Reproduction In Organisms

Class 12 | Pure Science — Zoology | Chapter 1

Class 12 Unit 1 Pure_Science

UNIT 1 | CHAPTER 1

Reproduction in Organisms

Paramecia are capable of both sexual and asexual reproduction.

Chapter Outline

1.1. Modes of reproduction

1.2. Asexual reproduction

1.3. Sexual reproduction

Learning Objectives

➢ Learns the modes of asexual reproduction in organisms.

➢ Understands the significance of asexual reproduction.

➢ Comprehends the different modes of sexual reproduction.

➢ Realizes the significance and advantage of sexual reproduction and its role in evolution.

Living organisms show a life cycle involving birth, growth, development, maturation, reproduction and death. Reproduction is the fundamental feature of all living organisms. It is a biological process by which organisms produce their young ones. The young ones grow and mature to repeat the process. Thus reproduction results in continuation of species and introduces variations in organisms, which are essential for adaptation and evolution of their own kind.

1.1 Modes of Reproduction

All modes of reproduction have some basic features such as synthesis of RNA and proteins, replication of DNA, cell division and growth, formation of reproductive units and their fertilization to form new individuals. Organisms exhibit two major modes of reproduction namely asexual and sexual reproduction. Reproduction by a single parent without the involvement of gamete formation is asexual reproduction and the offspring produced are genetically identical. Asexual reproduction is usually by amitotic or mitotic division of the somatic (body) cells, hence is also known as somatogenic or blastogenic reproduction. When two parents participate in the reproductive process involving two types of gametes (ova and sperm), it is called sexual reproduction.

1.2 Asexual Reproduction

Asexual reproduction is wide spread among different organisms. It is common in members of Protista, Bacteria, Archaea and in multicellular organisms with relatively simpler organisation. The offsprings show "uniparental inheritance" without any genetic variation. The different modes of asexual reproduction seen in animals are fission, budding, fragmentation and regeneration.

Fission

Fission is the division of the parent body into two or more identical daughter individuals. Five types of fission are seen in animals. They are binary fission, multiple fission, plasmotomy, strobilation and sporulation.

Binary Fission

In binary fission, the parent organism divides into two halves and each half forms a daughter individual. The nucleus divides first amitotically or mitotically (karyokinesis), followed by the division of the cytoplasm (cytokinesis). The resultant offsprings are genetically identical to the parent. Depending on the plane of fission, binary fission is of the following types:

1) Simple irregular binary fission

2) Transverse binary fission

3) Longitudinal binary fission

4) Oblique binary fission

Simple irregular binary fission is seen in Amoeba like irregular shaped organisms (Figure 1.1), where the plane of division is hard to observe. The contractile vacuoles cease to function and disappear. The nucleoli disintegrate and the nucleus divides mitotically. The cell then constricts in the middle, so the cytoplasm divides and forms two daughter cells.

In transverse binary fission, the plane of the division runs along the transverse axis of the individual. Example: Paramecium and Planaria. In Paramecium (Figure 1.2) the macronucleus divides by amitosis and the micronucleus divides by mitosis.

In longitudinal binary fission, the nucleus and the cytoplasm divides in the longitudinal axis of the organism (Figure 1.3). In flagellates, the flagellum is retained usually by one daughter cell. The basal granule is divided into two and the new basal granule forms a flagellum in the other daughter individual. Example: Vorticella and Euglena.

In oblique binary fission the plane of division is oblique. It is seen in dinoflagellates. Example: Ceratium.

Multiple Fission

In multiple fission the parent body divides into many similar daughter cells simultaneously. First, the nucleus divides repeatedly, later the cytoplasm divides into as many parts as that of nuclei. Each cytoplasmic part encircles one daughter nucleus. This results in the formation of many smaller individuals from a single parent organism. If multiple fission produces four or many daughter individuals by equal cell division and the young ones do not separate until the process is complete, then this division is called repeated fission. Example: Vorticella.

