Reproduction in organisms
CHAPTER OVERVIEW
1) Introduction
1.1 Life span
1.2 What is Reproduction
2) Asexual Reproduction
2.1 Binary Fission
2.2 Multible Fission
2.3 Budding
2.4 Spore Formation
2.5 Gemmule Formation
2.6 Vegetative propagation
3) Sexual Reproduction
3.1 Pre fertilization events
3.2 Syngamy and Fertilization
3.3 Post fertilization events
1) INTRODUCTION
1.1 Life Span
The living organisms do not survive indefinitely on this earth. Every individual dies after attaining a certain age or due to predation, accident, disease or failure to replace molecules to run the machinery of life. The duration between birth and death is called the life span, which may be a few minutes to a few hours in case of some microorganisms or may be several thousand years. The lifespan of living organisms (both plant and animals) varies greatly.
Plants generally have greater life span span life span as compared to animals. Some perennial trees (example :- Sequoia) live for thousands of of years. It is because generally the plants continue to grow at their tips even when a large part of their body consists of dead cells (i.e. the cells of cork, sclerenchyma and xylem).
The lifespan of an individual generally includes four stages. These are (i) juvenility, (ii) maturity and (iii) ageing and senescence, (iv) Death. The early period of life span, from the time of birth up to the stage when organism develops its capacity to reproduce is called juvenility.
Juvenility is followed by maturity during which the organism reproduce. Gradually the body of organisms starts deterioration. This period is called ageing. Ageing terminates into senescence when the deteriorating change is become irreversible finally the senescence leads to the death of an organism.
Death causes reduction in the number of individuals of a population which is restored by addition of new individuals through reproduction. Thus, reproduction is not only production of a new copy like itself, but it is the means for the survival of population of species on this earth. It is well known that life comes from Life.. All the living organisms have a property to produce more of its own kind by the process called reproduction.
1.2 What is Reproduction
Reproduction is one of the important process by which every living organism make a copy like itself. It is the means of multiplication and perpetuation of the species because the older individuals of each species undergoes senescence and die. There are so many many methods of reproduction in the living organisms. All the reproductive methods are broadly categorised into two types - asexual reproduction and sexual reproduction. In asexual reproduction, the new individuals are produced by any means other than the fusion of sex gametes. The sexual reproduction, on the other hand, involves fusion of two sex gametes to form the new individuals.
Table 1.1
Differences between Asexual and sexual Reproduction
2) ASEXUAL REPRODUCTION
Production of Spring by a by a single parent without the formation and fusion of gametes is called asexual reproduction. The young one receives all its all its genes from one parent. Sexual reproduction is also known as a agamogenesis and agamogeny. It is more primitive and, hence, simpler than sexual reproduction. It involves only mitotic cell divisions, and is also termed somatogenic reproduction. It ensures Rapid increase in numbers. Asexual reproduction produces identical offspring commonly referred as clone. The only genetic variation arises as a result of random mutation among the members of clone.
Asexual reproduction is a kind of reproduction involving only one parent in which meiosis, gamete production, fertilization (nuclear fusion), transfer of genetic material between two individual and parthenogenesis does not occur.
Occurance : Asexual Reproduction occur most commonly in unicellular organisms, such as monerans and protists and in plants and certain animals. It is absent among the higher in vertebrates and all vertebrates.
Types of asexual reproduction : sexual reproduction occurs in different ways in different organisms some common types are discussed below:
2.1 Binary Fission
It is a division of parent into two small, nearly equal sized daughter individuals.
Figure 1. 2
Example 1. Binary Fission in Bacteria Fig 1.2) : It is the most common method of asexual reproduction in actively growing bacteria. The cytoplasm and nucleoid divide equally into two, wnithout mitosis (similar to amitosis). During the division of nucleoid, the DNA replicates while attached to mesosome. A new mesosome develops which gets attached to the daughter chromosome. At the same time the cell wall and plasma membrane start growing transversely inwards from near the middle of the dividing cell between the two mesosomes. The dividing septum originates centripetally and separates the parent cell into two nearly equal daughter cells, each having a nuclear body. These cells may separate from each other or may remain attached for some time to form chain.
Under favourable condition the process of binary fission completes in the about 20 to 30 minutes. Thus, a very large number of bacterial cells may be produced in 24 hours (i. e., about 281,475,000,000,000 in number weighing about 110gms).
Example 2. Binary Fission in Protozoan Protists : In protozoans, the replicated chromosomes are separated by intranuclear (closed) mitosis and the nucleus divide by forrowing. Then cytoplasm gradually constricts between the two separating nuclei, ultimately forming two equal sides daughter individual, each with a nucleas. The offspring grow to the size of the parent before dividing again.
Binary fission is of three main kinds with regard to plane of the को division : irregular, longitudinal and transverse.
(i) Irregular Binary Fission : It occurs in Amebae. Plane of division cytoplasm is variable but it is always at right angles to the elongated dividing nucleus.
Irregular division in Amoeba
(ii) Longitudinal Binary Fission : It occurs in flagellates such as Euglena. The cytoplasm splits lengthwise forming two similar daughter individuals.
Longitudinal division in Euglena
(iii) Transverse Binary Fission : It occurs in ciliates such as paramecium. The cytoplasm divides transversely between two sets of nuclei, forming two dissimilar daughter individuals.
