Category : 12th Class
Reproduction : Reproduction is the ability of living organism to produce a new generation of living individuals similar to themselves.
Basic features of reproduction : All organisms reproduce. Modes of reproduction vary in different organisms. However, all modes have certain common basic features. These are
(1) Replication of DNA. This is the molecular basis of reproduction.
(2) Cell division, only mitotic, or both mitotic and meiotic. This is cytological basis of reproduction.
(3) Formation of reproductive bodies or units.
(4) Development of reproductive bodies into offspring.
Types of reproduction : These are of two main types
(1) Asexual (Non-gametic) (2) Sexual (gametic)
(1) Asexual reproduction
Definition : Production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. The young one receives all its genes from one parent.
Asexual reproduction is also known as agamogenesis or agamogeny. It involves only mitotic cell divisions, and also termed somatogenic reproduction. Asexual reproduction produces identical offspring commonly referred to as a clone. Today, the scientists have been able to produce clones of multicellular animals (e.g., boar calf names as Frosty, and Finn Dorset lamb named as the famous Dolly) artificially in the laboratory.
Occurrence : Asexual reproduction occurs in protozoans and some lower animals such as sponges, coelentrates, certain worms and tunicates. It is absent among the higher non-vertibrates and all vertibrates.
Types : Asexual reproduction takes place in five principal ways :
(i) Binary fission : Binary fission is the division of the parent into two small, nearly equalized daughter individuals. During binary fission nuclear divisions or karyokinesis, always followed by division of cytoplasm or cytokinesis. Examples - Protozoans (Amoeba, Euglena etc.) Bacteria and Planarians.
Modes of binary fission : In Binary fission, the nucleus divides first and the cytoplasm next. Subsequently, the mother cell splits into two equal sized daughter halves or cells. There are three modes of binary fission.
(a) Simple binary fission : If the plane of cytoplasmic division passes through any direction, the fission is called simple fission. Example - Amoeba.
(b) Transverse binary fission : If the plane of cytoplasmic division coincides with the transverse axis of the individual, the fission is termed transverse binary division. Example - Paramecium and Planaria.
(c) Longitudinal binary fission : If the plane of cytoplasmic division concides with the longitudinal axis of the individual. This kind of fission is designated as longitudinal binary fission. Example Euglena and vorticella.
Binary fission involves mitosis only and consequently, the resultant offspring's are genetically identical to the parent and each other.
(ii) Multiple fission : Multiple fission is the division of the parent into many small daughter individuals simultaneously. Examples – Multiple fission occurs in many protozoans such as Plasmodium, Amoeba and Monocystis, Foraminifera.
Mode of multiple fission : Sometimes, the nucleus divides several times by amitosis to produce many nuclei, is not followed immediately by cytokinesis. Later, each nucleus gathers a small amount of cytoplasm around it and the mother individual splits into many tiny daughter cells.
In course of time, each of these daughter cells starts a free life and transforms into an adult individuals. This kind of fission is called multiple fission.
(a) Encystation : In response to unfavourable living condition, an Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed as encystation.
(b) Sporulation : During favourable condition, the encysted Amoeba divides by multiple fission and produces many minute amoebae or pseudopodiospores; the cyst wall burst out, and the spores are liberated in the surrounding medium to grow up into many amoebae. This phenomenon is known as sporulation.
(c) Schizogony : It is a type of multiple fission present in plasmodium. Schizogonies are of two type. Liver schizogony and RBC schizogony.
(iii) Plasmotomy : Plasmotomy is the division of a multinucleate protozoan into several small, multinucleate daughters without nuclear division. The daughters grow and regain the normal number of nuclei by nuclear divisions. It takes place in Opalina and Pelomyxa.
(iv) Budding : Formation of a daughter individual from a small projection, the bud, arising on the parent body is called budding. It is a common method of asexual reproduction. In budding new individual form by mitosis. Examples - Budding occurs in some protozoans and certain lower animals such as sponges (Scypha), coelenterates (Hydra), annelids (Chaetopterus) and tunicates (Salpa).