During unfavorable conditions (increase or decrease in temperature, scarcity of food) Amoeba withdraws its pseudopodia and secretes a three-layered, protective, chitinous cyst wall around it and becomes inactive (Figure 1.4). This phenomenon is called encystment. When conditions become favourable, the encysted Amoeba divides by multiple fission and produces many minute amoebae called pseudopodiospore or amoebulae. The cyst wall absorbs water and breaks off liberating the young pseudopodiospores, each with a fine pseudopodia. They feed and grow rapidly to lead an independent life.

Strobilation

In some metazoan animals, a special type of transverse fission called strobilation occurs (Figure 1.5). In the process of strobilation, several transverse fissions occur simultaneously giving rise to a number of individuals which often do not separate immediately from each other. Example: Aurelia.

Plasmotomy

Plasmotomy is the division of multinucleated parent into many multinucleate daughter individuals with the division of nuclei. Nuclear division occurs later to maintain normal number of nuclei. Plasmotomy occurs in Opalina and Pelomyxa (Giant Amoeba).

Sporulation

During unfavourable conditions Amoeba multiplies by sporulation without encystment. Nucleus breaks into several small fragments or chromatin blocks. Each fragment develops a nuclear membrane, becomes surrounded by cytoplasm and develops a spore case around it (Figure 1.6). When conditions become favourable, the parent body disintegrates and the spores are liberated, each hatching into a young amoeba.

Budding

In budding, the parent body produces one or more buds and each bud grows into a young one. The buds separate from the parent to lead a normal life. In sponges, the buds constrict and detach from the parent body and the bud develops into a new sponge (Figure 1.7). When buds are formed on the outer surface of the parent body, it is known as exogenous budding. Example: Hydra.

In Hydra when food is plenty, the ectoderm cells increase and form a small elevation on the body surface (Figure 1.8). Ectoderm and endoderm are pushed out to form the bud. The bud contains an interior lumen in continuation with parent's gastro-vascular cavity. The bud enlarges, develops a mouth and a circle of tentacles at its free end. When fully grown, the bud constricts at the base and finally separates from the parent body and leads an independent life.

In Noctiluca, hundreds of buds are formed inside the cytoplasm and many remain within the body of the parent. This is called endogenous budding.

In freshwater sponges and in some marine sponges a regular and peculiar mode of asexual reproduction occurs by internal buds called gemmules (Figure 1.9). A completely grown gemmule is a hard ball, consisting of an internal mass of food-laden archaeocytes. During unfavourable conditions, the sponge disintegrates but the gemmule can withstand adverse conditions. When conditions become favourable, the gemmules begin to hatch.

Fragmentation

In fragmentation, the parent body breaks into fragments (pieces) and each of the fragment has the potential to develop into a new individual. Fragmentation or pedal laceration occurs in many genera of sea anemones. Lobes are constricted off from the pedal disc and each of the lobe grows mesenteries and tentacles to form a new sea anemone.

In the tapeworm, Taenia solium the gravid (ripe) proglottids are the oldest at the posterior end of the strobila (Figure 1.10). The gravid proglottids are regularly cut off either singly or in groups from the posterior end by a process called apolysis. This is very significant since it helps in transferring the developed embryos from the primary host (man) to find a secondary host (pig).

Regeneration

Regeneration is regrowth in the injured region. Regeneration was first studied in Hydra by Abraham Trembley in 1740. Regeneration is of two types, morphallaxis and epimorphosis.

In morphallaxis the whole body grows from a small fragment. Example: Hydra and Planaria. When Hydra is accidentally cut into several pieces, each piece can regenerate the lost parts and develop into a whole new individual (Figure 1.11). The parts usually retain their original polarity, with oral ends, by developing tentacles and aboral ends, by producing basal discs.

Epimorphosis (Figure 1.12) is the replacement of lost body parts. It is of two types, namely reparative and restorative regeneration. In reparative regeneration, only certain damaged tissue can be regenerated. Example: human beings (liver tissue). Whereas in restorative regeneration severed body parts can develop. Example: star fish, tail of wall lizard.

1.3 Sexual Reproduction

Sexual reproduction involves the fusion of male and female gametes to form a diploid zygote, which develops into a new organism. It leads to genetic variation. The types of sexual reproduction seen in animals are syngamy (fertilization) and conjugation.

