms.
(vii) The lower order of animals and plants that have a simple body organization, show this mode of reproduction.
(viii) Here, a single parent is involved, without the involvement of the fertilization process.
(ix) There is a rapid multiplication and growth that is seen, occurring in a small period of time.
(x) The newly formed individuals are genetically similar.
Modes of Asexual Reproduction
Asexual reproduction can occur in various modes. These are discussed below.
- Binary Fission: One of the modes of asexual reproduction is binary fission. In single-celled organisms, the body divides into half. Each half grows at a rapid pace to become an adult. Examples are Amoeba ,Paramecium.
- Fragmentation: In some multicellular organisms such as Spirogyra, asexual reproduction occurs through fragmentation. Here, the parent body is divided into fragments, and each fragment develops into a new individual.
3. Budding: Some organisms reproduce by budding, where small outgrowths called buds develop on the parent body. Each bud develops into a new organism. Examples of organisms exhibiting budding are yeast and Hydra.
4. Sporogenesis: Some organisms produce reproductive cells called spores. These spores grow into new organisms. The spores can be carried through wind and other animals. Many fungi reproduce through spores.
- Vegetative Propagation: This form of asexual reproduction is generally seen in plants, where a plant can grow a shoot. This shoot, now, has the ability to grow into a new organism.
(i) Vegetative propagation is something very peculiar to plants.
(ii) New plants can grow from older plants using vegetative propagation methods such as grafting and budding.
(iii) It is a form of asexual reproduction seen in plants.
(iv) This process involves only a single plant and the new individual that arises, is identical, both genetically and morphologically, to the parent plant.
(v) Vegetative propagation occurs through vegetative plant structures.
(vi) In non-vascular plants, the vegetative reproductive propagules are gemmae and spores whereas, in vascular plants, the roots , stems , leaves and nodes are the vegetative structures.
(vii) Also, we have learned about the meristem tissue in plants.This tissue helps in the vegetative propagation.
(viii) It has undifferentiated cells which divide.
Types of Vegetative Propagation
Vegetative Propagation by Roots
(i) In this process, new plants grow out of the modified roots called tubers.
(ii) In fact, in some plant species, roots develop adventitious buds.
(iii) These buds grow and form new plants/sprouts under the right conditions. For example, Sweet potato and Dahlia.
(iv) These sprouts can be separated from the parent plant and when planted in other areas, new plants are formed.
Vegetative Propagation by Stems
(i) Here, the new plants arise from nodes present in the stem.
(ii) This is where buds are formed, which grow into new plants.
(iii) Stems that grow horizontally on the ground are called runners.
(iv) As these runners grow, buds form at the nodes, which later develop the roots and shoots, resulting in the formation of a new plant.
Example – Cynodon, Oxalis, etc.
(v) The bulb is the round, swollen part of the underground stem.
(vi) Within the bulb lies the organ for vegetative propagation such as the central shoot that grows into a new plant.
(vii) Bulbs have a bud surrounded by layers of fleshy leaves. E.g. Onions, Garlic, and Tulips, etc.
(viii) Stem tubers are found in plants like potatoes. The apical part is swollen containing many nodes or eyes. Every eye has buds. New plants originate from these buds.
Vegetative Propagation by Leaves
(i) Plants like Bryophyllum, Begonia, etc., have adventitious buds coming out from the notches of the leaves.
(ii) These buds develop into new plants.
(i) Cuttings are most commonly used by gardeners to grow new plants.
(ii) A portion of the stem is cut and planted in the soil. It develops roots and further grows into a new plant.
(i) In grafting, we use two closely related plants to produce a new plant that has the desired, combined traits of both the parent plants.
(ii) One plant is the stock, of which we take the root system and the other is the scion, of which we use the shoot system.
(iii) In this method of artificial vegetative propagation, we attach the scion to the stock of the second plant.
(iv) In general, we use grafting for a variety of plants such as roses, apples, avocado, etc.
