Biology Notes for Class 12

Chapter 1: Reproduction in Organism

Chapter Summary

          Reproduction enables a species to live generation after generation. Reproduction in organisms can be broadly classified into asexual and sexual reproduction. Asexual reproduction does not involve the fusion of gametes. It is common in organisms that have a relatively simple organisation such as the fungi, algae and some invertebrate animals. The offspring formed by asexual reproduction are identical and can be referred to as clones. Zoospores, conidia, etc., are the most common asexual structures formed in several algae and fungi. Budding and gemmule formation are the common asexual methods seen in lower animals.

            Prokaryotes and unicellular organisms reproduce asexually by cell division or binary fission of the parent cell. In several aquatic and terrestrial species of angiosperms, structures such as runners, rhizomes, suckers, tubers, offsets, etc., are capable of giving rise to new offspring. This method of asexual reproduction is generally referred to as vegetative propagation.

           Sexual reproduction involves the formation and fusion of gametes. It is a complex and slower process as compared to asexual reproduction. Most of the higher animals reproduce almost entirely by sexual method. Events of sexual reproduction may be categorised into pre-fertilisation, fertilisation and post-fertilisation events. Pre-fertilisation events include gametogenesis and gamete transfer while post-fertilisation events include the formation of zygote and embryogenesis.

          Organisms may be bisexual or unisexual. Sexuality in plants is varied, particularly in angiosperms, due to the production of diverse types of flowers. Plants are defined as monoecious and dioecious. Flowers may be bisexual or unisexual flowers.

             Gametes are haploid in nature and usually a direct product of meiotic division except in haploid organisms where gametes are formed by mitosis.

            Transfer of male gametes is an essential event in sexual reproduction. It is relatively easy in bisexual organisms. In unisexual animals it occurs by copulation or simultaneous release. In angiosperms, a special process called pollination ensures transfer of pollen grains which carry the pollen grains to the stigma. Syngamy (fertilisation) occurs between the male and female gametes.

            Syngamy may occur either externally, outside the body of organisms or internally, inside the body. Syngamy leads to formation of a specialised cell called zygote.

               The process of development of embryo from the zygote is called embryogenesis. In animals, the zygote starts developing soon after its formation. Animals may be either oviparous or viviparous. Embryonal protection and care are better in viviparous organisms.

              In flowering plants, after fertilisation, ovary develops into fruit and ovules mature into seeds. Inside the mature seed is the progenitor of the next generation, the embryo.


Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself.

Life span of different organisms 

Name of Animal/Plant

Life span (apprx.)


60-70 yrs


10 years


25 Years


1-2 Weeks


15 Years

Banana Tree

25 Years


25 Years


140 Years


60 Years


50 Years

Fruit Fly

1 Months

Rice Plant

4 Months


100-150 Years

Banyan Tree

200-300 Years

Asexual reproduction:

Ø  Offsprings produced by single parents.

Ø  Without involvement of gamete formation

Ø Offsprings are genetically identical to their parents.


Asexual reproduction:

Ø  Cell division as a method of asexual reproduction as in Protista and monera.

Ø  Binary fission e.g. Amoeba, Paramecium.

Ø  Budding: e.g. yeast.

Ø  Asexual reproductive structures:

  • Zoospores: aquatic fungi, Chlamydomonas.
  • Conidia: Penicillium.
  • Bud: Hydra
  • Gemmules: sponges.

Vegetative propagation units in plant: (Vegetative propagules)

  • Runner, rhizome, sucker, tuber, offset, bulb.

Sexual reproduction:

Ø  Involvement of single or two individual.

Ø  Production of male and female gametes ( haploid)

Ø  Gametes fused to form a diploid zygote.

Ø  Zygotes developed into new organism.

Ø  The offsprings are not genetically identical with their parents.

Features of sexual reproduction:

Ø  Period between birth and sexual maturity is called juvenile phase. It is known as vegetative phase in plant.

Ø  Bamboo species flower only once in their life time generally after 50-100 yr.

Ø  Strobilanthus kunthiana (neelakranji) flowers once in 12 years.

