Reproduction In Organisms

Class 12 | Biology — Zoology | Chapter 1

Class 12 Unit 1 Biology

UNIT 1 | Chapter 1

Reproduction in Organisms

Paramecia is capable of both sexual and asexual reproduction

Chapter Outline

• 1.1 Modes of reproduction
• 1.2 Asexual reproduction
• 1.3 Sexual reproduction

Learning Objectives

1. Learns the modes of asexual reproduction in organisms.
2. Understands the significance of asexual reproduction.
3. Comprehends the different modes of sexual reproduction.
4. 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 simple 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. Four 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.

Transverse binary fission: 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.

Longitudinal binary fission: 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.

Oblique binary fission: 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 without the division of the cytoplasm, 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 unfavourable 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 gastrovascular 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.

Gemmules

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.

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): Epimorphosis 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, whereas in restorative regeneration severed body parts can develop. Example: star fish, tail of wall lizard.

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.

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 (Fertilization)

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.

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

Different Kinds of Syngamy

Different kinds of syngamy (fertilization) are prevalent among living organisms.

• 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 beings – 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 / Vegetative phase: Is the period of growth between the birth of the individual upto reproductive maturity.
• Reproductive phase / 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? — Types of Natural Parthenogenesis

Natural parthenogenesis are of different types:

• a) Arrhenotoky: In this type only males are produced by parthenogenesis. Example: Apis (honey bees).
• b) Thelytoky: In this type of parthenogenesis 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. These 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

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 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

Instructions: 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. Short Answer

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

Exercise 6. Short Answer

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

Exercise 7. Short Answer

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

Exercise 8. Short Answer

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

Exercise 9. Short Answer

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

Exercise 10. Short Answer

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

Exercise 11. Short Answer

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.

Exercise 12. Differentiate

12. Differentiate between the following:

a. External and internal fertilization

b. Regeneration in lizard and planaria

Exercise 13. Short Answer

13. How is juvenile phase different from reproductive phase?

Exercise 14. Long Answer

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

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: Apis (honey bees).

2. The mode of 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), where two individuals temporarily unite and exchange nuclear material (DNA).

3. In which mode of reproduction variations are seen

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

Answer: c) Sexual

Explanation: Sexual reproduction involves the 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.

IV. Assertion and Reasoning Questions

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

Answer: A — Both A and R are true and R is the correct explanation for A.

Explanation: In honeybees, fertilized eggs (zygotes) develop into queens and workers (diploid), whereas unfertilized eggs develop into drones (male) by parthenogenesis, making drones haploid. Hence R correctly explains A.

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

Answer: A — Both A and R are true and R is the correct explanation for A.

Explanation: Asexual reproduction is usually by amitotic or mitotic division of somatic cells, so the offsprings show uniparental inheritance without any genetic variation, making them genetically identical to the parent.

V. Answer Briefly

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

Answer: Amoeba. In Amoeba, binary fission (cell division) is itself a mode of reproduction. The parent organism divides into two halves and each half forms a daughter individual, making cell division the sole means of reproduction.

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

Answer: The phenomenon is Parthenogenesis — the development of an egg into a complete individual without fertilization. An avian example is the unfertilized eggs of poultry (hen), which are continuous breeders and can undergo development without fertilization.

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

Answer: Development of an egg into a complete individual without fertilization is known as parthenogenesis. It was first discovered by Charles Bonnet in 1745. Examples: (1) In honeybees (Apis), unfertilized eggs develop into drones (males) by natural parthenogenesis. (2) In Aphis gossypii (Amphitoky), parthenogenetic eggs may develop into individuals of any sex.

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. These variations are essential for adaptation and evolution. Asexual reproduction produces genetically identical offspring with no variation, limiting adaptability to environmental changes.

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 remain as a separate entity — it completely transforms into daughter cells. Since the parent organism is never truly "dead" but continues as two 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: The offspring formed by asexual reproduction are referred to as clones because they are genetically identical to the parent. Asexual reproduction involves only mitotic division of somatic cells (somatogenic reproduction) without gamete formation, so no genetic variation occurs and all offsprings are exact copies of the parent.

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) Honeybees are called parthenogenetic animals because unfertilized eggs in their life cycle develop into drones (males) without fertilization — a process called incomplete natural parthenogenesis (Arrhenotoky). Fertilized eggs develop into queens and workers, while unfertilized eggs develop into drones.
(b) In honeybees, fertilized eggs (diploid/zygotes) develop into queens and workers (32 chromosomes), whereas unfertilized eggs develop into drones by parthenogenesis. Since drones arise from unfertilized haploid eggs, they have only 16 chromosomes, half the number of females.

12. Differentiate between the following:
(a) External and internal fertilization
(b) Regeneration in lizard and planaria

Answer:

(a) External vs Internal Fertilization:
In external fertilization, the fusion of male and female gametes takes place outside the body of the female organism in a water medium. Examples: sponges, fishes and amphibians.
In internal fertilization, the fusion of male and female gametes takes place within the body of the female organism. Examples: reptiles, aves and mammals.

(b) Regeneration in Lizard vs Planaria:
Regeneration in the tail of a wall lizard is an example of epimorphosis (restorative regeneration), where severed body parts can develop back.
Regeneration in Planaria is an example of morphallaxis, where the whole body grows from a small fragment — each piece regenerates lost parts and develops into a whole new individual.

13. How is juvenile phase different from reproductive phase?

Answer: Juvenile phase (vegetative phase) is the period of growth between the birth of the individual up to reproductive maturity — the organism grows but does not reproduce. During the reproductive phase (maturity phase), the organisms reproduce and their offsprings reach maturity. Breeding animals in this phase may be seasonal breeders (frogs, lizards, deer) or continuous breeders (honey bees, poultry, rabbit).

VI. Answer in Detail

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

b) Exogamy: The male and female gametes are produced by different parents and fuse to form a zygote. It is biparental. Example: Human beings (dioecious/unisexual animals).

c) Hologamy: Lower organisms sometimes do not form gametes; the entire mature organisms behave as gametes and their fusion is called hologamy. Example: Trichonympha.

d) Paedogamy: 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). Example: Protozoa.

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

g) Anisogamy: The fusion of dissimilar gametes. It occurs in higher animals, and it is customary to use the term fertilization instead of anisogamy or syngamy. Example: Higher invertebrates and all vertebrates.

📝 Summary

1. Reproduction is a fundamental biological process by which organisms produce their young ones, resulting in the continuation of species and introducing variations essential for adaptation and evolution.
2. Organisms exhibit two major modes of reproduction: asexual reproduction (by a single parent without gamete formation, producing genetically identical offspring) and sexual reproduction (involving two parents and two types of gametes — ova and sperm).
3. Asexual reproduction is widespread among Protista, Bacteria, Archaea, and simple multicellular organisms, with offspring showing uniparental inheritance and no genetic variation; its main modes include fission, budding, fragmentation, and regeneration.
4. Fission is the division of the parent body into two or more identical daughter individuals, and it occurs in four 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/dinoflagellates), depending on the plane along which division occurs.