(i) In this method, we take a bud with a small portion of the bark from the desired plant.
(ii) Then we insert it into a small slit made in the bark of the other plant.
(iii) Next, we tie both the plants together and do not allow the buds to dry.
Sexual Reproduction and Pre-fertilisation Events
Common Pattern in Sexual Reproduction
(i) All organisms have to reach a certain stage of growth and maturity in life, before they can reproduce sexually.
(ii) The series of growth before onset of sexual maturity is named the juvenile phase. This phase is called vegetative phase in plants.
(iii) The end of the juvenile/vegetative phase marks the onset of the reproductive phase.
(iv) In many plants, vegetative phase and reproductive phase occur frequently in alternation.
(v) Both juvenile phase and reproductive phase are of variable durations in different organisms.
Cyclic Changes in Females of Placental Mammals
The females of placental mammals exhibit cyclic changes within the activities of ovaries and accessory ducts also as hormones during reproductive phase. Many mammals are reproductively active only during certain periods during a year. Some other mammals are reproductively active throughout the year.
- Oestrous Cycle: The cyclic changes in females of non-primates during the reproductive phase is represented by the oestrous cycle. In this case, the animal resorbs the endometrium just in case pregnancy doesn’t happen.
- Menstrual Cycle: The cyclic changes in females of primates during the reproductive phase are referred to as menstrual cycle. In this case, the animal sheds the endometrium just in case pregnancy doesn’t happen. Endometrium is shed in fragments alongside blood by means of menorrhea.
Senescence or Old Age: The end of the reproductive phase is taken into account together with the parameters of senescence or adulthood. Metabolism slows down and lots of other changes happen during adulthood.
Events in Sexual Reproduction
The events in amphimixis are often grouped into three distinct stages, viz. pre-fertilisation, fertilization and post-fertilisation events.
Pre-fertilisation Events: All events of amphimixis before the fusion of gametes come under pre-fertilisation events. The two main events during this stage are gametogenesis and gamete transfer.
The method of formation of male and female gametes is termed gametogenesis. Gametes are haploid cells.
Types of Gametes
(i) In some algae, the 2 gametes are so similar in appearance that it is impossible to categorise them into male and female gametes. Such a condition is called an isogamous condition.
(ii) But within the majority of sexually reproducing organisms, the gametes are of two morphologically distinct types.
(iii) Such a condition is called anisogamous or heterogamous condition.
(iv) The smaller gamete is the male gamete and is called antherozoid or sperm.
(v) The larger gamete is the female gamete and is called the egg or ovum.
Sexuality in Organisms
(i) Sexual reproduction generally involves the fusion of gametes from two different individuals.
(ii) But there are many organisms in which a single individual produces both types of gametes. This is the bisexual condition.
(iii) Many flowering plants produce bisexual flowers which produce both sorts of gametes. A bisexual animal is called hermaphrodite animal.
(iv) Other organisms show unisexual condition in which different gametes are produced by different individuals.
(v) In flowering plants, unisexual flowers can be pistillate or staminate.
A flower which produces only the male gametes is called a staminate flower.
A flower which produces only the female gamete is called a pistillate flower.
Cell Division During Gamete Formation
(i) Gametes are haploid.
(ii) The parent plant body can be either haploid or diploid. All animals are diploid.
(iii) A haploid parent produces gametes by mitotic division, whereas a diploid parent produces gametes by meiotic division.
(iv) Many organisms belonging to Monera, Fungi, Algae and Bryophyta have haploid plant bodies.
(v) But organisms belonging to Pteridophyta, Gymnosperms, Angiosperms and most of the animals have diploid parent bodies.
(vi) In diploid organisms, specialized cells called meiocytes (gamete mother cell) undergo meiosis to supply gametes.
(i) Male and female gametes have to be physically brought together to facilitate fertilization. In most organisms, the male gamete is motile and the female gamete is stationary.