Transverse division in Paramecium
Examples 3. Binary Fission in Planarians
Planarians divide by transverse binary fission. The worm fixes the posterior part of the body firmly to the substratum and Moves the anterior part forward. Thus, the anterior part exerts Pull over the posterior part, which holds on. Due to the pull, the middle part is stretched, constricts and finally breaks. The anterior part regenerates the posterior part, and the posterior part regrows the anterior part. Thus, two complete worms are formed from one.
Reproductive unit and Immortality : It should be noted that in binary fission, the patent body as whole constitutes the reproductive unit, and disappears when its division into daughter individual is completed. However, the parents cannot be said to have died as no dead body is left. In fact after binary fission, the parent continuous living as to daughter individuals. Thus, the organism that undergo binary fission are immortal.
2.2 Multiple Fission
Multiple Fission is the division of parent into many small daughter individuals small daughter individuals simultaneously.
Example 1. Multiple Fission in plasmodium.
In Plasmodium, malarial parasite, Multiple Fission take place in the active adults (schizont) as well as in the encysted zygote (Sporont). The process is called schizogony in the former case and is termed Sporogony in the latter case. The daughter individuals are called merozoites in the former case and sporozoites in the latter case.
Example 2. Multiple Fission in Amoeba
In Amoeba, the products of Multiple Fission become individually surrounded by by resistant coats, the cyst walls, before the release from the parent. The Multiple Fission is here called sporulation, and the encysted products are termed spores. The spores remain inactive during unfavourable conditions such as desiccation and extremes of temperature. On the return of favourable conditions, each cyst hatches into small Amoeba, which gradually grows into an adult. Sporulation not only brings about multiplication, but also enables organism to (i) survive during unfavourable conditions and (ii) diperse to new localities with air.
2.3 Budding
Formation of daughter individual from a small projection, the bud, arising on the parent body is called budding.
Examples 1 Budding in Yeast
Yeast is unicellular fungi. It reproduces asexually by the formation of spores. During the process of budding in yeast, the parent cell develops a small protuberance which grows in size. The parent nucleus divide and one of the two daughter nuclei goes into the bud. Finally the but gets detached and grows into a new individual.
Example 2. Budding in Hydra
In Hydra, the buds arise from the surface of the parent's body. Each bud grows and gradually assumes the form and size of the parent. When full grown, the new individual may separate from the parent and become an independent animal.
2.4 Spore Formation
Spores are propagules which germinate to produce new individual these are several kinds of Spores some are given below :
(i) Zoospores :- The zoospores are special kind of motile and flagellated spores produced inside the zoosporangia. They are usually naked (without cell wall). The flagella help them to swim in aquatic habitat for proper dispersal. The reproduction by zoospores occurs in some lower fungi phycomycetes (e.g., Achlya, sporolegnia, Albugo, Phytophthora, etc.)
(ii) Sporangiospores :- These are non motile spores produced inside the sporangia. Sometime these spores are also termed as endospores. They are usually dispersed by wind and germinate to produce new mycelium ( e.g rhizopus, Mucor, etc.).
A) Zoospores
B) Sporangiospores
(iii) Chlamydospores :- These are thick walled resting spores produce directly from hyphal cells. They store reserve food material are capable of withstanding long unfavourable conditions, e.g Chlamydomonas.
(iv) Oidia :- The Hypha breaks up into its component cells or small pieces which behave like spores. They are thin walled and do not store reserve foodq material. The oidia are usually formed under condition of excess water, sugar and certain salts. They germinate immediately after liberation and cannot survive under unfavourable conditions.
C) Chalmydospores
D) Oidia
(v) Conidia :- These are non motile spores produced singly or in the chains by constriction at the tip of lateral side of special hyphal branches, cord conidiospores example, Penicillium. They are produced exogenously, dispersed by wind and germinate directly by giving out germ tubes.
E) Conidia
2.5 Gemmule Formation
Gemmules are dormant structures produced asexually by the formation of specialised exogenous buds, at certain times of the year, in many freshwater sponge species and a few Marine species. To begin, archaeocytes accumulate nutrients by phagocytizing other cells and then cluster together within the sponge. Certain cells surrounding each cluster than secrete a thick, protective covering ; the formed structure is termed gemmule. After gemmule formation, the original sponge disintegrates. In favourable conditions the floating gemmules produce a new sponge.
Cross section of the Gemmule of a freshwater sponge, Ephydatia
Gemmules are far more resistance to desiccation, freezing and anoxia (lack of oxygen) then the sponges that produce them. Under appropriate environmental conditions, the living cells leave the Gemmule through a narrow escape opening and differentiate to a form functional sponge.
2.6 Vegetative propagation
The lower plants reproduce vegetatively through budding, fission, fragmentation, gammae, resisting buds, spores etc. Among flowering plants, every part of the body such as roots, stem, leaves and buds take part in vegetative propagation. Methods of vegetative propagation have been further divided into two types :- (i) Natural vegetative propagation and (ii) Artificial vegetative propagation.
