CBSE Class 12 Biology Chapter 1 Revision Notes Part 2

Chapter 1: Reproduction in Organsims Revision Notes Part 2

Sexual reproduction involves formation of the male and female gametes,either by the same individual or by different individuals of the oppositesex. These gametes fuse to form the zygote which develops to form thenew organism. It is an elaborate, complex and slow process as comparedto asexual reproduction.

Phases in sexual reproduction:

Juvenile Phase

  • The phase of growth between birth and reproductive maturity is called the juvenile phase.
  • It is known as vegetative phase in plants.
  • The end of the juvenile/vegetative phase marks the beginning of the reproductive phase.

Reproductive Phase

  • It is the phase of reproductive maturity.

  • A few plants exhibit unusual flowering phenomenon; some of them such as bamboo species flower only once in their lifetime, generally after 50-100 years, produce large number of fruits and die. Strobilanthus kunthiana (neelakuranji), flowers once in 12 years.

  • The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase:

    1. Oestrus Cycle: In non-primate mammals like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during reproduction is called the oestrus cycle.These creatures reproduce under favorable conditions just in a particular season. Hence, they are called seasonal breeders.

    2. Menstrual Cycle: In primates such as monkeys, apes, and humans, such cyclical changes during reproduction is called the menstrual cycle. These primates reproduce throughout their reproductive phase. Hence, they are called continuous breeders.

Senescent Phase

  • The end of the reproductive phase is considered as senescence or old age.
  • There are concomitant changes in the body (like slowing of metabolism, etc.) during this last phase of life span.
  • Old age ultimately leads to death.

Events in sexual reproduction:

  • Sexual reproduction is characterised by the fusion (or fertilisation) of the male and female gametes, the formation of zygote and embryogenesis.
  • For convenience these sequential events may be grouped into three distinct stages namely, the pre-fertilisation, fertilisation and the post-fertilisation events.

1. Pre-fertilisation Events

These include all the events of sexual reproduction prior to the fusion of gametes.

The two main pre-fertilisation events are:

(i) Gametogenesis

  • Gametogenesis refers to the process of formation of the two types of gametes – male and female.
  • Gametes are haploid cells.
  • In some algae the two gametes are similar in appearance. Hence, they are called homogametes (isogametes).
  • But, in a majority of sexually reproducing organisms the gametes produced are of two morphologically distinct types (heterogametes); the male gamete is called the antherozoid or sperm and the female gamete is called the egg or ovum.


(ii) Gamete Transfer

  • In several simple plants like algae, bryophytes and pteridophytes, water is the medium through which this gamete transfer takes place. A large number of the male gametes, however, fail to reach the female gametes.
  • In cross pollinating plants (including dioecious plants), a specialised event called pollination facilitates transfer of pollen grains to the stigma.
  • Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.

2. Fertilization Events

The most vital event of sexual reproduction is perhaps the fusion of gametes. This process called syngamy or fertilization results in the formation of a diploid zygote. However, in some organisms like rotifers, honeybees and even some lizards and birds (turkey), the female gamete undergoes development to form new organisms without fertilisation. This phenomenon is called parthenogenesis.

External vs Internal Fertilization

3. Post Fertilization Events

Events in sexual reproduction after the formation of zygote are called post-fertilisation events.

(i) The Zygote

  • Formation of the diploid zygote is universal in all sexually reproducing organisms.
  • In organisms with external fertilisation, zygote is formed in the external medium (usually water), whereas in those exhibiting internal fertilisation, zygote is formed inside the body of the organism.
  • In organisms belonging to fungi and algae, zygote develops a thick wall that is resistant to dessication and damage. It undergoes a period of rest before germination.
  • In organisms with haplontic life cycle, zygote divides by meiosis to form haploid spores that grow into haploid individuals.

(ii) Embryogenesis

  • Embryogenesis refers to the process of development of the embryo from the zygote.
  • During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation. While cell divisions increase the number of cells in the developing embryo; cell differentiation helps groups of cells to undergo certain modifications to form specialised tissues and organs to form an organism.
  • Animals are categorised into oviparous and viviparous based on whether the development of the zygote takes place outside the body of the female parent or inside. Due of proper embryonic care and protection, the chances of survival of young ones is greater in viviparous organisms.

Oviparous vs Viviparous

  • In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall off.
  • The pistil however, remains attached to the plant. The zygote develops into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function.
  • After dispersal, seeds germinate under favourable conditions to produce new plants.


  • The phases of sexual reproduction in organisms include – juvenile phase, reproductive phase and the senescent phase.
  • Sexual reproduction involve a set of events and can be divided into three stages: Pre-fertilization, Fertilization, and Post-fertilization.


1. What is the difference between external and internal fertilization?

In external fertilization, syngamy occurs in an external medium i.e., outside the body of the organism whereas, in internal fertilization, syngamy occurs inside the body of the organism

2. What are the differences between oviparous and viviparous organisms?

In oviparous organisms, the development of a zygote takes place outside the body of organisms. In the case of viviparous organisms, the development of a zygote takes place inside the body of organisms.

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  1. “Reproduction Methods | Boundless Biology.”, 2013, Accessed 2 Dec. 2021.
  2. Reproduction in Organisms – NCERT textbook for class 12.
  3. “13.1 How Animals Reproduce – Concepts of Biology-1st Canadian Edition.”, 2019, Accessed 2 Dec. 2021.
  4. “Asexual Reproduction | Boundless Biology.”, 2013, Accessed 2 Dec. 2021.

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