Types of budding : There are two types of budding
(a) Exogenous or External budding : Initially, a small outgrowth of the parent's body develops into a miniature individual. It then separates from the mother to lead a free life. This type of budding is recognised as exogenous budding. Example - Hydra.
(b) Endogenous or Internal budding : In fresh water sponges (e.g., Spongilla) and marine sponge (e.g., Sycon), the parent individual releases a specialised mass of cells enclosed in a common opaque envelope, called the gemmule, on germination. Each gemmule gives rise to an offspring gemmules are thought to be internal buds. This type of budding recognised as endogenous budding. Example - Sycon and Spongilla.
(v) Fragmentation : It is the breaking up of an animal's body into two or more pieces, each of which grows into a new individual. Examples - It occurs in the flatworm, microstomum.
Special asexual reproductive bodies : Archaeocytes of sponges are totipotent cells. They take part in the formation of gemmules. Gemmules form new sponges.
Reproductive units in asexual reproduction : Reproductive units vary in different forms of asexual reproduction. These are entire parent bodies in binary and multiple fission's and are small parts of parent body in budding and fragmentation. An asexual reproductive unit is called blastos.
Characteristics of asexual reproduction : All forms of asexual reproduction have certain common basic features. These are under :
(i) A single parent produces offspring, that is, asexual reproduction is uniparental.
(ii) Gametes are not formed.
(iii) Cell divisions are only mitotic.
(iv) The new individuals formed are usually genetically identical to the parent. Variability, if it occurs, is restricted to mutation only.
(v) Multiplication occurs rapidly.
(vi) The offspring are often formed in large numbers near the parent.
Significance of asexual reproduction : Asexual reproduction brings multiplication of the species only. It does not play a role in evolution as no variation is introduced into the new individuals formed by it. Asexual reproduction is theoretically most advantageous in stable, favourable environment because it perpetuates successful genotypes precisely.
(2) Sexual reproduction
Definition : It is the production of offspring usually by two parents, male and female. Involving four processes :
(i) Formation of special haploid cells, the gametes, by meiosis. (Gametogenesis)
(ii) Fusion of the gametes in pairs, forming diploid cells, the zygotes (Fertilization)
(iii) Repeated mitotic divisions of zygotes to form embryos (Embryogenesis)
(iv) Growth of Embryos into a new individuals (Development)
Occurrence : Sexual reproduction occurs nearly in all animals, including those which reproduce asexually. Some protozoans, such as Amoeba, Euglena lack sexual reproduction. In male and female, and the difference between them is determined genetically. In sexual reproduction offsprings resemble the parent.
Types : Sexual reproduction is of two main types -
(i) Amphigony : It involves the complete and permanent fusion of two gametes from differents or from the same parent to form a composite cells, the zygote. It is further of two kinds :
(a) Syngamy : It involves the fusion of two entire gametes to form a zygote. The fusion nucleus of zygote called synkaryon. It is further of two types with regard to the source of fusing gametes :
Special forms of syngamy : These are three special forms of syngamy :
(a) Neoteny : Neoteny refers to “retention of a larval or embryonic trait in adult body”. e.g., Retention of larval gills in some adult salamander.
(b) Paedogenesis or paedomorphosis : Paedogenesis is refers to “development of gonads and production of young ones by larva”. e.g., salamander Axolotl larva, liver fluke Redia larva, and gall fly.
(c) Polyembryony : The blastomeres formed by division of the zygote separate in early stages of development, each producing in a complete individual (fasciola liver fluke). Armadillo regularly produces 4-8 young ones per zygote. Identical twins in human beings is another example.
(ii) Conjugation : Some acellular protist animals (e.g., Paramaecium) exhibit sexual reproduction by forming male and female gamete nuclei, which they exchange through temporary cytoplasmic bridge; later, the cytoplasmic bridge disappears and the gamete nucleus of one individual fuses with that of the other to form zygote nuclei. This mode of sexual reproduction is known as conjugation.
Parthenogenesis (Virgin birth)
It is a modification of sexual reproduction in which an egg develops into a complete offspring without fertilization. It is monoparental. Parthenogenesis was discovered by Bonnet (1745).
