CBSE Class 12 Biology Reproduction In Organisms Notes Set A

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Reproduction In Organisms Class 12 Biology Revision Notes

Class 12 Biology students should refer to the following concepts and notes for Reproduction In Organisms in standard 12. These exam notes for Grade 12 Biology will be very useful for upcoming class tests and examinations and help you to score good marks

Reproduction In Organisms Notes Class 12 Biology




  Life Span 
  Period from birth till natural death.Every organism lives only for certain period of time Eg Elephant 60 -90 years, Fruit fly 4-5 weeks. Every organism live only for a certain period of time. 
• Reproduction –Producing young-ones of their kind, generation after generation. 

Types of reproduction: 
Asexual reproduction :single parent capable of producing offspring. Somatogenic reproduction 
Sexual reproduction :two parents are invovled in producing offspring. 

Modes of asexual reproduction 
 Binary fission: parent body divides into two halves, genetically identical to parent. 
• Amoeba: It is simple or irregular. Paramoecium: Transverse binary fission. Organisms considered immortal 
 Multiple fission: parent body divides into many daughter organisms : Plasmodium. 
 Budding: daughter organisms grow from small buds arising in parent body. Exogenous budding: out side the body eg. Hydra, Yeast. 

Budding in Yeast Endogenous budding : inside the body eg. Gemmule in sponge. 
 Conidia: non-motile, exogenous spores in chains eg. Fungi. 
 Zoospores: microscopic motile structures eg. Algae.
In plants : term vegetative reproduction 
frequently used instead of asexual reproduction, units of vegetative propagation called vegetative propagules.Eg runner, rhizome, sucker, tuber offset, bulb give rise to new plant 

  All organisms show remarkable similarity. Vast difference in their reproductive structure. Similar pattern or phases in their life cycles 

  Juvenile phase: The phase of growth before reproductive maturity. 
  Reproductive phase: Reproductive maturity. 
 Senescent phase: Phase between reproductive maturity and death. 

• The main events of sexual cycle are: 
i. Prefertilisation events: 

a.Gametogenesis : 
  The process of formation of male and female gametes by meiosis (cell-division). 
  Homogamete(Isogamete): - gametes similar eg. Algae 
  Heterogamete(an-isogamete): - morphologically dissimilar gamete ,male gamete (antherozoid or sperm) ,female gamete (egg or ovum) eg. Human. 

 Sexuality in organisms : In plants Bisexual term is used for Homothallic and Monoecious plants
Both male and female reproductive structures in same plant eg. Higher plants, cucurbits and coconut.

♦ Unisexual term used for Heterothallic and Dioecious plants Male and female reproductive-structure on different plants.
Flowering plants – male flower–staminate flower and female flower–pistillate flower eg. papaya and date-palm.

 Animals – Bisexual term is used for Hermaphrodite animals-eg. Earth-worm, Tape-worm, Leech, Sponge.

• Unisexual animals have male & female sexes in separate individuals-e.g. insects, frogs, humanbeings 

Cell division during gamete formation:

Haploid-parent (n) produces haploid gametes (n) by mitotic division, eg. Monera,fungi, algae and bryophytes.

Diploid parent (2n) produces haploid gametes(n) by meiosis division (possess only one set of chromosomes)and such specialized parent cell is called meiocyte or gamete mother cell.

CBSE Class 12 Biology - Reproduction In Organisms


b) Gamete transfer:- to facilitate fusion.
 Male gametes mostly motile and female non-motile, exception few fungi and in algae both gametes are motile in some cases
 Water medium for gamete transfer- in lower plants. Large number of male gametes produced to compensate loss
 Higher plants, pollen-grains are transferred by pollination.
 Fertilization: Fusion of male and female gametes diploid zygote.
 Parthenogenesis.-development into new organism without fertilisation eg. Rotifers, honey-bees, some lizard, bird(turkey).

Fertilization Two types- external and internal .
1. Release of large number of gametes into surounding medium
2. Large number of offsprings produced.