[A] Methods of vegetative propagation :
1) Natural methods of vegetative propagation :
a) Roots : Modified tuberous roots of sweet potato, Topioca, Yam, Dahlia and Tinospora can be propagated vegetatively when planted in soil. The buds present on the roots grow into leafy shoots (called slips) above ground and adventitious roots at their bases. Each slip gives rise to a new plant. Advantages buds develop on the ordinary root of Dalbergia sisso, populus, Guava, Murraya sp., Albizia lebbek etc. which grow to form New plants plants.
b) Underground stems. Underground modified stem such as the rhizomes, corms, bulbs and tubers possess but which grow into new plants and help in vegetative reproduction-
(i) Suckers. of mint and Chrysanthemum arise from the base of the new erect shoot, grow horizontally in the soil and then come out to form New aerial shoots. These should become independent plants when suckers break away from the parent plant.
In pineapple plant, propagation is carried out by 'crowns', 'slip' and 'suckers'. Planting suckers allow of fast growth of pineapples. Suckers are found between leaves of fully grown pineapple plants. These are pulled out with the hand and sown to propagate pineapple plant.
(ii) Rhizome serve as means of vegetative propagation by perennating under unfavourable conditions and producing new aerial shoots during the favourable season. Typha, Canna, Ginger, Turmeric, Lotus, Saccharum, water hyacinth and many other rhizome producing plants propagate vegetatively with the help of rhizome. A single plant of Eichhornia (water hyacinth) produces more than 65,000 plants in one growing season of about 8 months.
(iii) Corms are highly condensed and specialised underground stems which bear many buds. They perennate the unfavourable conditions and produce new plans under favourable conditions. Plants of Crocus gladiolus, Colocasia, Banana, etc., propagate with the help of Corms.
(iv) Bulbs of Onion, Lilies, Garlic, etc. serves as the means of vegetative propagation by producing new plants when Sown in the soil.
(v) Tubers are modified underground stem branches having several buds. Each eye of the potato Tuber is the bud which grows into a new potato plant when planted with a portion of the swollen tuber. The potato crop is raised by tubers and not by seeds.
(c) Creeping stems. Examples - runners, stolons and offsets-
(i) Runners are creeping modified stems which produce adventitious roots at nodes. Each node give rise to a aerial shoot which becomes a new plant example Cynodon (Doob grass), Oxalis (wood sorrel), Centella etc.
(ii) Stolons are arched runners which cross over small obstacles and develop small plantlets at their nodes, example Fragaria (strawberry) , Vallisneria etc.
(iii) Offsets are one internode long runners which develop tuft of leaves at the apex. example :- Pistia (water lettuce).
(d) Aerial stems. Aerial modified stems of cacti develop new plants in Segments fall on ground and act as means of vegetative propagation. examples :- opuntia.
(e) Leaves. Some plants develop adventitious buds on their leaves which get detached and develop new plants. Leaves of Bryophyllum daigremontianum develop small plantlets with adventitious roots along their margins. Leaves of Kalanchoe, Begonia, Streptocarpus and Saintpaulia develop buds when get injured or detached and placed on the moist soil.
(f) Bulbils. These are fleshy buds produced in the axil of foliage leaves in place of axillary buds. They grow to form New plants when shed and fall on the ground, example :- Oxalis, Allium sativum, Dioscorea, Lily, pineapple etc. In century plant ( agave sp.), the floral buds are modified into Bulbils which grow into new plants when shed from the mother plant.
(g) Turions. These are special type of fleshy buds that develop in aquatic plants (example:- Potemogeton, Utricularia etc.) for vegetative propagation.
2) Artificial Methods of Vegetative Propagation : Several methods of vegetative propagation are man made and developed by plant growers and horticulturists for commercial production of crops. They are called artificial methods. They also include several natural methods of propagation such as use of potato tubers for growing potato crops commercially. Similarly, rhizome of Ginger, corms of Banana, bulbs of onion, runners of mint, etc., are artificially used for commercial production of these crops some of the artificial methods are given below -
(a) Cuttings. The small piece of any flat organ (stem, root or leaf) used for propagation is called cutting. Leaf cutting are used to propagate Sansevieria, Begonia, Bryophyllum, Glocinia and Kalanchoe. Root cuttings are used to propagate citron and Tamarind. Stem cuttings are most commonly used for Artificial vegetative propagation. About 20 to 30 cm long pieces of stem are planted in the natural position in the soil for proper sprouting. Factors such as age of the plant, length and diameter of the cutting, season and the type of plans are taken into consideration for the propagation of a particular species. Sometimes the stem cuttings are treated with rooting hormone (IBA) for proper development of roots. Examples are plants propagated by cuttings are - Grapes, Sugarcane, Rose, Bougainvillea, Carnation, Coleus, Duranta, etc.
(b) Layering. It is the method, root or artificially induced on the stem branches before they are detached from the parent plant for propagation. There are two common types of Layering -
(i) Mound Layering :- It is the most common method of propagating Herbaceous plants. In this method, the lower branch of stem is bent down and partially Defoliated. An injury is made in the defoliated portion. The injured and the defoliated portion is covered with a light layer of moist soil in such a way that the growing tip of the branch remains above the soil surface. After a few days the pegged portion develops adventitious roots. The rooted branch is then cut, separated from the parent plants and grown into a new plant. Example :- Jasmine, grapevine, strawberry, Raspberry, Cherry, etc.