(ii) In a few fungi and algae both types of gametes are motile.
Steps in Sexual Reproduction
(i) The male gamete needs a medium through which it can reach the feminine gamete.
(ii) In many simple organisms, like algae, bryophytes and pteridophytes, water is the medium through which gamete transfer takes place.
(iii) But an outsized number of male gametes fail to fertilise the female gametes.
(iv) To compensate for this loss of male gametes during transport, organisms produce male gametes in several thousand times the number of female gametes produced.
(v) In seed-bearing plants, pollen grains are the carriers of male gametes and eggs are present in the ovule.
(vi) Hence, pollen grains need to be transferred to the stigma to facilitate fertilization. In bisexual plants which show self-pollination, anthers and stigma are close to each other.
(vii) Hence, transfer of pollen grains to the stigma is comparatively easy in these plants.
(viii) But in cross pollinating plants, pollen grains need the help of some agent of pollination to be transferred to the stigma.
(ix) It is important to note that cross-pollinating plants can be monoecious as well as dioecious.
(x) Once pollen grains reach the stigma, they germinate to produce pollen tubes.
(xi) Male gamete is then transferred to the ovule via pollen tube and ends up fertilizing the female gamete.
(xii) In dioecious animals; male and female gametes are formed in different individuals. Hence, such organisms need to evolve a special mechanism for gamete transfer.
Fertilization is the most critical event in sexual reproduction, and successful transfer of gametes is necessary for this event to materialize.
Fertilisation and Post-fertilisation Events
(i) Fusion of nuclei of male and feminine gametes is named syngamy or fertilization.
(ii) Fertilization leads to the formation of a diploid zygote.
The female gamete in some organisms develops to make a brand new organism without fertilization.
This phenomenon is called parthenogenesis, e.g. some lizards, rotifers, honeybees and some birds (turkey).
In most aquatic organisms, fertilization occurs in an external medium (water), i.e. outside the body of the organism. When fertilization happens outside the body of the organism, it is called external fertilization.
Organisms which exhibit external fertilization show great synchrony between the sexes while releasing an outsized number of gametes in water.
Release of an outsized number of gametes within the external medium enhances the probabilities of syngamy.
As a result, an outsized number of offspring are produced.
But such offspring are extremely susceptible to predators, and therefore the majority of them don’t survive up to adulthood.
In most terrestrial organisms, fertilization happens inside the body of the female organism. This type of fertilization is called internal fertilization.
E.g. reptiles, birds, mammals, bryophytes, pteridophytes, gymnosperms and angiosperms.
In organisms which exhibit internal fertilization, the amount of sperms is extremely large but there is a significant reduction within the number of eggs produced.
These events happen after the formation of zygote.
Further development of the zygote depends on the sort of life cycle of the organism and therefore the environment it is exposed to.
In fungi and algae, zygote develops a thick wall which is immune to desiccation and damage.
It undergoes a period of rest before germination.
In organisms with haplontic life cycle, the zygote undergoes meiosis to make haploid spores.
The spores then grow into haploid individuals.
The process of development of embryos from the zygote is called embryogenesis.
During this process, zygote completes mitosis and cell differentiation.
Cell divisions increase the amount of cells within the developing embryo.
Cell differentiation leads to formation of various sorts of tissues and organs which eventually leads to formation of an organism.
When the embryo develops inside the egg but outside the body of the organism, the organism is named oviparous animal.
In this case, the embryo is roofed by a tough calcareous shell and is laid during a safe place within the environment.
The egg undergoes a period of incubation.
Young ones come out from eggs after a time period .
E.g. Reptiles, Birds
When the embryo develops inside the body of the organism, the organism is called a viviparous animal.
In this case, the embryo develops into a young one inside the female organism.
After attaining a particular stage of growth, the young ones are delivered out of the body of the female organism.
Most of the mammals are viviparous animals.
Vivipary ensures better chances of survival.