 Disadvantage- offspring vulnerable to predators, natural disasters.
 Internal fertilisation- fusion occurs inside female body eg. majority of plants and animals. Egg non-motile and formed inside female body. Male gamete motile, produced in large numbers to reach egg and fuse with it. In seed plants, non- motile male gamete carried to female gamete by pollen-tube. 

Post -fertilisation events- formation of zygote.
a. Zygote. One celled , diploid, vital link between two generations.

• External fertilization –zygote formed in external medium water eg. Frog,
• Internal fertilization –zygote formed inside the body eg. Humanbeings. Development of zygote depends on type of life cycle and environment. Some develop thick wall ( prevent damage and desiccation) & undergo period of rest eg. Algae, fungi.
• Haplontic life cycle- zygote (2n) divides by meiosis to form haploid (n) spores.
• Diplontic life-cycle- zygote (2n) divides mitotically, develops into embryo (2n).
• Oviparous animals lay eggs out-side the female body.Eggs can be fertilized/ unfertilized. Fertilized eggs covered which hard calcareous shell, laid in safe place in the environment. Unfertilised eggs laid in water. Example- fishes, frogs, reptiles, birds.
• Viviparous animals bear and rear the embryo inside female body, give birth to youngones.

Advantage- proper embryonic care, protection, survival chances of young-ones greater.Example- cows, whales, humanbeings
• Embryogenesis: development of embryo from zygote by cell division (mitosis) and cell differentiation.
♦ Cell- division increases the number of cells in the developing embryo Cell differentiation - groups of cells undergo certain modifications for the formation of different kinds of tissues and organs.

• In flowering plants- zygote formed inside ovule
♦ Changes occur in flowering plants:

CBSE Class 12 Biology - Reproduction In Organisms


• Parthenogenesis: Female gamete develops into new organism.
• Seedless fruits formed by parthenogenesis
• Clone: A group of individuals of the same species that are morphologically and genetically similar to each other & their parents
• Turion :Fleshy overwintering buds in aqatic plants help in perrenation Eg potomegaton, utricularia
• Bulbil :Fleshy buds that produce new plant EgAgave and Oxalis

Very Short Answer Type Questions

Question. Mention two inherent characteristics of Amoeba and yeast that enable them to reproduce asexually.
Answer : The characteristics that enable Amoeba and yeast to reproduce asexually, are as follows
(i) Unicellularity (ii) Simple body organisation
(iii) Uniparental condition

Question. Why do we refer to offspring formed by asexual method of reproduction as clones?
Answer : The reproduction is called asexual, when offspring is produced by a single parent with or without the involvement of gamete formation.
As a result, the offspring that are produced are not only similar to one another, but are also exact copies of their parent. Such a group of morphologically and genetically similar individuals is called clone.

Question. 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.

Question. Between an annual and a perennial plant, which one has a shorter juvenile phase? Give one reason.
Answer : In a lifespan, the organism has to grow and develop (the juvenile phase). After that the organism mature sexually and enter into the reproductive phase, before it undergoes
senescence followed by death.
Since, the entire life cycle of an annual plant is shorter and has to be completed in one growing season, its juvenile phase is shorter than that of a perennial plant.

Question. Rearrange the following events of sexual reproduction in the sequence in which they occur in a flowering plant embryogenesis, fertilisation, gametogenesis, pollination.
Answer : The correct sequence of events of sexual reproduction in a flowering plant are as follows gametogenesis, pollination, fertilisation, embryogenesis.

Question. The probability of fruit set in a self-pollinated bisexual flower of a plant is far greater than a dioecious plant. Explain.
Answer : The probability of fruit set in a self-pollinated bisexual flower of a plant is far greater than a dioecious plant.
In self-pollinated bisexual plants transfer of pollen to stigma of flowers is easier than the dioecious plants. It is so because the anther and stigma lie close to each other and pollination
is not effected even in the absence of pollinator. But in dioecious plants pollinator is necessary to bring about effective pollination as the anther and stigma lie away from each other.

Question. Is the presence of large number of chromosomes in an organism a hindrance to sexual reproduction? Justify your answer by giving suitable reasons.
Answer : No, presence of 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 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 generation of haploid gametes.