Mound Layering
(ii) Air Layering (or Gootee) :- This method is commonly employed in case of shrubs and trees which do not possess branches near the ground. In this method, a ring of bark is removed (girdled) or a slit at an upward angle is made at the base of an aerial branch. The girdled portion in then covered with moist moss or grafting clay( 2 parts clay, 1 part cow dung, some pieces of hay, cotton and water) and wrapped with a polythene sheet. The wrap portion is called Gootee. The girdled portion of the branch inside the Gootee develops roots within a period of a month or two. Now the branch is cut and planted in the soil after removing the polythene. This method is used in vegetative propagation of Litchi, pomegranate, Orange, lemon, Lokat, Bougainvillea, etc.
(c) Grafting :- It is the most common method of vegetative propagation described by ancient gardeners long before the science of horticulture become established. In this method, parts of two plants are joined in such a way that they grow as one plant. Grafting is done between the two closely related dicotyledonous plants having vascular cambia. The rooted portion of one plant, called stock is joined with a twig of another plant called scion. Generally, the rootstock belongs to a wild variety which is resistant to diseases and pests and possess an efficient root system for absorbation of water and Minerals. The scion is derived from the plant possessing better characters. For example, a scion of the Dussehri mango is grafted on a stock of Desi mango. Similarly, Scion of high-quality roses are grafted on a wild Rose root stocks.
During grafting, about 4 - 12 inches long scion with all the buds intact is placed on the cut and of the stock and the joint is covered with a layer of wax or clay to prevent the evaporation of water or entry of pathogens. All the buds of rooted stock must be removed. Within a few days, the scion and the stock become the composite plant.
Grafting is of four types :- (i) Tongue or Whip grafting, Wedge grafting, Crown grafting and (iv) Side grafting. In case of tongue and wedge graftings, the scion and stock have almost the same diameter whereas in the case of crown and side graftings, the stock has more diameter than the scion.
(i) Tongue and Whip grafting :- Both the stock and scion are cut obliquely at about the same angle.
(ii) Wedge grafting :- A V shaped notch notch shaped notch notch is made on stock and a wedge shaped cut is made on scion.
(iii) Crown grafting :- tablets scions having wedge-shaped cut are grafted on the slits at the top of stock.
(iv) Side grafting :- Single scion on having wedge shaped cut is inserted in the lateral slit of the stock.
Examples of the plants propagated by grafting are :- Mango, Roses, Apple, Rubber, Citrus, Pear, Plum, Peach etc.
(d) Bud Grafting :- This method is similar to grafting except that scion in this case consists of bud along with a small portion of bark having intact cambium. The living tissue of scion bud is inserted into an incision made in the stock so that it reaches upto the cambium of stock.
The bud portion should remain exposed outside in the air. The joint is thoroughly sealed using bandage. The complete operation of bud grafting should be performed during the rainy season to ensure survival of the bud. After 3 - 5 weeks, the bud begins to grow. As soon as the bud sprouts, the stock is cut above the level of graft, e.g., Apple, Peach, etc.
(e) Propagation by Plant Tissue Culture (Micropropagation) :- This method includes propagation of plants by culturing the cells, tissues and organs, called tissue culture. Initially the culturing of cells or tissues results in the formation of an undifferentiated mass of cells, called callus, which later differentiate to produce a large number of plantlets. These plantlets are then transferred to separate pots and nursery beds to obtain a large number of plants. Tissue culture technique is useful in obtaining Virus free plants, disease free plants, homozygous diploids and in commercial micropropagation of Orchids, Carnation, Gladiolus, Chrysanthemum and other ornamental plants. This method is also employed for quick multiplication of plants.
[B] Importance of Vegetative Propagation
1) Vegetative propagation is the only method of reproduction in those plants which have lost their capacity to produce seeds, examples:- Banana, Seedless grapes, Oranges, Rose, Jasmine, Figs, Pineapple, Tulips, Dahlias, Carnation, etc.
2) Plants which produce small quantities of viable seeds. Example :- Cynodon dactylon = Bermuda grass or doob grass are mostly propagated by vegetatively.
3) Plants which have poor seed viability or prolonged seed dormancy reproduce mostly by vegetative methods because these methods are more Rapid, easier and less expensive.
4) The greatest advantage of vegetative propagation is that a plant biotype (original plant) can be retained and multiplied indefinitely without any change or variation. The variation are normally expected if the plants undergo meiosis and Syngamy due to the segregation and recombination of characters.
5) Effects of physicochemical treatments (such as application of some Chemicals and X-rays) and environmental variations (i.e., high temperature or low temperature) can be studied more conveniently in those plants which propagate vegetatively.
6) Vegetative propagation by plant tissue culture method has been applied for the production of disease free plants. This technique is also applied for production of an unlimited number of plants with in a relatively short time.
3) SEXUAL REPRODUCTION
Sexual Reproduction involves the production of male gametes and female gametes either by same same parents or by different parents. When the two opposite types two opposite types of gametes are produced by the same individual it is regarded as bisexual. When male gametes are produced from the male parent and female gametes are produced from the female parent the parents are regarded as unisexual (or heterosexual). During the process of sexual reproduction, the male and female gametes fuse to form the zygote, which develops into a new organism. The process of sexual reproduction is slow and elaborate as compared to asexual reproduction. Moreover, the offspring produced by the fusion of male and female gametes are not exactly identical to the parents or among themselves.
