CBSE CLASS 12 CHAPTER 1:Reproduction In Organisms

Reproduction is a process in which an organism produces young ones (offspring) similar to itself.

The period from birth to the natural death of an organism is known as its lifespan.

No individual is immortal, except unicellular organisms. There is no natural death in unicellular organisms.

Life spans of some organism


Based on the number of participants, reproduction is of 2 types: Asexual reproduction & Sexual reproduction.


It is the production of offspring by a single parent.

It is seen in unicellular organisms, simple plants & animals.

The offspring are identical to one another and their parents. Such morphologically and genetically similar individuals are known as clones.

Types of asexual reproduction

a. Fission:

In this, the parent cell divides (cell division) into two or more individuals. E.g. Protists and Monerans.

Fission is 2 types:

▪ Binary fission:

It is the division of the parent cell into two individuals. E.g., Amoeba, Paramecium.

▪ Multiple fission:

It is the division of parent cells into many individuals. E.g. Plasmodium, Amoeba.
Under the unfavorable condition, Amoeba withdraws its pseudopodia and secretes a 3-layered hard covering (cyst) around itself. It is called encystation.


Under favorable conditions, encysted Amoeba undergoes multiple fission to give many minute amoeba or pseudopodiospores. The cyst wall bursts out and spores are liberated to grow up into many amoebae. This is called sporulation.

b. Budding:

In this, a bud appears and grows in the parent body. After maturation, it is detached from the parent body to form a new individual. E.g. Hydra, Sponge, Yeast, etc.


c. Fragmentation:

In this, the body breaks into distinct pieces (fragments) and each fragment grows into an adult
capable of producing offspring. E.g. Hydra.


d. Vegetative propagation:

It is the production of offspring from vegetative propagules in plants. Vegetative propagules are units of vegetative propagation. Examples for vegetative propagules:
Buds (‘eyes’) of the potato tuber.
Rhizomes of banana & ginger.

Buds & Rhizomes arise from the nodes of modified stems.
The nodes come in contact with damp soil or water and produce roots and new plants.

Adventitious buds of Bryophyllum. They arise from the notches at the margins of leaves.
Bulbil of Agave.
▪ An offset of water hyacinth.
Runner, sucker, tuber, bulb, etc.


Other asexual reproductive structures:
E.g. zoospores (microscopic motile structures in some algae and protists), conidia (Penicillium), and gemmules (sponge).


Asexual reproduction is the common method in simple organisms like algae and fungi. During adverse conditions, they can shift to sexual methods.

Higher plants reproduce asexually (vegetative) & sexually. But most of the animals show only sexual reproduction.

Sexual Reproduction


In this mode of reproduction, a new offspring is produced by the participation of two parents of the opposite sex.

This type of reproduction is seen in all multicellular organisms including birds, reptiles, dogs, cats, cattle, elephants, etc.

The complete process of sexual reproduction consists of a set of events, including:

  • Pre-fertilization
  • Fertilization
  • Post-fertilization

Advantages of Sexual Reproduction:

  • The involvement of two parents results in the intermingling of genes resulting in the production of a new offspring
  • Genetically identical offspring are produced
  • Variations in species increase the chances of survival hence in the evolutionary advancements.

It is an elaborate, complex, and slow process as compared to sexual reproduction.

The period of growth to reach in maturity for sexual reproduction is called the Juvenile phase. in plants, it is called the vegetative phase.

In higher plants, flowering indicates the end of the vegetative phase and the beginning of the reproductive phase.

Annual and perennial plant Shows reproductive and senescent phases.

 in perennial plants, these phases are very difficult to identify.

some plants exhibit unusual flowering like 

Bamboo species Flower pot only once in their life after 50-100 years and produce a large number of fruits and die.

Strobilanthus kunthiana flowers once in 12 years.

In the animal Juvenile phase is followed by morphological and physiological changes prior to activity reproductive behavior.

Birds living in nature lay eggs only seasonally. forever, birds in captivity that is poultry to lay eggs throughout the year.

 The females of placental mammals exhibit cyclical changes in the ovaries, accessory ducts, and hormones during the reproductive phase.

The cyclical change in is called the oestrus cycle in non-primate-like Cat, sheep. dear, dog tiger, etc and menstrual cycle in primate-like monkeys Apes, and humans.

Best on breeding season mammals are of two types:-

Seasonal breeders -the mammal exhibit in reproductive cycle only during February season.

Continuous breeders – they are reproductively active throughout their reproductive phases.