Question. Is there a relationship between the size of an organism and its life span?
Give two examples in support of your answer.
Answer : There is no relationship between the size and life span of an organism. e.g.,
(i) The mango tree has a shorter life span as compared to a peepal tree though both are of the same size.
(ii) The size of crow and parrot is almost same but the life span is 15 years and 150 years respectively.

Question. In the figure given below the plant bears two different types of flowers marked ‘A’ and ‘B’. Identify the types of flowers and state the type of pollination that will occur in them.
Answer : In the figure given below the plant bears folllowing two types of flowers A—Chasmogamous flower (the flowers remain open, exposing anthers and stigmas).
B—Cleistogamous flowers (the flowers remain closed, so that anthers and stigmas are
never exposed) following are the types of pollination that will occur in these flowers.
(i) Autogamy (within same flower)
(ii) Geitonogamy (different flowers of same plant)
(iii) Xenogamy (different plants)
It is a case of cleistogamy (a type of autogamy) in which some plants, like, Commelina bengalensis possess both chasmogamous and cleistogamous flowers.
In chasmogamous flowers, the flowers may undergo self-pollination or cross-pollination, while in cleistogamous flowers, the flowers undergo only self-pollination.

Question. Give reasons as to why cell division cannot be a type of reproduction in multicellular organisms.
Answer : In unicellular animals, cell division is the means of reproduction to increase their number while, in case of multicellular organisms, they have well developed reproductive organs
which help in reproduction.
Their whole body does not participate in reproduction like unicellular organisms.

Question. In the figure given below, mark the ovule and pericarp.
Answer : In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall off. The pistil however, remains attached to
the plant.
The zygote develops into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function.

Question. Why do gametes produced in large numbers in organisms exhibit external fertilisation?
In most aquatic organisms, such as a majority of algae and fishes as well as amphibians, syngamy (fusion of gametes in sexual reproduction) occurs in the external medium (water) , i.e., outside the body of the organism. This type of genetic fusion is called external fertilisation.
Answer : Organisms that take part in such process produce large number of gametes because (i) In external fertilisation, there is a great chance that the sperm and the eggs released by the organisms can be affected by factors present in the environment like dessication, predators, etc. So, to make up for the high fatality rate of the gametes, the organism produces a lot of gametes.
(ii) Producing large number of gametes increase the chance for at least some eggs and sperms to meet in the environment ensuring that atleast a stable number of offspring
are able to survive and carry on.

Question. Which of the following are monoecious and dioecious organisms?
(a) Earthworm ............. (b) Chara ..........
(c) Marchantia ........... (d) Cockroach ...........
Answer : (a) Earthworm- Monoecious animal
(b) Chara- Monoecious plant
(c) Marchantia-Dioecious plant
(d) Cockroach- Dioecious animal
Note In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual condition (male and female reproductive structures in the same plant) and heterothallic and dioecious are the terms used to describe unisexual condition (male and female reproductive structures on different plants).
But in animals, individuals are either male or female (unisexual) or possess both the reproductive organs (bisexual).

Question. What do the following parts of a flower develop into after fertilisation?
(a) Ovary ....... (b) Ovules ......
Answer : (a) Ovary ........... fruit (b) Ovules .......... seed
After fertilisation, the zygote develops into the embryo and the ovules develop into the seed. 
The ovary develops into the fruit which develops a thick, protective wall called pericarp.

Short Answer Type Questions

Question. 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) because the zygote is the only diploid cell in the life cycle of such organisms. This meiosis an case of haploid organisms will occur of the fertilisation.

Question. The number of taxa exhibiting asexual reproduction is drastically reduced in higher plants (angiosperms) and higher animals (vertebrates) as compared with lower groups of plants and animals. Analyse the possible reasons for this situation.
Answer : Higher plants (angiosperms) and higher animals (vertebrates) have a more complex structural organisation as compared to the lower groups of plants and animals. They have evolved very efficient mechanism of sexual reproduction. These groups have resorted to reproduction by the sexual method for the following reasons (i) To ensure healthy progeny (ii) To produce genetically varied offsprings that adapt to changes in environment and surviveal in all climatic conditions. (iii) It ensures the genetic recombination that results in variation which gives rise to evolution.