Occurrence : Parthenogenesis is found in many non vertebrates such as rotifers, aphids, bees and crustaceans. It also occur in a few vertebrates.
Types : Parthenogenesis is of two main types :
(1) Natural parthenogenesis : It is a regular phenomenon in the life history at some animals. It may be three type.
(i) Complete (Obligatory) parthenogenesis : Males are absent, females develop parthenogenetically, e.g., rotifers, Typhlina brahmina (small lizard, 15 cm long), Lacerta saxicola-armeniaca (Caucasian Rock Lizard), Cnemidophorus (Whiptail Lizards of America).
(ii) Incomplete (cyclic) parthenogenesis : Some animals have both sexual and parthenogenetic individuals, which may alternate. In these animals, female can produce unfertilized or fertilized eggs, depending upon environmental conditions. In Daphnia, a fresh water crustacean, female lays unfertilized eggs that develop parthenogenetically under favourable conditions, and fertilized eggs during times of environmental stress.
In honeybee, unfertilized eggs develop into male bees (drones) with haploid cells, and fertilized eggs give rise to females (queen bees and worker bees) with diploid cells.
(iii) Paedogenetic parthenogenesis : In certain insects, larvae lay eggs which develop parthenogenetically into a new generation of larvae. Parthenogenesis in larvae is called paedogenesis.
(2) 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 may be (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, to water.
On the basis of chromosome sets parthenogenesis is of two types –
(i) Arrhenotoky (Haploid parthenogenesis) : Haploid eggs grow to form haploid males e.g., Arachnids, some insects (honey bees).
(ii) Thelotoky (Diploid parthenogenesis) : Diploid eggs grow without fertilization in to diploid individuals, generally females. e.g., Gall fly.
(i) This avoids the wastage of germplasm as sperms and ova. Adult organism is devoted exclusively to feeding and reproduction so is a mode of high reproduction e.g., aphids.
(ii) There is no chance of separation of useful combination of genes by crossing over and are transmitted as such.
(iii) The offsprings are exactly similar to parents.
(iv) Haploid parthenogenesis is the direct proof of chromosomal theory of sex-determination.
Disadvantages of parthenogenesis : It stops the chances of new combinations of genes and thus avoids selection in population. It decreases the chances of adaptability followed by extinction.
Reproductive unit in sexual reproduction
The reproductive units in sexual reproduction are specialised cells called gametes. The gametes are generally of two kinds -
(1) Microgametes or Spermatozoa
(2) Macrogametes or Ova
Both are well developed 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 gametes are usually large, non motile and often have a store of food to nourish the developing embryo.
Maintenance of chromosome number : 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 formed by the fusion of two haploid gametes in naturally diploid, i.e., has double or normal number (2n) of chromosomes.
(1) External fertilization and External development : This pattern is found in many aquatic animals, such as Obelia, Nereis, all bony fishes and frogs. Parents release sperms and eggs into the surrounding water, where fertilization occurs and zygotes develop into offspring.
(2) Internal fertilization and External development : Sperms are passed from the male into the female with an intromittent organ, such as claspers in male shark, hemi penis in lizard, snake, crocodile and penis in mammals, or otherwise, for example, by cloacal apposition in birds, with modified arm in cuttle fish. Internal fertilization has several advantages.
Animals with internal fertilization usually produce fewer zygotes because of protection provided by egg shells or 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 predators who attack externally developing eggs.
Characteristics of sexual reproduction : Sexual reproduction has the following important basic features –
(i) It is generally biparental.
(ii) It involves formation of male and female gametes.
(iii) Mostly there is fusion of male and female gametes (fertilization).
(iv) Cell divisions are meiotic during gamete formation and mitotic during development of zygote into an offspring.
(v) The offspring are not genetically identical to the parents. They show variation as they receive characters (chromosomes) from two different parents. Sexual reproduction is, thus, a source of variety in population.
Significance of sexual reproduction : Sexual reproduction has a dual significance for the species :
(i) It results in multiplication and perpetuation of the species.
(ii) It contributes to evolution of the species by introducing variation in a population much more rapidly than asexual reproduction.
