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NCERT Biology Class 12 NCERT solutions.
Important points to remember :
- 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.
- Reproduction becomes a vital process without which species cannot survive for long. Each individual leaves its progeny by asexual or sexual means. Sexual mode of reproduction enables creation of new variants, so that survival advantage is enhanced.
- When offspring is produced by a single parent with or without the involvement of gamete formation is called asexual reproduction.
- When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction.
- Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organisations eg. Amoeba, Paramecium).
- Organisms choose to reproduce asexually by different modes or ways like fragmentation, budding, binary fission, spore formation, Conidia formation, Gemmules formation.
- Vegetative Reproduction : Plants reproduce asexually through their vegetative parts. Hence, the new plants formed are genetically identical to their parents.
- Sequential events that occur in sexual reproduction are grouped into three distinct stages are 1. Pre-fertilization, 2. Fertilization and the 3. Post-fertilization.
- 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 andovules mature into seeds. Inside the mature seed is the progenitor of the next generation, the embry
1. Why is reproduction essential for organisms?
Answer : Reproduction is essential for organisms because Reproduction is a fundamental feature or process of all living organisms. It is a biological process by which living organisms produce offspring’s similar to them by fusion of male and female gametes(sexual reproduction) or without fusion(asexual reproduction). Reproduction ensures the continuity of each and every species on the Earth. If the reproduction is absent the species will not be able to exist for a long time and may soon get extinct. Reproduction also helps to create recombination by the process of meiosis. Reproduction is the source of genetic recombination. It is vital process in the evolution of life/ organisms.
2. Which is a better mode of reproduction: sexual or asexual? Why?
Answer : Sexual reproduction is a better mode of reproduction as compared to asexual reproduction because in asexual reproduction there is no fusion of male and female gamete, but it involves in only single parent gives rise another offspring which is exactly similar to the parent because it passes its genetic material to the offspring. So, there are no chances of variations in progeny.
While in sexual reproduction it involves in the fusion of male and female gamets and leads to the formation of new variants by the combination of the DNA from two different individuals gametes. In sexual reproduction Variations are introduced during meiosis which is an important tool for evolution. It adds survival value to particular species.
3. Why is the offspring formed by asexual reproduction referred to as clone?
Answer : In asexual reproduction there is no fusion of male and female gamete, but it involves in only single parent gives rise another offspring which is exactly similar to the parent because it passes its genetic material to the offspring.
The offspring formed by asexual reproduction referred to as clone because the offspring which is form by single parent is morphologically and genetically similar to there parent.
4. Offspring formed due to sexual reproduction have better chances of
survival. Why? Is this statement always true?
Answer : As we know Sexual reproduction involves the fusion of the male and the female gamete. This fusion allows the formation of new variants by the combination of the two parental DNA. The variations allow the individuals to adapt under varied environmental conditions for better chances of survival. And As per the theory of natural selection given by charles darwin nature selects only useful variations and thus ensures better chances of survival. Hence, we can said that offspring formed due to sexual reproduction have better chances of survival.
5. How does the progeny formed from asexual reproduction differ from
those formed by sexual reproduction?
Answer : Asexual reproduction does not involve the fusion of the male and the female gamete. Organisms undergoing this kind of reproduction produce offspring’s that are morphologically and genetically identical to them and we Offsprings thus produced do not show variations and we called them as clones.
Sexual reproduction involves the fusion of the male and the female gamete of two individuals, typically one of each sex. Organisms undergoing this kind of reproduction produce offspring’s that are not identical to them.
6. Distinguish between asexual and sexual reproduction. Why is vegetative
reproduction also considered as a type of asexual reproduction?
- sexual reproduction involves the fusion of the male and female gamete while asexual reproduction does not involves the fusion of the male and the female gamete.
- In sexual reproduction gametes are produce in asexual reproduction gametes are not produce.
- sexual reproduction requires two (usually) different individuals of same species. In asexual reproduction requires only one individual.
- In sexual reproduction individuals produced are not identical to their parents and show variations from each other and also, from their parents. While in asexual reproduction individuals produced are identical to the parent and are hence, called clones.
- In sexual reproduction Most animals reproduce sexually. Both sexual and asexual modes of reproduction are found in plants. Asexual modes of reproduction are common in organisms having simple organizations such as algae and fungi.
- sexual reproduction is a slow process. Asexual reproduction is a fast process.
- In sexual reproduction Meiosis and mitosis both takes place. In asexual reproduction Only mitosis takes place.
And just because in vegetative propagation stem, leaf. etc give rise to new plant without the involvement of two parents it’s considered as asexual reproduction.
7. What is vegetative propagation? Give two suitable examples.
Answer : Vegetative propagation is a type of asexual reproduction Plants reproduce asexually through their vegetative parts. Hence, the new plants formed are genetically identical to their parents. progeny produced by any vegetative propagule are rhizome, tubers, suckers etc.
A tuber of potato has ‘eyes’ on it. Each eye is capable of producing a new plant of potato. Cutting of stem of rose is used to produce a new plant of rose. These are good examples of vegetative propagation.
