NCERT Solutions for Class 12 Biology Chapter 1 Reproduction in Organisms

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NCERT Solutions for Class 12 Biology Chapter 1 Reproduction in Organisms, NCERT Solutions for Class 12 Biology Chapter 1 pdf, class 12 biology chapter 1 questions and answers


ncert solutions for class 12 biology Chapter 1

Class12th 
Chapter No01
ProvidingQuestions And Answers, Notes & Numericals PDF
Chapter NameReproduction in Organisms 
BoardCBSE
Book NCERT
SubjectBiology
Medium English
Study MaterialsFree VVI Study Materials are Available
Download PDF Class 12 Biology Ncert Solutions PDF CH-1

Reproduction in Organisms 


1. The time period for which an organism is functional, from birth to natural death, is called its lifespan. During its lifespan, an organism grows, develops, attains maturity and gives rise to new individuals of the same type. This process of making new individuals is called reproduction.

2. Reproduction is a biological process in which an organism gives rise to young ones (offspring) similar to itself. It involves passing the genetic material to the next generation and ensures the continuation of species.

3. Reproduction is of two types

(i) Asexual reproduction occurs in organisms of relatively simple organisations, e.g., Fungi, algae and a few Liga y invertebrates.

It is characterised by

  • The involvement of only one parent.
  • No formation or fusion of gametes.
  • Offspring are identical to the parent cell and also to each other. Genetically and morphologically identical offspring. produced by the parent through asexual reproduction are referred to as clones.

(ii) Sexual reproduction involves the fusion of male and female gametes.

4. Asexual Reproduction in Protistan and Monera 

(i) Fission Parent cell makes one or more copies of its DNA by mitosis and then undergoes cytokinesis (cytoplasmic division) to make two or more daughter cells. 

(ii) Binary fission Parent cell divides into two halves and each half grows into a new individual, e.g., Bacteria, Amoeba.

(iii) Multiple fission Parent cell makes multiple copies of DNA and then many daughter cells, e.g., Amoeba, under unfavourable conditions makes multiple spores inside the parent cell and makes a protective covering around the parent body. Only under favourable conditions, these spores will come out and grow into new amoebae.

(iv) Budding Nucleus makes a copy and the cell divides unequally to produce a small projection (bud) on a side. This eventually gets separated and grows as a new yeast cell, e.g., Yeast.

5. Asexual Reproductive Structures in Fungi and Simple Plants (algae) 

(i) Zoospores are flagellated, motile spores produced inside a parent cell called zoosporangia. The parent nucleus divides repeatedly to form many daughter nuclei that later develop a cell membrane around them. The parent cell bursts open to release zoospores in the aquatic habitat, eg., In fungi, Phycomycetes and algae Chlamydomonas.

(ii) Conidia are non-motile spores produced singly or in chains by a constriction at the tip or laterally on special hyphal branches called conidiophores. These are exogenous spores that disperse by wind and germinate in suitable habitats by producing germ tubes, e.g., Penicillium.

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(iii) Buds form as a small outgrowth on the body of the parent containing a copy of the parental nucleus. It may split away from the parent body and grow up as a new individual, eg., Hydra.

(iv) Gemmules are endogenous buds that are simply a copy of the parent nucleus, with a small amount of cytoplasm and a protective covering around them. During favourable conditions, each bud comes out and grows as an individual, e.g., Freshwater sponges.

6. Sexual Reproduction Generally Involves

  • Two prospective parents. 
  • Gamete formation by meiosis.
  • Fusion of gametes (fertilisation).
  • Genetic and morphological variation of offspring from the parents.

Gamete fusion leads to the formation of the zygote which is the first cell of the new individual. Most of the higher animals and plants reproduce sexually.

7. Life Cycle of an Organism Involves Three Phases 

  • Juvenile phase The stage of growth and maturity in life before reproducing sexually.
  • Reproductive phase The stage where one can reproduce sexually and is able to give birth to offspring. 
  • Senescent phase The end of the reproductive phase till death. If refers to old age. 