It is the last phase of life span and end of the reproductive phase.

 during this on comitantes change according to the body example slowing of metabolism it ultimately leads to death.

In plants and animals hormone causes transition between the juvenile and productive and senescence phase.



It is the formation of male and female gametes.

Gamete are haploid and of two types- homogamete (isogamete) Heterogamete.

Homogametes(isogamete):-Similar gametes they cannot categorize into male and female gametes. For example some algae like cladophora

Heterogamete:- The male and female camel chart distinct types. Mil Gaya mat is called antherozoid sperm and female gamete is called the egg (ovum) example in fucus an alga and human beings.

Fertilization – Internal And External Fertilization

Living organisms ensure their continuance on the earth by reproduction. Reproduction may occur either asexually or sexually.

There is an evident difference between sexual reproduction and asexual reproduction. The sexual mode is a more complex process than the asexual mode.

One crucial difference is the fertilization. It is the main stage of sexual reproduction, which is absent in asexual mode. Let’s learn more about types of fertilization.

Fertilization in Animals

The process of fusion of sperm with egg (ovum) to produce a zygote is called fertilization. It is the crucial and primary stage of sexual reproduction. During sexual intercourse, the penis ejaculates millions of sperms into the vagina of the woman.

Sperms will travel through the uterus to oviducts. At the oviduct, one out of million sperms fertilizes the released ovum. The fertilized egg develops into a zygote. Without the fusion of gametes, sexual reproduction is futile. It doesn’t occur in asexual reproduction

Fertilization in most of the animals is similar to that in humans. Animals also produce gametes for fusion. But the fusion of gametes may take place inside or outside the body.

Based on this, fertilization is of two types – internal and external fertilization.

Internal Fertilization

In sexual reproduction, the male inserts the sperms into the female reproductive tract to fuse with the egg. If the fusion takes place within the female parent, it is called internal fertilization.

In humans, most of the animals like cats, lions, pigs, dogs, hens, etc., the fusion of gametes takes place internally. In this type, a zygote is formed within the mother and gets its nourishment from her.

External Fertilization

When the fusion of sperm and egg takes place outside the female parent, it is called external fertilization. Only a minority of organisms exhibit this type of gamete fusion.

For example, fish, frogs, etc. Here the female parent deposits her eggs in a place and later, the male parent ejects his sperms over them, then fusion takes place.

Gametes that fuse externally have to face many challenges. Since eggs and sperms are deposited in the external environment, the chances of fusion are very less.

Predators may eat eggs and the zygote that is formed. To compensate for this loss, organisms like fish and frogs lay hundreds of eggs at a time.


Fertilization results in diploid zygote formation. Eventually, the zygote divides mitotically and develops as an embryo. This process is called embryogenesis.

During embryogenesis, the cell differentiates and modifies accordingly. Zygote development depends on the organism and its life cycle.

Animals are classified into oviparous and viviparous based on whether the zygote develops outside or inside the body respectively. In angiosperms, the zygote develops into the ovary and the ovary transforms into the fruit while ovules develop into seeds.

Embryo Development


Embryo development refers to the different stages in the development of an embryo. Embryonic development of plants and animals vary. Even in animals, every species undergoes different stages during embryonic development

  • Embryogenesis is defined as the process of development of the embryo from the zygote. During embryogenesis, the zygote undergoes cell division (mitosis) and cell differentiation.
  • Cell divisions will lead to the increase in the number of cells in the developing embryo, while cell differentiation helps the group of cells undergo certain modifications to form specialized tissues and organs to form organisms.
  • In animals, if the development of the zygote takes place in the body of the female parent, it is called viviparous. 
  • In egg-laying animals such as reptiles and birds, fertilized eggs, which are covered by a hard calcareous shell, are deposited in a safe place in the environment. After an incubation period, the young hatch. It is called Oviparus
  • On the other hand, in viviparous animals such as mammals, including humans, the zygote develops into a cub that emerges from the mother’s body. The chances of survival of the young are greater with live-bearing organisms due to adequate embryonic care and protection.

In the case of Angiospermic plants, the zygote is formed, inside the ovule. Once fertilization takes place, different parts like the sepals, petals, and stamens of the flower fall off. The pistil is the only part that remains attached to the plant.

In plants:

  1. The zygote develops into an embryo.
  2. Ovule develops into a seed
  3. The integument of the ovule develops into a seed coat.
  4. The ovary develops into a fruit.
  5. Ovary wall develops into pericarp and is protective in function.
  • After dispersal, seeds germinate under favourable conditions to produce new plants