(a) Juvenile phase
Answer : The period of growth before onset of sexual maturity is called juvenile phase. This phase is called vegetative phase in plants.
(b) Reproductive phase
Answer: The phase of life when an organism is capable of sexual reproduction is called reproductive phase.
(c) Senescent phase.
Answer: It is the period of ageing in an organism.
9. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
Answer : Higher organisms undergo sexual reproduction it is much complex process in nature because the organisms are produced by the fusion of two different male and female gametes and therefore show genetic variations. These variations allow the individual to cope with the different environmental factors, and diffrent conditions. Variations also lead to the evolution of better organisms by sexual reproduction and hence usually have higher rates of survival as compare to asexual reproduction.
10. Explain why meiosis and gametogenesis are always interlinked?
Answer : Gametogenesis is process of gamete formation. Meiosis is the process of cell division that reduces the number of chromosomes.
In order to produce gamete which are haploid in nature the cells have to undergo meiosis. In meiosis chromosome number is reduced to half in the two successive meiotic cycles which involves in two successive nuclear and cell divisions with a single cycle of DNA replication to form the haploid gametes. Thus meiosis and gametogenesis are always interlinked.
11. Identify each part in a flowering plant and write whether it is haploid
(n) or diploid (2n).
Answer : It is the female part of the plant which has ovules. It is a diploid (2n) structure.
Answer : It is the male part of the flower and contains pollen grain. It is a diploid structure (2n).
Answer : Ovules are known as egg. It is female gamete present in the ovary. It is a haploid structure (n).
Answer : Pollens are male gamete. It is a haploid (n) gamete.
(e) Male gamete
Answer : Pollens are male gamete. It is a haploid (n) gamete.
Answer : It is formed by the fusion of male and female gamete. It is a diploid structure (2n).
12. Define external fertilisation. Mention its disadvantages.
Answer : External fertilization is a mode of reproduction in which fertilization occurs outside of the female’s body is called external fertilization.
Or External fertilization is the process in which the fusion of male and female gamete takes place outside the female body in an external medium, generally water. For example, fish, frog, starfish etc. undergo external fertilization.
Some disadvantages of external fertilization
- A large number of gametes are left unfertilized and wasted.
- Predators and other environmental hazards reduce the chances of fertilization.
- The sperm might not necessarily come in contact with the eggs.
- The gametes or the zygote dessicate.
13. Differentiate between a zoospore and a zygote.
- A zoospore is a motile asexual spore that utilizes the flagella for movement. A zygote is a non-motile diploid cell formed as a result of fertilization.
- Zoospore It is an asexual reproductive structure. While zygote It is formed as a result of sexual reproduction.
- Zoospore may be haploid or diploid while zygote is always diploid.
14. Differentiate between gametogenesis from embryogenesis.
- Gametogenesis is the formation of male and female gamete While embryogenesis is the formation of embryo from the zygote
- Gametes are the haploid cells. Embryo is a diploid cell.
- In gametogenesis both mitotic & meiotic cell division take place. In embryogenesis Only mitotic division occur.
- Gametogenesis is of two types: a) spermatogenesis (formation of male gametes). b) Oogenesis (formation of female gametes). While embryogenesis It involves cell division to increase the number of cells, cell growth and cell differentiation.
15. Describe the post-fertilisation changes in a flower.
Answer : Post-fertilization is a series of events that takes place after fertilization to develop a seed from an ovule and a fruit from an ovary. The major events in post-fertilisation of a flower include development of endosperm and embryo, maturation of ovules into seeds and ovary into fruit.
Post-fertilization changes in flower are:
- The sepal, petal and stamen of the flower withers and fall off only pistil remains attached to the plant.
- The ovules develop into the seed.
- The zygote develops into the embryo.
- The ovary develops into the fruit.
- The ovary wall is transformed into a protective layer called the pericarp which covers the fruit.
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.
Answer : A flower which bears both male (androecium) and female (gynoecium) sex organs is called as bisexual flower. Examples : Lily, Rose, Sunflower,Tulip, Daffodil, Mustard, Brinjal,Hibiscus, Tomato, Long bean, Chille,Country bean, mango.
Five common examples of bisexual flowers:
- Water lily ( Nymphaea odorata)
- Rose (Rosa multiflora )
- Hibiscus (Hibiscus Rosa-sinensis )
- Mustard ( Brassica nigra)
- Petunia (Petunia hybrida).
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?
- Cucurbit has unisexual flower which carry either male reproductive organ or female reproductive organs.
- Flowers with male reproductive organs (stamen) are termed as staminate flowers. while flowers carrying only carpel, are termed as pistillate flowers.
- Other examples of unisexual flowers are papaya, watermelon, maize etc.
18. Why are offspring of oviparous animals at a greater risk as compared
to offspring of viviparous animals?
Answer : Oviparous animals lay their eggs outside their body. As a result the egg of these animals are under continuous threat from various environmental factors. Other the other hand, in viviparous animals, the development of the egg takes place inside the body of the female. Hence, the off-spring of an egg laying or oviparous animal is at greater risk as compared to the off-spring of a viviparous animal, which gives birth to its young ones.