8. Events of Sexual Reproduction are

  • Pre-fertilisation Gametogenesis or formation of gametes.
  • Fertilisation Syngamy or fusion of gametes to form zygote. 
  • Post-fertilisation Embryogenesis through division and differentiation of the zygote.

9. Bisexual or Unisexual Organisms 

  • Bisexual animals have both male and female reproductive organs in the same individual, e.g., earthworms, tapeworms, leeches, sponges, etc.  They are also called hermaphrodites. 
  • Unisexual animals have either male or female reproductive organs in one individual, e.g., humans. 
  • Bisexual plants have both male (stamen) and female (pistil) parts on the same flower, e.g., in roses, lilies, jasmine, sweet peas, etc. They are also called homothallic or monoecious. 
  • Unisexual plants have flowers with only male or only female parts. Unisexual male flowers will have stamens and unisexual female flowers will have only pistils. They are also called heterothallic or dioecious.

10. Gametes These are the cells that make sexual reproduction possible. The male gamete is known as a sperm antherozoid. The female gamete is known as the egg or ovum.

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(i) If male and female gametes are similar in appearance, they are called homogametes or isogametes. 

(ii) If male and female gametes are different in appearance, they are called heterogametes.

(iii) In most of the sexually reproducing species including humans, gametes look drastically different. The female gamete is much bigger in size and non-motile, whereas the male gamete is very tiny and is generally motile.



11. Cell Division During Gametogenesis

Gametes are produced by meiotic cell division of the diploid mother gamete cell (meiocyte 2n) to reduce the chromosome number to half. This means that each gamete cell has only one set of chromosomes (n) which are not paired any more.

(i) In haploid organisms, gametes are produced by mitosis as the chromosomes already exist as a single, non-paired set.

(ii) Transfer of male gamete is an important event in sexual reproduction and different ways are used by the organisms to accomplish this.

(iii) Biotic ways of gamete transfer make use of animals, whereas abiotic ways are those that employ water or wind.

12. Fusion between male and female gametes is called syngamy or fertilisation. This results in the formation of a diploid zygote that will further develop and differentiate to form the new individual. 

13. Types of Fertilisation 

(i) Syngamy may occur inside the female body (internal fertilisation) that ensures safe development or outside the female body (external fertilisation). This makes the embryo vulnerable to predators or changing environmental conditions.

(ii) Parthenogenesis is a phenomenon, where a female gamete undergoes development to form new organisms without fertilisation with a male gamete. It is a way by which a female alone can produce offspring without the need for a male. In complex animals and plants, it’s a way to clone themselves. Its disadvantage is low genetic diversity and susceptibility to harmful mutations that can persist through generations. 

14. Embryogenesis is the process, where the zygote divides repeatedly and differentiates to form an embryo, eventually resulting in a new organism. 

Changes After Fertilisation- In flowering plants, non ve althovenop 

(i) The ovary develops into a fruit.

(ii) ovules mature into seeds. 

The embryo is present inside the mature seed that will germinate to give rise to the new plant, under proper environmental conditions. 

15. During embryogenesis, the zygote undergoes cell division (mitosis) and cell differentiation.

16. Oviparous animals lay eggs, e.g., reptiles and birds. Eggs are covered by a hard calcareous shell. 

17. Viviparous animals give birth to young, e.g., the majority of mammals. In flowering plants, the zygote is formed inside the ovule. The zygote develops into the embryo and the ovules develop into the seed.


class 12 biology chapter 1 Questions and answers


Question 1. Why is reproduction essential for organisms?

Answer Reproduction is essential because it enables the continuity of the species generation after generation.

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

Ans- Sexual reproduction is considered a better mode as it gives rise to genetic variation.

(1) This genetic variation may confer some survival advantages on the offspring, under stressful environmental conditions. (ii) It also contributes to evolution.

