CBSE Class 12 - Biology - Chapter 1: Reproduction in Organisms (Questions and Answers)
Q1: Define life span.
Answer: Life span is the period from birth to the natural death of an organism.
Q2: What is the life span of the following organisms?
(a) Mayfly
(b) Human Being
(c) Parrot
(d) Rice Plant
(e) Sequoia Trees
(f) Butterfly
Answer:
(a) Mayfly - 1 day
(b) Human Being - 100 years
(c) Parrot - 140 years
(d) Rice Plant - 3 to 4 months
(e) Sequoia Trees - 3000 to 4000 years
(f) Butterfly - 1 to 2 weeks
Q2: Define reproduction.
Answer: Reproduction is a process in which an organism gives rise to young ones (offspring) through which the cycle of life is maintained.
Q3: Why do we say there is no natural death in single-celled organisms? Do they die?
Answer: No individual is immortal but unicellular organisms are said to be biologically immortal. This is because they do not grow old and die instead. They reproduce asexually and make offspring. Thus they lose their parental identity and become daughter cells.
Single-celled organisms also die due to harsh condition. e.g. huge changes in temperature.
Q4: What is ageing?
Answer: It is progressive deterioration in the body of a living being is called ageing.
Q5(NCERT): Why is reproduction essential for organisms?
Answer: Reproduction, as the characteristic feature of all living organisms, is the ability to produce a young one similar to itself. It is a fundamental feature of all living organisms and ensures the continuance of various species on the earth. In its absence, the species will not be able to exist for a long time and may soon get extinct from the earth.
Q6: Name the three modes of reproduction.
Answer: Living organisms reproduce by three different modes of reproduction:
(i) Asexual reproduction
(ii) Vegetative reproduction
(iii) Sexual reproduction
Q7: What is an asexual reproduction?
Answer: Asexual reproduction is a process of producing individuals whose genes come from single-parent only with the help of mitotic division.
Q8: What is vegetative propagation?
Answer: Vegetative propagation occurs in plants. New plants can arise from the vegetative parts of a plant. In plants, the units of vegetative propagation like a runner, rhizome, sucker, tuber, bulb are called vegetative propagules. All are capable of producing new offsprings.
Q9: Define sexual reproduction.
Answer: Sexual reproduction involves the fusion of male and female gametes (sex cells). The fusion of gametes leads to the formation of a zygote that develops into an embryo.
Q10: 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.
It is because of the following reasons:
① Asexual reproduction allows very little or no variation throughout their generation, due to which the individuals produced are exact copies of their parents and themselves.
② Sexual reproduction allows the formation of new variants by combining the DNA of two different parents. The offspring produced from the fusion are not identical to their parents and to themselves. Due to such variation, the individuals constantly adapt to the changing and challenging environment for their survival and leads to the evolution of better-suited organisms which ensures the greater survival of a species.
Q11: Why is the offspring formed by asexual reproduction referred to as clone?
Answer: In asexual reproduction, offsprings produced are not only identical to one another but also exact copies of their parent. Such offsprings are genetically identical. The term clone is referred to describe such morphologically and genetically similar individuals.
Q12: In asexual reproduction, what is the unit or reproduction?
Answer: Unit of reproduction in the asexual mode of reproduction can be:
i. whole parent body, or
ii. a bud, or
iii a body fragment, or
iv. a single somatic cell.
Q13: Give examples of organisms that reproduce asexually?
Answer: Examples are:
Protistan protozoans e.g. Amoeba, Paramecium,
Sponges (Scypha), Coelenterates (Hydra, Tubularia etc.)
Some Flatworms (Planaria) etc.
Q14: Offspring formed due to sexual reproduction have better chances of survival, why?
Answer:
① Sexual reproduction introduces variations in offspring, that has evolutionary significance.
② It helps the offsprings to adjust according to changes in the environment, thus increases chances of survival.
③ As a result of new character combination, better offsprings are produced thus result in improved varieties.
Q15: Is there any disadvantage of sexual reproduction? If yes, what is that?
Answer: It is not always necessary that the offspring produced due to sexual reproduction has better chances of survival. It may lead to genetic disorders in offsprings. e.g., haemophilia.
Q16: Name an organism where cell division is itself a mode of reproduction.
Answer: Amoeba (unicellular)
Q17: What is vegetative propagation? Give two suitable examples.
Answer: The process of multiplications, in which parts or fragments of the plant act as reproductive unit or propagule to form new individuals is called vegetative propagation.
Examples are:
(i) Buds (eyes) of potato
(ii) Rhizome of Ginger
Q18: How does the progeny formed from asexual reproduction are different from those formed by sexual reproduction?