Sexual reproduction is the kind of reproduction that occurs between two individuals ( male and female) involving meiosis, i.e., halving of parental chromosomes inside the male and female gametes and their subsequent fusion resulting in the Restoration of the original parental number of chromosomes causing and increased Genetic diversity.
A survey of mode of sexual reproduction in diverse organisms ( i.e., plants, animals or fungi) shows that through these organism differ, greatly in their external morphology, internal structures and physiology, the pattern of their sexual reproduction is almost the same. All the organism grow to a certain maturity in their life before they started producing sexually. The period of growth between their growth upto their reproductive maturity is called Juvenile phase. In plants, the period of growth between seed germination upto initiation of flowering is called vegetative phase. The later part, when the organism start reproducing sexually, is called reproductive phase. The Juvenile phase is of variable duration in different organisms.
In case of plants, the vegetative phase start immediately after seed germination. The young Seedling grows and develop root system and shoot system. The shoot system continues to develop stem, its branches and leaves. The phase is of variable duration. The annuals start flowering at the end of the season. The biennials produce flowers in second year of growth. Both annuals and biennials die after flowering. Thus, the annuals and biennials have clear cut vegetative, reproductive and senescent phases. The senescent phase is characterized by yellowing of leaves that ultimately leads to the death of the plants.
In perennial plants the duration of vegetative phase varies from few years to several years. They start flowering after attaining their reproductive maturity. Some perennials attain reproductive maturity only after few years of vegetative phase and then flower every year either in some specific season or throughout the year. Some exceptional plants flower only once in their lifetime. For example, bamboo species flower only once in their life time and that too nearly after 50-100 years of vegetative growth. Once they flower, they produce large number of fruits and die and die. Another strange plant, Strobilanthus Kunthiana (Neelakuranji) flowers once in 12 years. These plants flowered during September - October 2006 in hilly areas of Kerala, Karnataka and Tamilnadu.
Curiosity Question :- It is common observation that some plants flower in winters and some in summers. Why?
Answer :- You must have also observed that some plants flowers in winters while other flowers in summers. some plants however, flower crew of the Year. The flowering in various seasons is regulated by the duration of light during the day and duration of dark during the night. It is because the days are longer in summer and shorter in winters.
Accessory (External) Sex characters in Human Male and Female
In animals, the reproductive phase begins with the Marked changes in external and internal morphology and physiology. All these changes are regulated by hormones and enzymes. Some animals, specially human beings, develop secondary sexual characters at the onset of reproductive phase. In primates (monkeys, apes and humans), characteristic cyclical changes start in females. These are called menstrual cycle. In non primate mammals (cows, sheep, rats, deers, dogs, tiger, etc.), such cycle is called Oestrus cycle.
EVENTS IN SEXUAL REPRODUCTION
In spite of their morphological, Anatomical and physiological differences, the fundamental events in sexual reproduction are same. These events can be categorised as follows:
1) Pre fertilization events
2) Fertilization
3) Post fertilization events
3.1 Pre Fertilization Events
All the preparatory events which occur in organism before fusion of gametes are called pre fertilization events. These include : Gametogenesis and Gamete Transfer.
1) Gametogenesis :- Formation of two types of gametes - male and female, inside the gametangia, is called gametogenesis. The reproductive units in sexual reproduction are specialised cells called gametes. The gametes are generally of two kinds : male and female. Gametes of all the organisms are usually haploid cells, i.e, possess is single set ( or n number) of chromosomes. In some lower plants (i.e., in some algae) the two gametes are morphologically similar, they appear similar in size, shape and activity. Such gametes are called isogametes or homogametes. It is not possible to distinguish them into male and female gametes. However, in majority of sexually reproducing organisms the male and female gametes are morphologically distinct (heterogametes). In such organisms, the male gametes are called microgametes, or spermatozoa, and female gametes termed macrogametes, or ova. Both are well adapted for their role in reproduction. The male gametes are mostly minute and motile so that they may swim to the female gametes for fertilization. The female gamete are usually large, non motile and often have a store of food to nourish the the developing embryo.
Sexuality in Animals : The male and female gametes are in most of the animals produced by male and female parents parents respectively. Such animals are said to be unisexual, or dioecious. However, in some animals, such as liver fluke, earthworm and leech, both kinds of gametes are produced by a single individual. Such animals are said to be bisexual or monoecious, or hermaphrodite. Advantage of hermaphroditism is that it Doubles the reproductive capacity as all individuals can produce eggs in contrast to the unisexual animals in which only females (50% of individuals) produce eggs. More over, hermaphroditism permit self fertilization, which is essential for some endoparasites, such as a tapeworm, that leads a solitary life.
some animals show show sequential hermaphroditism in which an individual reserves its sex during its lifetime. The coral reef fish, called wrasse, lives in harems consisting of a single male and several females. If the male dies, the largest female in the harem changes sex and become a new functional male. A large male and look after the harem properly. In oyster species that are sequential hermaphrodites, the large males change into females which can produce more eggs a necessity for a sedentary animals.
Cell division during gamete formation :- The gametes are usually formed by meiotic divisions. Therefore, they are haploid, i.e., have halved or reduced (n) number of chromosomes.