Question. Honeybees produce their young ones only by sexual reproduction. Inspite of this, in a colony of bees we find both haploid and diploid individuals. Name the haploid and diploid individuals in the colony and analyse the reasons behind their formation.
Answer : (i) Sterile diploid females as workers (ii) One fertile diploid female as queen (iii) Fertile haploid males as drones. In case of honeybees, both haploid and diploid individuals formad as a result of incomplete (cyclic) parthenogenesis, i.e., both sexual reproduction and parthenogenesis. Fertilised eggs (zygote) give rise to queen and workers (both are females) by the process of sexual reproduction and unfertilised eggs (ova) develop into drones (males) by the process of parthenogenesis.

Question. With which type of reproduction do we associate the reduction division? Analyse the reasons for it.
Answer : Reduction division (meiosis) is associated with sexual reproduction. The reasons are (i) Since, sexual reproduction involves the fusion of two types of gametes male and female), they must have haploid number of chromosomes. (ii) The cell (meiocyte) which gives rise to gametes often has diploid number of chromosomes and it is only by reducing the number by half that we can get haploid gametes. (iii) Reduction division also ensures maintenance of constancy of chromosome number from generation to generation.

Question. 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 : The formation of new plants from vegetative units (vegetative propagules) such as buds, tubers, rhizomes. etc., is called vegetative propagation (vegetative reproduction). It 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 morphological variations.

Question. ‘Fertilisation is not an obligatory event for fruit production in certain plants’. Explain the statement.
Answer : Fertilisation is not an obligatory event for fruit production in certain plants. Some fruits are developed from unfertilised ovary called parthenocarpic fruits. These are seedless fruits, such as pomegranate, grapes, etc. Flowers of these plants are sprayed by a growth hormone that induces fruit development even though fertilisation has not occurred. The ovules of such fruits, however, fail to develop into seeds.

Question. In a developing embryo, analyse the consequences if cell divisions are not followed by cell differentiation.
Answer : Cell divisions increase the number of cells in the developing embryo, while cell differentiation helps group of cells to undergo certain modifications to form specialised tissues and organs to form an organism. At many stages of embryogenesis, if cell differentiation does not occur, the embryo cannot develop into a new organism. It will only remain as a mass of cells.

Question. List the changes observed in an angiosperm flower subsequent to pollination and fertilisation.
Answer : In an angiosperm flower, the post-fertilisation changes occur as follows
Sepal                      Fall off
Petal                       Fall off
Stamen                   Fall off
Zygote                    Embryo
Primary endosperm  Endosperm (3n) 
Synergid                   Disintegrate
Antipodals                 Disintegrate
Ovary                        Fruit
Ovule                        Seed

Question. Suggest a possible explanation why the seeds in a pea pod are arranged in a row, whereas those in tomato are scattered in the juicy pulp.
Answer : In pea, fruit is legume. The pea pod is developed from monocarpellary, unilocular and half superior ovary. At maturity, the fruit splits along the dorsal and ventral sutures and discharge its seeds. In gynoecium with single carpel, ovules are always attached to the ventral suture. This results in the fruit with marginal placentation. Thus, the seeds are arranged in a row in legume (pea) pod. (Image 21) In tomato, the fruit is berry. It is fleshy fruit develop from superior or inferior ovary. In this, the margins of the carpels grow inward to the centre of the ovary dividing the central chamber into compartments called locules. So that, the ovules are arranged radially on the axis, attached by placenta that is called axial placentation. That’s the reason the seeds are embedded in the juicy pulp.

Question. Draw the sketches of a zoospore and a conidium. Mention two dissimilarities between them and atleast one feature common to both structures.
Answer : The feature common to zoospores and conidia is that both of these are the asexual reproductive structures, which facilitate the process of reproduction in asexually reproducing organisms.