Syngamy

In syngamy, the fusion of two haploid gametes takes place to produce a diploid zygote. Depending upon the place where the fertilization takes place, it is of two types.

In external fertilization, the fusion of male and female gametes takes place outside the body of female organisms in the water medium. Example: sponges, fishes and amphibians.

In internal fertilization, the fusion of male and female gametes takes place within the body of female organisms. Example: reptiles, aves and mammals.

Do You Know?

Power of Regeneration: Sponge when macerated and squeezed through fine silk cloth, the cluster of cells pass through, and these can regenerate new sponges. This technique is used for cultivation of sponges.

Different Kinds of Syngamy (Fertilization)

a) Autogamy: The male and female gametes are produced by the same cell or same organism and both the gametes fuse together to form a zygote. Example: Actinosphaerium and Paramecium.

b) Exogamy: The male and female gametes are produced by different parents and they fuse to form a zygote. So it is biparental. Example: Human dioecious or unisexual animal.

c) Hologamy: Lower organisms, sometimes the entire mature organisms do not form gametes but they themselves behave as gametes and the fusion of such mature individuals is known as hologamy. Example: Trichonympha.

d) Paedogamy: It is the sexual union of young individuals produced immediately after the division of the adult parent cell by mitosis. Example: Actinophrys.

e) Merogamy: The fusion of small sized and morphologically different gametes (merogametes) takes place. Example: Protozoa.

f) Isogamy: The fusion of morphological and physiological identical gametes (isogametes) is called isogamy. Example: Monocystis.

g) Anisogamy: The fusion of dissimilar gametes is called anisogamy (Gr. An — without; iso — equal; gam — marriage). Anisogamy occurs in higher animals but it is customary to use the term fertilization instead of anisogamy or syngamy. Example: higher invertebrates and all vertebrates.

Conjugation

Conjugation is the temporary union of the two individuals of the same species. During their union both individuals, called the conjugants exchange certain amount of nuclear material (DNA) and then get separated. Conjugation is common among ciliates. Example: Paramecium, Vorticella and bacteria (Prokaryotes).

Phases of Life Cycle

Organisms have three phases: Juvenile phase, reproductive phase and senescent phase.

Juvenile phase or vegetative phase is the period of growth between the birth of the individual upto reproductive maturity.

During reproductive phase or maturity phase the organisms reproduce and their offsprings reach maturity period. On the basis of time, breeding animals are of two types: seasonal breeders and continuous breeders. Seasonal breeders reproduce at particular period of the year such as frogs, lizards, most birds, deers etc. Continuous breeders continue to breed throughout their sexual maturity. Example: honey bees, poultry, rabbit etc.

Senescent phase begins at the end of reproductive phase when degeneration sets in the structure and functioning of the body.

Parthenogenesis

(Gr. Parthenos — virgin, Genesis — produce) Development of an egg into a complete individual without fertilization is known as parthenogenesis. It was first discovered by Charles Bonnet in 1745. Parthenogenesis is of two main types namely, Natural Parthenogenesis and Artificial Parthenogenesis. In certain animals, parthenogenesis occurs regularly, constantly and naturally in their life cycle and is known as natural parthenogenesis.

Do You Know?

Natural parthenogenesis is of different types:

a) Arrhenotoky: In this type only males are produced by parthenogenesis. Example: honey bees.

b) Thelytoky: In this type only females are produced by parthenogenesis. Example: Solenobia.

c) Amphitoky: In this type parthenogenetic egg may develop into individuals of any sex. Example: Aphis gossypii.

Natural parthenogenesis may be of two types, viz., complete and incomplete. Complete parthenogenesis is the only form of reproduction in certain animals and there is no biparental sexual reproduction. There are no male organisms and so, such individuals are represented by females only. Incomplete parthenogenesis is found in some animals in which both sexual reproduction and parthenogenesis occurs. Example: in honeybees; fertilized eggs (zygotes) develop into queen and workers, whereas unfertilized eggs develop into drones (male).

In paedogenetic parthenogenesis (paedogenesis) the larvae produce a new generation of larvae by parthenogenesis. It occurs in the sporocysts and Redia larvae of liver fluke. It is also seen in the larvae of some insects. Example: Gall fly.