In sexual reproduction, the male and female gametes fuse to form a single cell, the zygote. This process is called fertilization. The zygote formed by the fusion of two haploid gametes is naturally diploid, i.e., has double or normal number (2n) of chromosomes. Zygote would give rise to the offspring by Mitotic divisions. Thus, the offspring is also diploid like its parents which formed haploid gametes by meiosis for its creation. Meiosis land fertilization are the two important events in sexual reproduction that keep the number of chromosome constant from generation to generation. The number of chromosomes must be maintained generation after generation. Only then the characteristics of species, being determined by the chromosomes, would remain unchanged.
Meiosis has another advantage. During this process random segregation of chromosomes (independent assortment) and exchange of genetic material between homologous chromosomes (crossing over) result in New combinations of genes in the gametes, and this reshuffling increases is Genetic diversity. The coming together of two unique sets of chromosomes in the zygote formed the genetic basis of variation within the species.
2) Gamete transfer :- Once the male and female gametes are formed inside the gametangia, they are allowed to come in contact with each other to facilitate their fusion ( Syngamy/fertilization). In a majority of organisms, male gametes are motile and female gametes are non motile. In case of most of the lower plants like algae, bryophytes and pteridophytes, the male gametes are small and flagellated. They are produced inside the antheridia. At maturity, the flagellated male gametes (called sperms or antherozoids) are liberated from antheridia into the external medium of water. They are transferred to female gamete through water to reach the archegonia which possesses female gamete, the egg. During such a gamete transfer, a large number of them fail to reach the female gamete. To compensate this loss of male gamete during transport, they are produced in several thousand times the number of the female gametes.
In higher plants (seed plants) the male gametes are non flagellated. They are produced inside the germinated pollen grains. The microspores and pollen grains are produced inside the microsporangia or Pollen sacs of anthers. At maturity, they are librated into air or transferred to the stigma of carpel through some other agency. Transfer of pollen grains from anther to stigma is called pollination. In case of some bisexual, self fertilizing plants (example:- Peas), transfer of pollen grain to stigma occurs within the same flower. However in case of cross pollination, the pollen grains are transferred from the anthers of male flowers to the stigma of female flower through the agency of air, water or animals particularly insects. Once the Pollen grains reach the correct stigma stigma, they start germinating. Each Pollen Grain germinates to produce a long tube, called pollen tube. This tube carries the male gametes upto the female gamete (the egg) to complete the sexual Fusion.
Chromosome number in the cell of few living organisms
In case of animals, the male gametes are transferred to female gamete by three distinct patterns depending on whether fertilization or embryonic development occur within or outside the maternal body :
1) External Fertilization and External Development :- This pattern is found in many aquatic animals, such as Obelia, Nereis, Labeo and frog. Parents release sperms and eggs into the surrounding water, where fertilization occur then zygotes develop into offspring. For this pattern to the succeed, male and female must shed their gametes at the same time and place and in a large numbers. The parents may or may not make a physical contact for releasing the gametes.
2) Internal Fertilization and External Development :- Sperm has passed from the male into the female with an intromittent organ, such as penis as in Shark and lizard, or otherwise, for example, by cloacal apposition in birds, with modified Arm in cuttle fish. Internal fertilization has several advantages. The female reproductive tract provides a confined, protected place where sperm and egg can easily meet without the danger of being eaten up by predators or washed away by water currents. That zygote passing down the female reproductive tract to the exterior can acquire secretions, membranes, or shell for the protection of the developing embryo.
Animals with internal fertilization usually produce fewer zygotes because of protection provided by egg shells on internal development.
3) Internal Fertilization and Internal Development :- Internal development provides additional advantages to the embryo. The Mother's body provides exactly the right chemical conditions and, in mammals, warmth and nourishment also. As the mother carries the embryo wherever she goes, it is not vulnerable to the predators who attacked externally developing eggs.
The fertilization that occurs outside the body of an organism is called external fertilization and the fertilization that occurs inside the body of an organism is called internal fertilization.
3.2 Syngamy and Fertilization
Syngamy is the fusion of two entire gametes (one male and the other female) to form a zygote. It involves close Association of gametes and all their acts resulting in the formation of zygote including the fusion of nuclei. The fertilization, however, includes all the associated events that ultimately leads to Syngamy.
With regard to the source of the fusing gametes, the Syngamy may be divided into two types : Endogamy and Exogamy.
(i) Endogamy. It involves self fertilization. i.e., The fusion of two gametes of the same parent. It is, thus, uniparental. It is not common. It is found in Taenia, a tapeworm. Tapeworm is a bisexual (hermaphrodite, monoecious) animal.
(ii) Exogamy. It involve cross-fertilization cross-fertilization, i.e., the fusion of two gametes formed by different parents. It is, thus, biparental. It is very common. It is found in the frog frog, rabbit and man.
Cross fertilization also occur in many hermaphrodite animals, example:- earthworm. The reason for cross fertilization in such animals is that their male and female reproductive organs mature at different times.
Syngamy is also of two kinds with regard to the structure of the fusing gametes : Isogamy, Anisogamy or Hetrogamy.
(i) Isogamy. Diffusing gametes are similar morphologically as in Monocytis, of Protozoan. Such gametes are known as isogametes, and their fusion is termed isogamy. Although the isogametes are similar in structure, they have behavioral differences. The gametes produced by one parents do not fuse with each other. Biologist recognise mating type in such organisms.
