CBSE Class 12th Biology Notes for Chapter 1 Reproduction In Organisms

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Chapter 1 Reproduction In Organisms
In this method, a single individual parent is capable of producing offspring. The offspring that are produced are not only identical to one another but are also exact copies of their parents.
Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism. Because of the fusion of male and female gametes, sexual reproduction results in offspring that are not identical to the parents or amongst themselves.
Events in sexual reproduction: The events in sexual reproduction are grouped into three stages: pre-fertilization, fertilization, post-fertilization.

  • Pre-fertilization events: The events that occur prior to the fusion of gametes are called the pre-fertilization events.
    • Gametogenesis: It refers to the process of formation of the two types of gametes- male and female. Gametes are haploid cells.
    • Gamete transfer: After their formation, male and female gametes are physically brought together to facilitate fusion (fertilization). This is called the gamete transfer.


  • Fertilization: The fusion of the male of female gametes is called fertilization. It is also known as syngamy and it results in the formation of diploid zygote.


  • Post-fertilization events: The events in sexual reproduction after the formation of the zygote are called post-fertilization events.
    • The zygote: Zygote is the vital link that ensures continuity of species between organisms of one generation and the next. Every sexually reproducing organism, including human beings, begins life as a single cell- the zygote.
    • Embryogenesis: It refers to the development of embryo from the zygote. During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation.

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