1
Reproduction
in Organisms
Important Points
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 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
invertebrates.
It is characterised by
(a) involvement of only one parent.
(b) no formation or fusion of gametes.
(c) offsprings are identical to 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.
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4. Asexual Reproduction in Protistan and Moneran
(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 cell divides
unequally to produce a small projection (bud) on a side.
This eventually gets separated and grows as 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. Parent nucleus divides
repeteadly 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,
e.g., In fungi, Phycomycetes and algae Chlamydomonas.
(ii) Conidia are non-motile spores produced singly or in
chains by constriction at the tip or laterally on special
hyphal branches called condiophores. These are exogenous
spores that disperse by wind and germinate in suitable
habitat by producing germ tubes, e.g., Penicillium.
(iii) Buds form as small outgrowth on the body of the parent
containing the copy of the parental nucleus. It may split
away from the parent body and grow up as a new
individual, e.g., Hydra.
(iv) Gemmules are endogenous buds that are simply a copy of
parent nucleus, with a small amount of cytoplasm and a
protective covering around. During favourable conditions,
each bud comes out and grows as an individual,
e. g. , Freshwater sponges.
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NCERT Class XII Biology
Conidia
(a)
(b)
Mouth
Bud
(c)
(d)
Asexual reproductive structure (a) Zoospores of Chlamydomonas (b) Conidia of
Penicillium (c) Buds in Hydra (d) Gemmules in Sponge
6. Sexual Reproduction Generally Involves
(i) two prospective parents.
(ii) gamete formation by meiosis.
(iii) fusion of gametes (fertilisation).
(iv) genetic and morphological variation of offsprings from the
parents.
Gamete fusion leads to formation of the zygote that 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
(i) Juvenile phase The stage of growth and maturity in life
before reproducing sexually.
(ii) Reproductive phase The stage where one can
reproduce sexually as is able to give birth to offsprings.
(iii) Senescent phase The end of reproductive phase till
death. If refers to old age.
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8. Events of Sexual Reproduction are
(i) Pre-fertilisation Gametogenesis or formation of gametes.
(ii) Fertilisation Syngamy or fusion of gametes to form
zygote.
(iii) 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., earthworm, tapeworm,
leech, sponge, 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 rose, lily, jasmine, sweet
peas, etc. They are also called homothallic or monoecious.
Unisexual plants have flowers with only male or only female
parts. Unisexual male flower will have stamens and unisexual
female flower will have only pistil. They are also called
heterothallic or dioecious.
10. Gametes These are the cells that make sexual reproduction
possible.
Male
gamete
is
known
as
sperm
or
antherozoid. Female gamete is known as egg or ovum.
(a)
(b)
(c)
Types of gametes (a) Isogametes of Cladophora (an alga) (b) Heterogametes
of Fucus (an alga) (c) Heterogametes of Homo sapiens (human beings)
(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. Female
gamete is much bigger in size and non-motile, whereas
male gamete is very tiny and is generally motile.
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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 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 in the zygote divides
repeatedly and differentiates to form embryo, eventually
resulting in to a new organism.
Changes After Fertilisation
In flowering plants,
(i) the ovary develops into 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, zygote undergoes cell division (mitosis)
and cell differentiation.
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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., majority of
mammals. In flowering plants, zygote is formed inside the
ovule. The zygote develops into the embryo and the ovules
develop into the seed.
Exercises
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?
Answer Sexual reproduction is considered a better mode as it gives rise
to genetic variation.
(i) 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 clone?
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 inherits 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 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 genetic and
morphological resemblance to the parent generation. In case to sexual
reproduction, progeny formed has genetic and morphological variations
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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 as a type of asexual
reproduction?
Answer Differences between sexual and asexual reproduction
S.N. Sexual Reproduction
1.
Two parents of opposite sex are
involved in the process.
2.
Gamete formation and fertilisation
always takes place.
3.
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Offsprings show genetic and
morphologic variation from the
parents.
The process involves both meiotic
and mitotic cell division.
Asexual Reproduction
Only one parent is involved.
Generally gamete formation and
fertilisation not required.
Offsprings are genetically and
morphologically same as the parents.
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.
What is vegetative propagation? Give two suitable
examples.
Answer The process wherein the vegetative parts of plants give rise to
Question 7.
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.
Define
(a) Juvenile phase
(c) Senescent phase
Answer
Question 8.
(b) Reproductive phase
(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 vegetative stage.
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(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 the changes, e.g., slower
metabolism, break down 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 and
contribute to evolution.
Question 10.
interlinked?
Explain why meiosis and gametogenesis are always
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 to
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 chromosomes number in daughter cells does
not reduce to half.
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
Question 11.
(a)
(b)
(c)
(d)
(e)
Ovary
Anther
Ovum/egg
Pollen
Male gamete
—
—
—
—
—
Diploid
Diploid
Haploid
Haploid
Haploid
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NCERT Class XII Biology
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 external medium, e.g., water.