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

Answer Offsprings produced through asexual reproduction are called clones as they are genetically and morphologically similar to the parent that produced them and to each other.

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

Answer Offsprings formed through sexual reproduction inherit genetic material from two genetically different parents and that too, with certain recombination. This genetic variation may confer survival advantages on the offspring. This statement is true in the majority of cases provided embryonic safety and care is ensured.

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

Answer The progeny formed by asexual reproduction has a genetic and morphological resemblance to the parent generation. In the case of sexual reproduction, progeny formed has genetic and morphological variations due to genetic recombination. It takes place during gamete formation and then again during fertilisation. 

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

Answer Differences between sexual and asexual reproduction are-

Sexual ReproductionAsexual Reproduction
1. Two parents of the opposite sex are involved in the process. Only one parent is involved.
2. Gamete formation and fertilisation always take place.Gamete formation and fertilisation not required.
3. Offsprings show genetic and General gamete formation and morphologic variation from the parents.Offsprings are genetically and morphologically the same as the parents!
4. The process involves both melodic and mitotic cell division.Only mitotic cell division takes place.

Vegetative reproduction is also considered as a form of asexual reproduction since, the formation of the offspring from vegetative propagules doesn’t involve

(i) two parents.

(ii) formation or fusion of gametes.

(iii) genetic variation.

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

Answer- The process wherein the vegetative parts of plants give rise to new organisms is called vegetative reproduction.

Examples

(i) Eyes (buds) on the potato tuber. 

(ii) Adventitious buds on the leaves of Bryophyllum are capable of giving rise to new plants.

Question 8. Define 

(a) Juvenile phase

(b) Reproductive phase)

(c) Senescent phase

Answer

(a) Juvenile phase is a period of growth from birth up to a stage, where the organism will start undergoing changes leading to reproductive maturity.  No reproduction can take place in this phase. In plants, it is called the vegetative stage.

(b) Reproductive phase is a period during which an organism is sexually mature and can produce gametes that are used to give rise to new organisms. In higher plants, this phase is marked by the production of flowers. 

(c) Senescent phase is the period from the end of the reproductive phase till death. It is marked by changes, e.g., slower metabolism, breakdown of proteins, immobilisation of nutrients, etc, In plants, leaves become yellow and fall off.

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

Answer Higher organisms have resorted to sexual reproduction despite its complexity, as it allows for

(i) genetic variability through new genetic combinations in gametes.

(ii) improvement of genetic makeup in the offspring. Both these factors may confer survival advantages on species that contribute to evolution.

Question 10. Explain why meiosis and gametogenesis are always interlinked.

Answer – Meiosis and gametogenesis are always interlinked as haploid gametes can be produced only through meiosis, the type of cell division that reduces the number of chromosomes to half, by separating the pairs into different cells.

The resulting gamete cells will have only one set of chromosomes, not a paired set.

The other type of cell division (mitosis) does not separate the pairs during cell division and hence, the chromosome number in daughter cells does not reduce to half.

Question 11. Identify each part in a flowering plant and write whether it is haploid (n) or diploid (2n).

  • Ovary
  • Anther
  • Pollen
  • Egg
  • Male gameteborgail
  • Zygote

Answer

(a) OvaryDiploid
(b) AntherDiploid
(c) Ovum/eggHaploid
(d) PollenHaploid
(e) Male gameteHaploid

Question 12. Define external fertilisation. Mention its disadvantages.

Answer- External fertilisation is defined as the type of syngamy that occurs outside the female body, in an external medium, e.g., water. that Example Bony fish and frogs. The major disadvantage in this type of fertilisation is that the offspring are

(i) exposed to environmental pressures.

(ii) extremely vulnerable to predators.

This threatens their survival till adulthood.

Question 13. Differentiate between a zoospore and a zygote.