In artificial parthenogenesis, the unfertilized egg (ovum) is induced to develop into a complete individual by physical or chemical stimuli. Example: Annelid and sea urchin eggs.

Summary

Reproduction is a process by which the living beings propagate or duplicate their own kind. Reproduction can be broadly classified into asexual reproduction and sexual reproduction. In asexual reproduction fusion of gametes are not involved, but in sexual reproduction the formation and fusion of gametes occur.

Different modes of asexual reproduction are fission, budding, fragmentation and regeneration. Fission is further divided into binary fission, multiple fission, sporulation and strobilation. According to the plane of fission different kinds of binary fission have been identified in different organisms. They are simple irregular binary fission, transverse binary fission, longitudinal binary fission and oblique binary fission.

Multiple fission is the division of the parent into many small daughter cells simultaneously. Budding is another mode of asexual reproduction. The parent body produces one or more buds; each bud grows into a young one and may separate from the parent to lead a normal life. When many buds are formed on the outer surface of the parent, it is known as exogenous budding. Hundreds of buds are formed inside the cytoplasm and remain within the body of the parent, this process is called endogenous budding.

Fragmentation is another mode of asexual reproduction. In fragmentation the body of the parent breaks into fragments (pieces). Each fragment has the potential to develop into a new individual. Regeneration is the development of the whole body of an organism from a small fragment. It is of two types namely restorative regeneration and reparative regeneration.

Various modes of sexual reproduction is seen in animals. In syngamy the fusion of two haploid gametes takes place to produce a zygote. The following kinds of syngamy is prevalent among the living organism. They are autogamy, exogamy, hologamy, paedogamy, merogamy, isogamy, anisogamy and conjugation.

Parthenogenesis is the special type of sexual reproduction seen in animals. It is of two main types namely natural parthenogenesis and artificial parthenogenesis.

Evaluation

Exercise 1. MCQ Questions

1. In which type of parthenogenesis are only males produced?

a) Arrhenotoky b) Thelytoky c) Amphitoky d) Both a and b

2. The mode of sexual reproduction in bacteria is by

a) Formation of gametes b) Endospore formation c) Conjugation d) Zoospore formation

3. In which mode of reproduction are variations seen?

a) Asexual b) Parthenogenesis c) Sexual d) Both a and b Exercise 4. Assertion and Reasoning Questions

In each of the following questions there are two statements. One is assertion (A) and other is reasoning (R). Mark the correct answer as:

a) If both A and R are true and R is correct explanation for A

b) If both A and R are true but R is not the correct explanation for A

c) If A is true but R is false

d) If both A and R are false

1. Assertion: In bee society, all the members are diploid except drones.
Reason: Drones are produced by parthenogenesis.

a) Both A and R are true and R is correct explanation for A b) Both A and R are true but R is not the correct explanation for A c) A is true but R is false d) Both A and R are false

2. Assertion: Offsprings produced by asexual reproduction are genetically identical to the parent.
Reason: Asexual reproduction involves only mitosis and no meiosis.

a) Both A and R are true and R is correct explanation for A b) Both A and R are true but R is not the correct explanation for A c) A is true but R is false d) Both A and R are false Exercise 5–7. Short Answer Questions

5. Name an organism where cell division is itself a mode of reproduction.

6. Name the phenomenon where the female gamete directly develops into a new organism with an avian example.

7. What is parthenogenesis? Give two examples from animals.

Exercise 8–10. Short/Medium Answer Questions

8. Which type of reproduction is effective — Asexual or sexual and why?

9. The unicellular organisms which reproduce by binary fission are considered immortal. Justify.

10. Why is the offspring formed by asexual reproduction referred as a clone?

Exercise 11. Give Reasons

11a. Some organisms like honey bees are called parthenogenetic animals.

11b. A male honey bee has 16 chromosomes whereas its female has 32 chromosomes.

Exercise 12. Differentiate Between

12a. External and Internal fertilization

12b. Regeneration in lizard and Planaria

Exercise 13–14. Long Answer Questions

13. How is juvenile phase different from reproductive phase?

14. Explain the different kinds of syngamy in living organisms.

Exercise Concept Map

Complete the concept map for Reproduction in Organisms based on what you have learned in this chapter.