Difference between sexual and asexual reproduction
It is always uniparental.
It is generally biparental.
It invariably results in increase in the number of individuals.
It may not result in increase in the number of individuals.
Gametes are not formed.
It always involves the formation and fusion of gametes.
There is no fertilization.
Fertilization generally occurs.
It involves only mitotic cell divisions.
It involves meiotic divisions during gamete formation and mitotic divisions during development of zygote into an offspring.
Daughter individuals are genetically identical to the parent.
Daughter individuals genetically differ from the parents.
It occurs in only lower invertebrates and lower chordates.
It occurs nearly in all animals.
It contributes little to evolution.
It contributes to evolution by introducing variation in offspring.
It often causes rapid increase in number.
It causes slower increase in number.
Blastogenesis and Embryogenesis : Development of the offspring from reproductive units, such as buds or fragments, in asexual reproduction is called blastogenesis. Development of the embryo from the zygote in sexual reproduction is termed embryogenesis.
Unisexual or dioecious : Organism in which the two sexes occur in different individuals, e.g., humans, mammals, birds, lizards.
Bisexual / Hermaphrodite or monoecious : Organism in which the two types of sex organs (testes and ovaries) occur in the same individual, e.g., Earthworm, Taenia, Leech, Fasciola, Myxine, Herdmania.
Deviations in the reproductive strategies : Although asexual and sexual reproductions are the two major trends of breeding, many deviations are also observed in the reproductive strategies of animals. One such variation in reproductive strategy in hermaphroditism, found in tapeworms and earthworms. Tapeworms are self-fertilising; the sperm produced in the testes of one individual can fertilise the eggs produced by the same individual. The earthworms employ cross, fertilisation; the sperm of one individual fertilises the eggs of the other.
Sexual dimorphism : Differentiation in morphology of the two sexes of the same species is called sexual dimorphism. Example - Ascaris, Oryctolagus and humans etc.
In human beings, reproduction takes place by sexual method and the sexes are separate.
(1) Sex organs : Human are unisexual. The reproductive system of each sex consists of many organs. The latter are distinguishable into primary and secondary sex organs. Besides these, there are some accessory sex characters -
(i) Primary sex organs : Gonads which form gametes are called primary sex organs - testis (plural testes) in males and ovary (plural ovaries) in females. Testis produces sperms and secrets testosterone. Ovary produces ova. Maturing Graffian follicles secrete estrogens.
(ii) Secondary sex organs : Sex organs, glands and ducts which do not produce gametes but are otherwise essential for sexual reproduction are known as secondary sex organs. In human male reproductive system, the secondary sex organs are vasa efferentia, epididymes, vasa deferentia, ejaculatory ducts, urethra, accessory sex glands are prostate glands, Cowper's glands seminal vesicle and penis. Secondary sex organs of a human female include fallopian tubes, uterus, vagina, external genitalia, Bartholin's gland and mammary glands are accessory sex glands.
(iii) Accessory / External / Secondary sex characters : They are traits which do not have any direct role in reproduction but provide specific features and structures to the two sexes. The important external / accessory sex characters of human male are beard, moustaches, body hair on shoulder and chest, pubic hair on both lateral and vertical directions, comparatively more height with more muscular body, larynx apparent externally, voice low pitched with breathing more by means of diaphragm. The important accessory sex character of human females are high pitched voice, breast, broader pelvis, lateral pubic hair, rounded body contours with more subcutaneous fat in thighs, buttocks and face and sternal breathing.
(2) Characteristics of human reproduction :
(i) Human beings are non-seasonal breeders.
(ii) There is no oestrus / heat.
(iii) In human females the ability to produce young ones begins at menarche (beginning of menses) and ends at menopause (stoppage of menses).
(iv) In human females the reproductive phase has 28 day repeated menstrual cycle.
(v) Fertilization is internal.
(vi) There is vivipary, i.e., giving birth to young ones.
(vii) Foetus develops inside uterus and is nourished by joint special structure called placenta.
(viii) Infants can be fed on mother's milk.
(ix) Parental care is very well developed.
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