Example Bony fish and frog.
The major disadvantage in this type of fertilisation is that the offspring are
(i) exposed to the 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. Zoospore
Zygote
1.
It is an asexual reproductive structure,
usually flagellated and motile.
2.
3.
It can be haploid or diploid.
It germinates to directly give rise to
new plants.
Question 14. Differentiate
It is single cell formed as a result of
fertilisation in sexual reproduction. It is
non-flagellated and non-motile.
It is diploid.
It develops into an embryo that further
differentiates to form a new individual.
between
gametogenesis
from
embryogenesis.
Answer
S.N. Gametogenesis
Embryogenesis
1.
It is the process of generation of
haploid gametes.
It is the process of formation of
embryo.
2.
Meiotic cell division of mother gamete
cell is required.
3.
Further transformation may be
required to make male gametes
motile.
Repeated mitotic cell divisions are
required to increase the number of
cells of the zygote.
Further cell differentiation 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 pistil
remains attached to the plant.
(ii) The zygote develops into the embryo and ovules develop into the
seeds.
(iii) Ovary develops into the fruit and a thick protective wall called
pericarp, covers the fruit.
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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)
Question 16.
parts.
Scientific Name
Cucumis sativus
Cocos nucifera
Rosa alba
Lathyrus odoratus
Hibiscus rosa-sinensis
Common Name
Cucumber
Coconut
Rose
Sweet pea
Gurthal, China flower
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 :
Question 17.
(i) The staminate flowers show presence of only stamens and no
carpels.
(ii) The pistillate flowers show presence of only female parts, the pistils
or carpels and no stamens.
Example plants that bear unisexual flowers: pumpkin, papaya, date palm.
Why are offspring of oviparous animals at a greater
risk as compared to the offspring of viviparous animals?
Answer The offsprings of oviparous animals are at a greater risk because
Question 18.
they develop outside the female body and are exposed to varying
environmental conditions and predator threat.
Whereas in viviparous animals, young ones develop inside the female body,
which ensures proper embryonic care and protection.
Selected NCERT Exemplar Problems
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.
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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 is the
correct sequence.
Gametes need to be formed first of all, pollination ensures their transfer so
that fertilisation can take place. Fertilised zygote divides and differentiates
during embryogenesis.
Question 3. 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 and
higher organisms have evolved meiotic cell division as the process of
making haploid gametes.
Question 4. Between an annual and a perennial plant, which one
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 that has relatively long lifespan.
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
Question 5.
example,
(i) Parrot (lifespan 140 yrs) and crow (lifespan 15 yrs) are roughly of the
same size but have drastically different lifespan.
(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).
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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 specific division of labour in higher organisms to run all the
functions smoothly and harmoniously so only specific type of cells are
assigned the job of reproduction.
Which of the following are monoecious and dioecious
organisms?
(a) Earthworm
(b) Chara
(c) Marchantia
(d) Cockroach
Answer
Question 7.
(a) Earthworm—monoecious  (one individual has both

(b) Chara—monoecious
 male and female sex organs)
(c) Marchantia—dioecious
(d) Cockroach—dioecious
 (one individual has either male
 or female sex organs)

Short Answer Type Questions
haploid organisms that undergo sexual
Question 1. In
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.
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In a developing embryo, analyse the consequences, if
cell divisions are not followed by cell differentiation.
Question 3.
Answer Cell divisions only increase the number of cells in the developing
embryo. But, to help these group 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 tomato are scattered in
the juicy pulp.
Answer Pea pod develop from monocarpellary superior ovary. Pea seeds
are non albuminous, which have no residual endosperm as it is completely
consumed during embryo development. Whereas in tomato, the fruit is
fleshy and seeds are scattered in its juicy pulp.
Long Answer Type Questions
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.
Question 1.
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 chromosomal pairs.
(i) Crossing Over
(a) Gametes form through meiotic cell division of a diploid mother
gamete cell.
(b) During meiotic division, each pair of chromosomes (pair is one
from father and one from mother) may cross over and exchange
a segment.
(c) The exchange may occur in one chromatid and not the other.
At the end of cell division, four haploid (having only one of the
pairs) gametes formed will have the following genetic
composition
l All black chromosome.
l Black chromosome with gray segment.
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Gray chromosome with black segment.
All gray chromosome.
Hence, each gamete has a different genetic composition.
l
l
(i) Independent assortment
All chromosomal pairs assort themselves independent of the others.
Which means that gametes can have the following genetic
compositions:
(a) All are mother chromosomes.
(b) All are father chromosomes.
(c) Combination of mother and father chromosomes.
l The last one is the most common outcome and it has
innumerable combination possibilities.
l The higher the chromosome number, the greater the
permutations and combinations.
l That is why all gametes in a parent can never be the same.
l Additionally, in males, 50% of the gametes will end up having
X-chromosome and 50% will end up having Y-chromosome.
l Therefore, each gamete will have a different set of
chromosomes or a different genetic composition.