Ø  Oestrus cycle: cyclical changes during reproduction in non-primate mammal like cows, sheep, rats, deers, dogs, tiger etc.

Ø  Menstrual cycle: cyclical changes during reproduction in primate mammals like monkeys, ape, and humans.

Ø  Seasonal breeders: reproductive cycle takes place in favourable seasons as in wild animals.

Ø  Continuous breeders: reproductively active throughout their reproductive phase.

Pre-fertilization events:

Ø  Process of gamete formation is gametogenesis.

Ø  Two gametes are similar in appearance are called homogametes (isogametes).

Ø  Gametes produced are of two morphologically distinct types called heterogametes.

Ø  Male gamete is called antherozoids or sperm and the female gamete is called ovum or egg.

Sexuality in organism:

Ø  Plant having both male and female sex organ called homothallic or monoecious.

Ø  Plants having only one sex organ is called heterothallic or dioecious.

Ø  In flowering plants, the unisexual male flower is staminate, i.e. bearing staments, while the female is pistillate or bearing pistils.

Ø  Animal having one type of reproductive system, called unisexual.

Ø  Animal having both male and female reproductive system, called hermaphrodite or bisexual.

Cell division during gamete formation:

Ø  Gametes in all heterogametic species two types namely male and female.

Ø  Gametes are always haploid irrespective of parent’s ploidy.

Ø  A haploid parent produces gametes by mitotic division.

Ø  Diploid parent produces gametes by meiotic division.

Ø  In diploid organisms specialized cells called meiocytes (gamete mother cell) undergo meiosis to produce haploid gametes.

Gamete transfer:

Ø  Male and female gamete must be physically brought together to facilitate fusion called fertilization.

Ø  In most cases male gametes are motile, female gametes are non-motile.

Ø  In case of few fungi and algae, both male and female gametes are motile.

Ø  In most cases water is the medium for gamete transfer.

Ø  Male gametes are produced in several thousand times the number of female gametes produced to compensate the loss during transfer.


Ø  Fusion of male and female gamete is called fertilization or syngamy.

Ø  The female gamete undergoes development to form new organism without fertilization. This phenomenon is called parthenogenesis.

Ø  Gametic fusion takes place outside the body i.e. water is called external fertilization.

Ø  Their must be synchrony of gamete release, large number of gametes released to enhance the chance of fertilization.

Ø  Enable the individual to produce large number of offsprings.

Ø  A major disadvantage is that the offsprings are extremely vulnerable to predators.

Ø  Fertilization takes place inside the body is called internal fertilization.


Ø  Formation of zygote after fertilization is universal in all sexually reproducing organisms.

Ø  Zygote is formed usually in water in case of external fertilization.

Ø  Zygote is formed inside the body of the organism in internal fertilization.

Ø  Zygote of fungi and algae develops a thick wall that is resistant to dessication and damage.

Ø  Organism with haplontic life cycle, zygote undergoes meiosis to produce haploid spores.


Ø  Development of zygote into an embryo is called embryogenesis.

Ø  Zygote undergoes cell division (mitosis) and cell differentiation.

Ø  Oviparous animal which lays eggs and development takes place inside egg.

Ø  Viviparous animal gives birth to the young. The development takes place inside the body of the female.

In plants:

  • Zygote developed into embryo.
  • Ovule developed into seed.
  • Integument of the ovule developed into seed coat.
  • Ovary developed into fruit.
  • Ovary wall developed into pericarp.

Disclaimer: All contents are originally prepared by Shri K C Meena Ji, Principal, KVS. 

Amazon Affiliate Disclaimer: is a part of Amazon Services LLC Associates Program, an affiliate advertising program  designed to provide a means for sites to earn advertising fees by advertising and linking to As an amazon associates we earn from qualifying purchases.

Stamen, Microsporangium and Pollen grain:

Notify of

1 Comment
Newest Most Voted
Inline Feedbacks
View all comments
intraday tips

Mind blown! Your expertise shines through this article.

error: Content is protected !!
Would love your thoughts, please comment.x
Scroll to Top