Book-back Exercises — Questions & Answers

Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.

I. Choose the Correct Answer

1. In which type of parthenogenesis are only males produced?

a) Arrhenotoky b) Thelytoky c) Amphitoky d) Both a and b

Answer: a) Arrhenotoky

Explanation: In Arrhenotoky, only males are produced by parthenogenesis. Example: honey bees.

2. The mode of sexual reproduction in bacteria is by

a) Formation of gametes b) Endospore formation c) Conjugation d) Zoospore formation

Answer: c) Conjugation

Explanation: Conjugation is common among ciliates and bacteria (Prokaryotes). During conjugation, two individuals of the same species temporarily unite and exchange nuclear material (DNA).

3. In which mode of reproduction are variations seen?

a) Asexual b) Parthenogenesis c) Sexual d) Both a and b

Answer: c) Sexual

Explanation: Sexual reproduction involves fusion of male and female gametes, which leads to genetic variation. In asexual reproduction, offsprings are genetically identical to the parent with no genetic variation.

II. Assertion and Reasoning Questions

4(1). Assertion: In bee society, all the members are diploid except drones.
Reason: Drones are produced by parthenogenesis.

a) If both A and R are true and R is correct explanation for A b) If both A and R are true but R is not the correct explanation for A c) If A is true but R is false d) If both A and R are false

Answer: a) If both A and R are true and R is correct explanation for A

Explanation: In honeybees, unfertilized eggs develop into drones (males) by parthenogenesis, making them haploid, while fertilized eggs develop into queens and workers, which are diploid.

4(2). Assertion: Offsprings produced by asexual reproduction are genetically identical to the parent.
Reason: Asexual reproduction involves only mitosis and no meiosis.

a) If both A and R are true and R is correct explanation for A b) If both A and R are true but R is not the correct explanation for A c) If A is true but R is false d) If both A and R are false

Answer: a) If both A and R are true and R is correct explanation for A

Explanation: Asexual reproduction is usually by amitotic or mitotic division of somatic cells, producing offsprings that are genetically identical to the parent without any genetic variation.

III. Answer Briefly

5. Name an organism where cell division is itself a mode of reproduction.

Answer: Amoeba is an organism where cell division (binary fission) is itself a mode of reproduction. The parent organism divides into two halves through simple irregular binary fission and each half forms a daughter individual.

6. Name the phenomenon where the female gamete directly develops into a new organism with an avian example.

Answer: The phenomenon is Parthenogenesis — development of an egg into a complete individual without fertilization. An avian example is the turkey, where unfertilized eggs develop into new individuals (natural parthenogenesis).

7. What is parthenogenesis? Give two examples from animals.

Answer: Parthenogenesis (Gr. Parthenos – virgin, Genesis – produce) is the development of an egg into a complete individual without fertilization. It was first discovered by Charles Bonnet in 1745. Examples: (1) Honey bees – unfertilized eggs develop into drones. (2) Solenobia – females are produced by parthenogenesis (Thelytoky).

8. Which type of reproduction is effective — Asexual or sexual and why?

Answer: Sexual reproduction is more effective because it involves the fusion of male and female gametes, leading to genetic variation in offsprings. These variations are essential for adaptation and evolution. Asexual reproduction produces genetically identical offsprings with no variation, limiting adaptability to changing environments.

9. The unicellular organisms which reproduce by binary fission are considered immortal. Justify.

Answer: In binary fission, the parent organism divides into two halves and each half forms a daughter individual. The parent body does not die; instead, it completely transforms into two new daughter individuals. Since the parent organism never truly dies but continues to exist as new individuals, unicellular organisms reproducing by binary fission are considered immortal.

10. Why is the offspring formed by asexual reproduction referred to as a clone?

Answer: Offspring formed by asexual reproduction are referred to as clones because they are genetically identical to each other and to their parent. Since asexual reproduction involves only mitotic division of somatic cells without gamete formation, there is no genetic variation among the offsprings.