The two dissimilarities between these (zoospore and conidium) are as follows 
          Zoospore                           Conidium
(Found usually in Algae)      (Found usually in Fungi)
Flagellated                                Non-flagellated
Formed inside a sporangium    Formed at the tip of conidiophores
(endogenously)                             (exogenously)

Question. Justify the statement ‘vegetative reproduction is also a type of asexual reproduction’.
Answer : In flowering plants, the units of vegetative reproduction such as runners, stolons, suckers, offsets, rhizome, corm, tuber, etc., are capable of giving rise to new offsprings. These structures are called vegetative propagules. In all these plants the formation of these structures does not involve two parents, the process involved is asexual. So, it can be said that vegetative reproduction is also a type of asexual reproduction.

Long Answer Type Questions

Question. Enumerate the differences between asexual and sexual reproduction. 
Describe the types of asexual reproduction exhibited by unicellular organisms.
Answer : The differences between asexual and sexual reproduction are mentioned below.  (Image 23)  
Asexual reproduction occurs usually in unicellular organisms, such as monerans and protists and in plants and certain animals. It takes place in the following ways
(i) Binary Fission In this type of asexual reproduction, the parent organism divides into two halves, each half forming an independent daughter organism. e.g., Amoeba, Euglena, Paramecium.
(ii) Budding In this type of asexual reproduction, a daughter individual is formed from a small projection, the bud, arising from the parent body. e.g.,yeast, Hydra.
(iii) Fragmentation In this type of asexual reproduction, the parent body breaks into two or more fragment. Each body fragment develops into an organism. e.g., sponges, Selaginella.
(iv) Gemmule In this type of asexual reproduction, internal buds, called gemmules are involved. Gemmules are asexually reproduced mass of cells, that is capable of developing into a new organism. e.g., sponges.
(v) Sporulation In this type of asexual reproduction, dispersive structures called spores are released from the parent body that germinate under favourable conditions form new individuals.
(a) Motile spores are called zoospores and are found in aquatic animals. e.g., Albugo, Chlamydomonas.
(b) Non-motile spores are named as sporangiospores (e.g., Rhizopus, Mucor) and conidia e.g., Penicillium.

Question. 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 suitable explanation.
Answer : No, all the gametes formed from a parent organism do not have the same genetic composition.
It can be better understand with the help of the explanation given below Sexual reproduction in organisms generally involves the fusion of gametes from two different individuals. These gametes form by the process of gametogenesis. In the heterogametic species, gametes are of two types namely male and female. Gametes are haploid though the parent body from which they arise may be either haploid or diploid.
(a) A haploid parent like Monera, fungi, algae and bryophytes produce gametes by mitotic division. The number of chromosomes, i.e., the genetic composition remain same after such type of division.
(b) The diploid parent like pteridophytes, gymnosperms, angiosperms and most of the animals including human beings produces gametes by meiosis. In such organisms (diploid), specialised cells called meiocytes (gamete mother cell) undergo meiosis. At the end of meiosis only one set of chromosomes gets incorporated into each gamete. It means the gametes formed contain a haploid number of chromosomes in contrast to the number of chromosomes in mother cells.

Question. Although sexual reproduction is a long drawn, energy-intensive complex form of reproduction, many groups of organisms in kingdom–Animalia and Plantae prefer this mode of reproduction. Give atleast three reasons for this.
Answer : Follwing are the three reasons for the mode of sexual reproduction in higher group of organisms
(i) The sexual mode of reproduction ensures creation of new variants.
(ii) Genetically varied offsprings are produced that adapt to changes in environment and survive in all climatic conditions. (iii) Sexual reproduction ensures the genetic recombination that results in variation which gives rise to evolution.

Question. Rose plants produce large, attractive bisexual flowers, but they seldom produce fruits. On the other hand a tomato plant produces plenty of fruits though they have small flowers. Analyse the reasons for failure of fruit formation in rose.
Answer : Rose plants produce large, attractive bisexual flowers, but they seldom produce fruits. The reasons for failure of fruit formation in rose are as follows
(i) Rose plants may not produce viable pollens, hence, no fertilisation can take place.
(ii) Rose plants may not have functional eggs.
(iii) Rose plants may have defective and non-functional ovule ,which is the female gametophyte generator.
(iv) There may be self-incompatibility.
(v) There may be internal barriers for pollen tube growth or fertilisation.
(vi) As rose plants are hybrids and reproduce vegetatively, there are chances for them to be sterile.

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