Answer

S.N.ZoosporeZygote
1It is an asexual reproductive structure, usually flagellated and motile.It is single cell formed as a result of fertilization in sexual reproduction. it is non-flagellated and non-motile. 
2It can be haploid or diploid. It is diploid.
3It germinates to directly give rise to new plants.It develops into an embryo that further differentiates to form a new individual.

Question 14. Differentiate between gametogenesis from embryogenesis.

Answer

S.N. GametogenesisEmbryogenesis
1It is the process of generation of haploid gametes.It is the process of the formation of the embryo.
2mitotic cell divisions are required to increase the number of cells is required.Repeated Meiotic cell division of mother gamete required to increase the number of cells of the zygote.
33. Further transformation may be required to make male gametes motile.Further cell differentiation is required for the formation of specialised tissues and organs.

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

Answer

Post-fertilisation changes in a flower are as follows:

(i) The sepal, petal and stamen of the flower withers and fall off, but the pistil remains attached to the plant. 

(ii) The zygote develops into the embryo and ovules develop into the seeds.

(ii) Ovary develops into the fruit and a thick protective wall called a pericarp, covers the fruit. dem of go for a to novib yard 

Question 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 bisexual flower has both the male (stamen) and female (pistil) parts.

Scientific Name Common Name
Cucumis sativus- Cucumber
Cocos nucifera- Coconut
Rosa alba- Rose
Lathyrus odoratus- Sweet pea
Hibiscus rosa-sinensis- Gurthal, China flower

Question 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?

Answer Cucurbit plants show two different types of flowers:

(1) The staminate flowers show the presence of only stamens and no carpels. 

(ii) The pistillate flowers show the presence of only female parts, the pistils or carpels and no stamens.

Examples of plants that bear unisexual flowers: are pumpkin, papaya, and date palm.

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

Answer: The offspring of oviparous animals are at a greater risk because they develop outside the female body and are exposed to varying environmental conditions and predator threats. Whereas in viviparous animals, young ones develop inside the female body. which ensures proper embryonic care and protection.


Very Short Answer Type Questions


Question 1. Mention two/three inherent characteristics of Amoeba and yeast that enable them to reproduce asexually.

Answer

(i) Unicellularity 

(ii) Simple body organisation

(iii) Easy division of the parent cell to make two daughter cells.

Question 2. Rearrange the following events of sexual reproduction in the sequence in which they occur in a flowering plant Embryogenesis, fertilisation, gametogenesis, pollination

Answer- Gametogenesis, pollination, fertilisation, embryogenesis correct sequence.

Gametes need to be formed first of all, pollination ensures their transfer so that fertilisation can take place. The fertilised zygote divides and differentiates during embryogenesis.

Question 3. Is the presence of a large number of chromosomes in an organism a hindrance to sexual reproduction? Justify your answer by giving suitable reasons.

Answer No, the presence of a large number of chromosomes in an organism is not a hindrance to sexual reproduction. Ophioglossum (a fern) has chromosome number 1260, still, it can reproduce sexually.

In higher organisms, the chromosomes are present in a compartment called the nucleus, within the cell. Whether the number is small or large, the chromosomes are duplicated and then segregated inside this compartment, during cell division. 

The basis of sexual reproduction is the generation of haploid gametes and higher organisms have evolved meiotic cell division as the process of making haploid gametes.

Question 4. Between an annual and a perennial plant, has a shorter juvenile phase? Give one reason.

Answer: An annual plant will have a shorter juvenile phase because of its shorter lifespan. In a lifespan, the organism has to grow and develop (the juvenile phase) and then mature sexually and enter into the reproductive phase before it undergoes senescence followed by death. 

So, an annual plant has limited time to complete all these phases as compared to the perennial plant which has a relatively long lifespan.

Question 5. Is there a relationship between the size of an organism and its lifespan? Give two examples in support of your answer. 

Answer No, the size of the organism is not related to its lifespan. 

For example

(i) Parrots (lifespan 140 yrs) and crows (lifespan 15 yrs) are roughly of the same size but have drastically different lifespans.