(ii) Anisogamy or Hetrogamy. The fusing gametes are different in form, size structure, and behaviour as in frog and humans. Such gametes are known as anisogametes, or heterogametes, and their fusion is termed anisogamy, or heterogamy.
Both are well adapted for their role in reproduction.
Parthenogenesis
It is a modification of sexual reproduction in which develops into complete offspring without fertilization. It is Monoparental. Parthenogenesis is found in many nonvertebrates such as rotifers, aphids, bees and crustaceans. It is it also occurred in few vertebrates. Parthenogenesis is of two main types : natural and artificial.
A) Natural Parthenogenesis. It is a regular phenomenon in the life history of some animals. It may be complete, incomplete or paedogenetic.
(i) Complete (Obligatory) Parthenogenesis :- It occurs in some rotifers and few vertebrates. There are no male at all. Hence, there is no biparental sexual reproduction. Females developed exclusively by parthenogenesis. The Caucasian Rock lizard, Lacerta sexicola armaniaca, shows complete parthenogenesis.
It has no males, and the female is developed by developed by parthenogenesis. In the small (15 cm long) Indian snake Typhlina brahmina also females developed by parthenogenesis and there are no males.
(ii) Incomplete (cyclic) Parthenogenesis. Some animals have both sexual and parthenogenetic individuals, which May alternate. In these animals, females can produce unfertilized or fertilized eggs, depending upon environmental conditions. In Daphnia, a freshwater crustacean, female Lays unfertilized eggs that develop parthenogenetically under favorable conditions, and fertilized eggs during the times of environmental stress. In Aphids, the insect pest of crops, females produced many parthenogenetic generation from unfertilized eggs alternating with a biparental generation from fertilized eggs.
In Honey Bee, unfertilized eggs developed into male bees (drones) with haploid cells, and fertilized eggs give rise to females (Queen bees and worker bees) with diploid cells. Spermatogenesis in drone is peculiar in lacking reduction division. In Turkey, about 40% males are produced by parthenogenesis whereas 60% males and all females develop from fertilized egg.
(iii) Paedogenetic Parthenogenesis. In certain insects, larvae lay eggs which develop parthenogenetically into a new generation of larvae. Parthenogenesis in Larvae is called paedogenesis (G. pais= child, genesis = origin).
B) Artificial Parthenogenesis. Eggs of certain animals, such as annelids, mollusks, starfish, frog, hen, rabbit etc., can be induced to develop parthenogenetically by artificial stimuli. Artificial stimuli maybe (i) physical, viz., prick of a needle, electric shock, change in temperature or pH, or (ii) chemical such as addition of Urea, fatty acids, Ether, chloroform, water, etc.
Post Fertilization Events
All the events that occurs during sexual reproduction after the fusion of gametes are called post fertilization events.
1) The Zygote. Fusion of two haploid (n) gametes result in the formation of single diploid (2n) zygote. The zygote is first cell cell of diploid organism including human beings. However, there are certain organisms where zygote undergoes meiosis. Whether the zygote divides by meiosis to form haploid spores or divides by mitosis to develop diploid body depends upon the type of life cycle occurs in the organism.
2) Embryogenesis. The process of development of embryo from the zygote is called embryogenesis. Development of the offspring from reproductive units, such as Buds or fragments, in asexual reproduction is called blastogenesis. Both embryogenesis and blastogenesis have the same target to develop a new individual by the process of cell division and differentiation.
In animals the development of the zygote take place either inside the body of a female parent or outside. On this basis, the animals are grouped into two categories : oviparous and viviparous.
(i) Oviparous animals : These animals lays fertilised or infertilized eggs. For example, reptiles and birds lay fertilized eggs in a safe place in the environment. The eggs are covered by hard Calcareous shell. After passing a variable incubation period, the young one hatch out from the eggs.
(ii) Viviparous animals : The animals give rise to young ones. The zygote develops into young one inside the body of female.
In flowering plants (angiosperms), The zygote develops into an embryo inside the embryo sac. After fertilization the different parts of the flower such as sepals, petals and stamens wither and fall off. In some cases the sepals remains persistent. The ovules are transformed into seeds and the ovary become the fruit. The fruit habit is of great advantage to the plants. It is not only provides nourishment and protection to the developing seeds but also helps in seed diapersal. The seeds are dispersed by various mechanisms and germinate to produce new plants.
Characteristics. Sexual reproduction has the following important basic features :
1) It is generally biparental.
2) It involves formation of male and female gametes
3) mostly there is fusion of male and female gametes (fertilization).
4) Cell divisions are meiotic during gamete formation and mitotic during development of zygote into an offspring.
5) The offspring are not genetically identical to the parents. They show variation as they receive the characters characters (chromosomes) from two different parents. Sexual reproduction is, thus, a source of variety in population.
6) Multiplication is slower than in Asexual Reproduction Reproduction.
7) Sexual reproduction may result in the increase in the number of individuals.
Significance. sexual reproduction has a dual significance for significance for has a dual significance for significance for the species.
1) It results in the multiplication and perpetuation of the species.
2) It contributes to evolution of the species by introducing variation in a population much more rapidly than asexual reproduction.
REVIEW AT A GLANCE
1) Reproduction is the continuous production of new individuals. It is the means of multiplication and perpetuation of the species.