11. Give reasons for the following:
(a) Some organisms like honey bees are called parthenogenetic animals.
(b) A male honey bee has 16 chromosomes whereas its female has 32 chromosomes.

Answer: (a) Honey bees are called parthenogenetic animals because they exhibit incomplete parthenogenesis. In honeybees, fertilized eggs (zygotes) develop into queen and workers, whereas unfertilized eggs develop into drones (males) by parthenogenesis.
(b) Male honey bees (drones) are produced from unfertilized eggs by parthenogenesis, making them haploid with 16 chromosomes. Female honey bees (queen and workers) develop from fertilized eggs and are therefore diploid with 32 chromosomes.

IV. Differentiate Between the Following

12(a). Differentiate between External and Internal fertilization.

Answer:

External Fertilization	Internal Fertilization
Fusion of male and female gametes takes place outside the body of female organisms.	Fusion of male and female gametes takes place within the body of female organisms.
Takes place in a water medium.	Takes place inside the female's body.
Examples: Sponges, fishes and amphibians.	Examples: Reptiles, aves and mammals.

12(b). Differentiate between Regeneration in lizard and Planaria.

Answer:

Regeneration in Lizard	Regeneration in Planaria
It is restorative regeneration (a type of epimorphosis) where a severed body part can develop.	It is morphallaxis where the whole body grows from a small fragment.
Only the lost part (tail) is regenerated; the fragment alone cannot form a whole new individual.	Each fragment develops into a whole new individual.
Example: Tail of wall lizard regenerates after being severed.	Example: Planaria cut into several pieces, each piece regenerates into a whole organism.

V. Answer in Detail

13. How is juvenile phase different from reproductive phase?

Answer: The juvenile phase (also called vegetative phase) is the period of growth between the birth of the individual up to reproductive maturity. During this phase, the organism grows and develops but does not reproduce. The reproductive phase (maturity phase) is the period during which organisms reproduce and their offsprings reach maturity. Animals that breed at a particular period of the year are called seasonal breeders (e.g., frogs, lizards, deer), while those that breed throughout their sexual maturity are called continuous breeders (e.g., honey bees, poultry, rabbit). After the reproductive phase ends, the senescent phase begins, during which degeneration sets in the structure and functioning of the body.

14. Explain the different kinds of syngamy in living organisms.

Answer: Syngamy is the fusion of two haploid gametes to produce a diploid zygote. The different kinds of syngamy are:

a) Autogamy: Male and female gametes are produced by the same cell or organism and fuse together. Example: Actinosphaerium and Paramecium.

b) Exogamy: Male and female gametes are produced by different parents and fuse to form a zygote. It is biparental. Example: Humans (dioecious animals).

c) Hologamy: Entire mature organisms do not form gametes but themselves behave as gametes and fuse. Example: Trichonympha.

d) Paedogamy: Sexual union of young individuals produced immediately after division of the adult parent cell by mitosis. Example: Actinophrys.

e) Merogamy: Fusion of small sized and morphologically different gametes (merogametes). Example: Protozoa.

f) Isogamy: Fusion of morphologically and physiologically identical gametes (isogametes). Example: Monocystis.

g) Anisogamy: Fusion of dissimilar gametes. It occurs in higher animals. Example: Higher invertebrates and all vertebrates.

📝 Summary

1. Reproduction is a fundamental biological process by which organisms produce young ones, resulting in continuation of species and introducing variations essential for adaptation and evolution.
2. Organisms exhibit two major modes of reproduction: asexual reproduction (single parent, no gamete formation, genetically identical offspring) and sexual reproduction (two parents, involving two types of gametes — ova and sperm).
3. Asexual reproduction is widespread in Protista, Bacteria, Archaea, and simpler multicellular organisms, with offspring showing uniparental inheritance without genetic variation; its main modes include fission, budding, fragmentation, and regeneration.
4. Fission is the division of the parent body into daughter individuals and occurs in five types: binary fission, multiple fission, plasmotomy, strobilation, and sporulation.
5. Binary fission can be simple irregular (as in Amoeba), transverse (as in Paramecium and Planaria), longitudinal (as in Euglena and Vorticella), or oblique (as in Ceratium), depending on the plane along which division occurs.