(ii) Tortoise (lifespan is 100-150 yrs) is much smaller in size as compared to a horse which is much bigger but its lifespan is much shorter (25-30 yrs).

Question 6. Give reasons as to why cell division cannot be a type of reproduction in multicellular organisms. 

Answer: A simple cell division cannot be the type of reproduction in multicellular organisms because of their complex organisation. There is a specific division of labour in higher organisms to run all the

functions smoothly and harmoniously so only specific types of cells are assigned the job of reproduction.

Question 7. Which of the following are monoecious and dioecious organisms?

  • Earthworm
  • Chara
  • Marchantia 
  • Cockroach

Answer

(a) Earthworm-monoecious (one individual has both male and female sex organs)
(b) Chara-monoecious(one individual has both male and female sex organs)
(c) Marchantia dioecious(one individual has either male or female sex organs)
(d) Cockroach-dioecious (one individual has either male or female sex organs)

Short Answer Type Questions


Question 1. In haploid organisms that undergo sexual reproduction, name the stage in the life cycle when meiosis occurs. Give reasons for your answer.

Answer -Meiosis can take place only in a diploid stage (post-zygotic stage) only.

If the organism is haploid, no meiosis is required for gametogenesis.

Question 2. Is it possible to consider vegetative propagation observed in certain plants like Bryophyllum, water hyacinth, ginger, etc., as a type of asexual reproduction? Give two/three reasons. 

Answer Vegetative propagation can be considered as a type of asexual reproduction as it involves the production of new individuals.

(i) By a single parent.

(ii) Without the formation and fusion of gametes. 

(iii) Without resulting in any genetic or morphologic variation. 

Question 3. In a developing embryo, analyse the consequences, if cell divisions are not followed by cell differentiation. 

Answer Cell divisions only increase the number of cells in the developing embryo. But, to help these groups of cells get organised into specialised tissues and organs requires cell differentiation.

So, if cell differentiation doesn’t occur, a new organism cannot develop. It will only remain as a mass of cells.

Question 4. Suggest a possible explanation why the seeds in a pea pod are arranged in a row, whereas those in a tomato are scattered in the juicy pulp.

Answer: Pea pods develop from the monocarpellary superior ovary. Pea seeds are non-albuminous and have no residual endosperm as it is completely consumed during embryo development. Whereas in tomatoes, the fruit is fleshy and seeds are scattered in its juicy pulp.


Long Answer Type Questions


Question 1. Do all the gametes formed from a parent organism have the same genetic composition (identical DNA copies of the parental genome)? Analyse the situation with the background of gametogenesis and provide or give a suitable explanation.

Answer No, all the gametes formed from a parent organism do not have the same genetic composition for two major reasons:

(i) Crossing over

(ii) Independent assortment of chromosome pairs.

(i) Crossing Over

  • Gametes form through the meiotic cell division of a diploid mother gamete cell.
  • During meiotic division, each pair of chromosomes (pair is one from the father and one from the mother) may cross over and exchange a segment.
  • The exchange may occur in one chromatid and not the other.

At the end of cell division, four haploids (having only one of the pairs) gametes formed will have the following genetic composition. 

  • All black chromosomes.
  • Black chromosome with the grey segment.
  • Grey chromosome with the black segment.
  • All grey chromosomes.

Hence, each gamete has a different genetic composition. 

(1) Independent assortment

All chromosomal pairs assort themselves independently of the others. This means that gametes can have the following genetic compositions:

  • All are mother chromosomes. 
  • All are father chromosomes.
  • Combination of mother and father chromosomes. 

The last one is the most common outcome and it has innumerable combinations possibilities. The higher the chromosome number, the greater the permutations and combinations. That is why all gametes in a parent can never be the same. Additionally, in males, 50% of the gametes will end up having an X-chromosome and 50% will end up having a Y-chromosome.  

Therefore, each gamete will have a different set of chromosomes or a different genetic composition.


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