2) All the reproductive methods of living organisms are broadly categorised into two types :- asexual reproduction and sexual reproduction.
3) Asexual Reproduction involve the participation of single parent and occurs without the formation of the sex organs. It does not involve meiosis and fusion of gametes. It is also called apomixis.
4) Asexual reproduction is common in unicellular or multicellular simple organisms such as algae, fungi protozoans and some invertebrates.
5) The types of Asexual Reproduction are :- Fission, budding, spore formation, gemmule formation and vegetative propagation.
6) Most of the prokaryotes (like bacteria) and unicellular protist reproduce by cell division or binary fission of the parent cell.
7) Budding is common in yeast (a unicellular fungi).
8) Spore formation is the most common means of asexual reproduction in fungi and algae. The common types of spores are - Zoospores, Sporangiospores, chlamydospores, oidia and conidia. Gemmule Formation in sponge is also means of asexual reproduction.
9) Plants are unique because the parts of other than seeds (viz. root, stem, leaves etc.) are capable of regenerating into whole new plants. This type of reproduction is called vegetative reproduction. It may occur naturally or can be produced artificially.
10) Artificial methods of vegetative propagation have been deployed by plant growers and horticulture for commercial production of economically important plants. This method includes - cutting, layering, grafting and micropropagation.
11) Vegetative propagation is the only method of reproduction in plants which have lost their capacity to produce seeds.
12) The technique of grafting is used to improve the varieties of many plants of of horticultural importance.
13) Sexual reproduction involves meiosis and fusion of gametes
14) There are three events of sexual reproduction :- pre fertilization events events (includes gametogenesis and gamete transfer) Syngamy ( or fertilization) and post fertilization ( includes formation of zygote and embryogenesis).
Context
1) NCERT EXERCISES WITH ANSWERS
Coming soon...
2) VERY SHORT ANSWER QUESTIONS
3) SHORT ANSWER QUESTIONS
4) LONG ANSWER QUESTIONS
5) HOTS
6) VALUE BASED QUESTIONS
7) MCQ
8) ASSERTION REASONS TYPE QUESTIONS
9) COMPETITION FOCUS NEET AIIMS SPECIAL
10) MOCK TEST
1) NCERT EXERCISE
Q1 why is Reproduction essential for organisms?
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Q2 Which is the better mode of reproduction?
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Q3 Why is offspring formed by Asexual Reproduction referred to as clone?
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Q4 Offspring Formed due to sexual reproduction have better chances of Survival, why? Is this statement always true?
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Q5 How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
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Q6 Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
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Q7 What is vegetative propagation? Give two suitable examples.
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Q8 Define : (i) Juvenile phase, (ii) Reproductive phase, (iii) Senescent phase.
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Q9 Higher organisms have restored to sexual reproduction in spite of its complexity, why?
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Q10 Explain why meiosis and gametogenesis are always interlinked?
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Q11 Identify Each part and write whether it is haploid (n) or diploid (2n), in flowering plant.
(a) Ovary (b) Anther (c) egg (d) Pollen (e) male gamete (f) zygote
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Q12 Define external fertilization. Mention its disadvantages.
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Q13 Differentiate between zoospores and a zygote.
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Q14 Differentiate between gametogenesis and embryogenesis.
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Q15 Describe the post fertilization changes in a flower.
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Q16 What is bisexual flower? Collect five bisexual flowers from your neighbourhood with the help of your teacher find out their common and scientific names.
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Q17 Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
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Q18 Why is offspring of oviparous animals are at a greater risk as compared to offsprings of viviparous animals?
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2. SHORT ANSWER QUESTIONS
(very short answer questions contains 1 mark)
Q1 What is Reproduction
Q2 What is life span?
Q3 Give two examples which reproduce asexually by binary fission.
Q4 Name a unicellular fungi which reproduces asexually by budding. Q5 Name a multicellular invertebrate which reproduces asexually by budding.
Q6 How many kindes of natural vegetative reproduction take place in flowering plants?
Q7 Name the artificial means of vegetative reproduction.
Q8 Fill in the blanks : a) Fusion of two.... results in the formation of zygote. (b) Development of.... from the.... is called embryogenesis. (c).... animals lay eggs. (d) After fertilization,.... are transformed into seeds.
Q9 What are secondary sexual characters?
Q10 What is clone?
Q11 Select the organisms which reproduce by binary fission : Amoeba, Hydra, Penicillium, Paramecium, Sponge.
Q12 What is vegetative propagules?
Q13 What is the major difference you observe in the offsprings produced by asexual reproduction and in the progeny produced by sexual reproduction?
Q14 What are the main causative factors which regulate the reproductive processes and related behavioral expressions in organisms?
Q15 Name the last phase of life span that ultimately leads to death. Give the important changes that occur in the body during this phase.
Q16 Mention the site where Syngamy occurs in amphibians and reptiles.
Q17 Mention a characteristic feature and a function of zoospores in some algae.
Q18 Name the phase all organisms have to pass through before they can reproduce sexually.
Q19 Name the organism where cell division in itself is a mode of reproduction.
Q20 Name an alga that reproduces asexually through zoospores. Why are these reproductive unit so called?
Q21 Name the phenomenon and one bird where the female gamete directly develops into a new organism.
Q22 Name any two means of vegetative propagation in pineapple plant?
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