NCERT Book Class 12 Biology Reproduction in Organisms

REPRODUCTION IN ORGANISMS

Each and every organism can live only for a certain period of time. The period from birth to the natural death of an organism represents its life span. Life spans of a few organisms are given in Figure 1.1. Several other organisms are drawn for which you should find out their life spans and write in the spaces provided. Examine the life spans of organisms represented in the Figure 1.1. Isn’t it both interesting and intriguing to note that it may be as short as a few days or as long as a few thousand years? Between these two extremes are the life spans of most other living organisms. You may note that life spans of organisms are not necessarily correlated with their sizes; the sizes of crows and parrots are not very different yet their life spans show a wide difference.

Similarly, a mango tree has a much shorter life span as compared to a peepal tree. Whatever be the life span, death of every individual organism is a certainty, i.e., no individual is immortal, except single-celled organisms. Why do we say there is no natural death in single-celled organisms? Given this reality, have you ever wondered how vast number of plant and animal species have existed on earth for several thousands of years? There must be some processes in living organisms that ensure this continuity. Yes, we are talking about reproduction, something that we take for granted.

Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle  of birth, growth and death. Reproduction enables the continuity of the species, generation after generation. You will study later in Chapter 5 (Principles of Inheritance and Variation) how genetic variation is created and inherited during reproduction.

There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offspring. The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces. Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual. When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction.

1.1 ASEXUAL REPRODUCTION 

In this method, a single individual (parent) is capable of producing offspring. As a result, the offspring that are produced are not only identical to one another but are also exact copies of their parent. Are these offspring likely to be genetically identical or different? The term clone is used to describe such morphologically and genetically similar individuals.

Let us see how widespread asexual reproduction is, among different groups of organisms. Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organisations. In Protists and Monerans, the organism or the parent cell divides into two to give rise to new individuals (Figure1.2). 

EXERCISES

1. Why is reproduction essential for organisms?

2. Which is a better mode of reproduction sexual or asexual? Why?

3. Why is the offspring formed by asexual reproduction referred to as clone?

4. Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?

5. How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?

6. Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction? 

7. What is vegetative propagation? Give two suitable examples.

8. Define

 (a) Juvenile phase,

 (b) Reproductive phase,

 (c) Senescent phase.

9. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?

10. Explain why meiosis and gametogenesis are always interlinked?

11. Identify each part in a flowering plant and write whether it is haploid

 (n) or diploid (2n).

 (a) Ovary ———————————

 (b) Anther ———————————

 (c) Egg ———————————

 (d) Pollen ———————————

 (e) Male gamete ———————————

 (f ) Zygote ———————————

12. Define external fertilisation. Mention its disadvantages.

13. Differentiate between a zoospore and a zygote.

14. Differentiate between gametogenesis from embryogenesis.

15. Describe the post-fertilisation changes in a flower.

16. What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.

17. Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?

18. Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?

Please refer to attached file for NCERT Class 12 Biology Reproduction in Organisms