Answer: In asexual reproduction, offsprings are produced by a single parent. It has no involvement in gamete formation and fusion of gametes. It involves only mitotic cell divisions and thus offsprings produced to resemble the parent genetically and morphologically. Offsprings do not show any variation.
In sexual reproduction, a fusion of gametes occurs and fertilisation and meiosis are essential events. It involves meiotic cell divisions producing haploid nuclei. Here, offsprings differ from parents due to the appearance of variations.
Q19: Why is vegetative reproduction also considered a type of asexual reproduction?
Answer: Vegetative reproduction is considered as a type of asexual reproduction because here also characters of parent plants are preserved. Here propagation takes place whom a new individual arises from any vegetative part of the parent (root, stem, leaf etc.), and possesses exactly the same characteristics of the parent plant from whit it was sampled. Moreover, the formation of new individuals does not involve two parents, the process involved is asexual.
Q20: State the difference between meiocyte and gamete with respect to chromosome number.
Answer: In meiocyte, the chromosome number is diploid (Zn) while in gamete, haploid (n)
Q21: Although potato tuber is an underground part, it is considered as a stem. Give two reasons.
Answer: Potato tuber is considered as a stem because of the following reasons:
(i) The tuber has nodes and internodes,
(ii) Leafy shoots appear from the nodes.
Q22: Define
(a) Juvenile phase,
(b) Reproductive phase,
(c) Senescent phase.
Answer:
(a) Juvenile phase:
It is the period of growth in an individual organism after its birth and before it reaches reproductive maturity.
(b) Reproductive phase:
It is the period when an individual organism reproduces sexually.
(c) Senescent phase:
It is the period when an organism grows old and loses the ability to reproduce.
Q23: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
Answer: It is because sexual reproduction induces genetic variability (adaptations). This makes the offsprings better equipped for the struggle of existence. These variations allow the individual to cope with various environmental conditions and thus, make the organism better suited for the environment. Variations also lead to the evolution of better organisms and therefore, provide better chances of survival.
Q24: Explain why meiosis and gametogenesis are always interlinked?
Answer: Meiosis is a process of reductional division in which the amount of genetic material is reduced. Gametogenesis is the process of the formation of gametes. Gametes produced by organisms are haploids (containing only one set of chromosomes), while the body of an organism is diploid. Therefore, for producing haploid gametes (gametogenesis), the germ cells of an organism undergo meiosis. During the process, the meiocytes of an organism undergo two successive nuclear and cell divisions with a single cycle of DNA replication to form the haploid gametes.
Q25: 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 ———————————
Answer:
(a) Ovary Diploid (2n)
(b) Anther Diploid (2n)
(c) Egg Haploid (n)
(d) Pollen Haploid (n)
(e) Male gamete Haploid (n)
(f) Zygote Diploid (2n)
Q26: Define external fertilisation. Mention its disadvantages.
Answer: When fusion of the gametes takes place outside the body of the organisms in an external medium, generally water, it is called external fertilisation or external syngamy.
External fertilisation is commonly found in many aquatic animals like fishes, amphibians, majority of algae. During the process, parents release eggs and sperms in the surrounding water, then fertilisation and development of offspring occur externally.
Disadvantages of external fertilisation
(i) It occurs only in an aquatic medium.
(ii) A chance factor is involved requiring synchronous release of gametes nearby and absence of turbulence of the water.
(iii) There is no protection for young ones which results in a low rate of survival of the progenies and are vulnerable to a number of predators.
(iv) Eggs have fewer chances of fertilisation. This can lead to the wastage of a large number of eggs produced during the process.
Q27: What is a bisexual flower?
Answer: Bisexual or perfect flowers have both male (androecium) and female (gynoecium) reproductive structures, including stamens and an ovary. Flowers that contain both androecium and gynoecium are called hermaphroditic. Examples. Pea, Mustard, Petunia tomato. etc
Q28: Differentiate between a zoospore and a zygote.
Answer:
➊ Zoospores are the microscopic, flagellated (motile) special asexual reproductive structures found in certain members of the kingdom fungi and simple plants like algae whereas zygote is a diploid cell formed by the fusion of male and female gametes. The zygote is usually non flagellated.
➋ Zoospores are the structures that give rise to new organism whereas zygote is formed after fertilisation which develops into a mature organism.
➌ Zoospores are haploid while zygote is diploid.
Q29: Differentiate between gametogenesis from embryogenesis.
Answer:
| Gametogenesis | Embryogenesis |
|---|---|
| It is the process of the formation of haploid male and female gametes from diploid meiocytes through the process of meiosis. | It is the process of the development of the embryo from the repeated mitotic divisions of the diploid zygote. |
| Formation of gametes | Formation of embryo |
| Gametes - Haploid | Embryo - diploid |
| Cell division - Meiosis | Mitosis |
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