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LESSON
REPRODUCTION IN ORGANISMS
Introduction
Life Span: The period from birth to the natural death of an organism represents its life span. It may
be as short as a few days or as long as a few thousand years. Every organism has a certain life span,
for example: Mayfly – 1 day; Man – 100 years; Banyan tree – 200 years etc. Whatever be the life
span, death of every individual is a certainty, i.e., no individual is immortal, except single-celled
organisms. Thus, there must be some process in living organisms that ensure their continuity
i.e.reproduction.
Reproduction
 Definition: Reproduction is defined as a biological process in which an organism gives rise to
young ones (offspring) similar to itself.
 Purpose/significance of reproduction:- Reproduction enables the continuity of species
generation after generation.
 Types of Reproduction:- Reproduction can be broadly classified into the following two types:
(a) Asexual Reproduction
(b) Sexual Reproduction
Table: 1 Differences between Asexual and Sexual Reproduction
Asexual Reproduction
Sexual Reproduction
(i) A single parent is involved
(i) Two parents (a male and a female) are
involved.
(ii) There is no formation or fusion of
(ii) There is formation and fusion of
gametes.
gametes
(iii) It involves mitotic divisions only
(iii) It involves meiosis at some stage
(iv) All individuals produced are genetically
(iv) All individuals produced exhibit
identical, i.e., clone
variation i.e., offspring.
Asexual Reproduction
 Definition: When offspring is produced by a single parent, without the involvement of gamete
formation, the reproduction is asexual.
 Significance of Asexual Reproduction:- In this method, a single individual (parent) is capable
of producing offspring. The offspring that are produced are identical to one another and exact
copies of their parent. A group of such morphologically and genetically similar individuals is
called as – clone.
 Occurrence: Asexual reproduction is common among single-celled organisms and in plants and
animals with relatively simple organizations.
Modes of Asexual Reproduction in Animals: The three common modes of asexual reproduction
are fission, budding and fragmentation.
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Table: 2
1.
Fission: It is a type of asexual reproduction in which a full grown parental organism divides
into daughter cells. Based on the number of daughter cells produced fission is of two typesbinary fission and multiple fission.
(i) Binary fission: In binary fission, the mother cell divides into two equal sized daughter
halves or cells. Based on the plane of cytoplasmic division, the binary fission is of different
types. .
(a)
Simple Binary fission: The plane of cytoplasmic division passes through any
direction, e.g., Amoeba. (Figure 1a)
Figure 1(a) Binary Fission in Amoeba
(b)
Transverse Binary fission: The plane of cytoplasmic division coincides with the
transverse axis of the individual e.g., Paramecium (Figure 1b)
Figure 1(b) Transverse Binary Fission in Paramecium
(c)
Longitudinal Binary fission : The plane of cytoplasmic division coincides with the
longitudinal axis of the individual e.g., Euglena (Figure 1c)
Figure 1 (c) Longitudinal Binary Fission in Euglena
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(ii) Multiple fission - When the nucleus divides into many nuclei and each nucleus gathers a
small quantity of cytoplasm and transforms into an adult individual, the phenomenon is
called multiple fission e.g., Plasmodium,Monocystis
2.
Budding: It is a type of asexual reproduction where individuals are formed by mitosis of some
cells in the mature individual. The structure formed is called a bud. The bud separates from the
parent and establishes itself as an independent individual. Based on the nature of the bud,
budding can be of two types:
(i) Exogenous budding: Here a small outgrowth of the parent’s body develops into a
miniature individual. It then separates from the mother to lead a free life. The type of
budding is recognized as exogenous budding. e.g., Hydra
Figure 2(a) Budding in Hydra
(ii) Internal budding: In some others, the parent individual releases a specialized mass of cells
enclosed in a common opaque envelope. These are called the internal buds (gemmule).
On germination, each gemmule gives rise to an offspring. e.g., Spongilla (fresh water
sponges) and Sycon (marine sponges).
Figure 2(b) Gemmule
3.
Fragmentation: The body of the parent breaks into distinct pieces, each of which can grow into
an independent individual, e.g., planaria and Hydra. The parent organism does not exist after
fragmentation.
Note: Members of the kingdom fungi and simple plants such as algae reproduce through special
asexual reproductive structures.
 In yeast, the cell division results in a large cell and a small cell, called bud, attached to the large
cell; the bud gets separated and grows into an adult.
 In fungi and algae specialized asexual reproductive units are formed; they are as follows:
(i) Zoospores in many algae and certain aquatic fungi.
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Figure 3(a) Zoospores of Chlamydomonas
(ii) Conidia in penicillium, Aspergillus, etc.
Figure 3 (b) Conidia of Penicillium
Modes of Asexual Reproduction in Plants:
Asexual reproduction in plants is commonly referred as vegetative reproduction, because it occurs
through a vegetative part of a plant such as stem, root or leaf that develops into an independent unit.
These structures are called vegetative propagules. Since the formation of structures does not
involve two parents, the process involved is asexual.

Natural methods of vegetative propagation: Some of the natural means of
propagation are discussed below:-
Figure 4(a) Vegetative propagation through the root of Dalbergia
vegetative
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(i) Underground Stems:
(a) Sucker: Mint (runner) and chrysanthemum bear suckers at the base of aerial shoots.
After growing for some distance the suckers grow out and produce new crowns. When
suckers break due to mechanical disturbance or decay, a number of independent plants
are formed.
(b) Rhizomes: Rhizomes are thick underground stems which store food for perennation.
They possess buds for forming new shoots during favourable periods e.g., Banana,
Turmeric, Ginger, Aspidium, Adiantum, etc
(c) Bulb: Bulb is an underground condensed shoot having buds. When sown in the soil
each bulb gives rise to new plants, e.g., garlic, Narcissus, Onion, etc.
(d) Stem Tubers: Stem tubers are found in Potato and Artichoke. They have buds in the
region of nodes or eyes for vegetative multiplication.
(ii) Offset: Creeping stems like offsets (e.g., Eichhornia) develop from the base of an old shoot
and after growing horizontally for some distance give rise to new plants.
(iii) Leaves: Leaves of a number of plants develop or possess adventitious buds for vegetative
propagation, e.g.,Bryophyllumm
(iv) Bulbils : They are fleshy buds which develop into new plants after falling on the soil, e.g.,
Agave
(b) Runner -- Oxalis
(d) Potato Tuber
(c) Rhizome – Ginger
(e) Offset -- Pistia
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(f) Buds of Bryophyllum leaf
(g) Bulb -- Onion
(h) Bulbil of Agave
Figure 4
Merits of Vegetative Propagation
 It is the only known method of multiplication in seedless varieties and species e.g., Banana,
Sugarcane, Pineapple, Seedless Orange, Seedless Grape, etc.
 Vegetative propagation is a quick method of multiplication.
 It helps in rapid spread of a plant over an area, ex. Aquatic plant ‘water hyacinth’ propagates
vegetatively at a phenomenal rate and spread all over the water body in a short period of time.
 Good qualities of the race can be preserved as it gives rise to genetically uniform population or
clone.
Demerits of Vegetative Propagation:
 Variability is absent, so adaptability to changed environment decreases.
Sexual Reproduction
Definition: Sexual reproduction involves formation of the male and female gametes of the opposite
sex and their fusion to form the zygote which develops to form the new organism.
It is an elaborate, complex and slow process as compared to asexual reproduction.
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Significance of Sexual Reproduction: Sexual reproduction has the biological advantage of making
possible the recombination of the inherited traits of the two parents. Evolution can proceed much
more rapidly and effectively with sexual reproduction than it can with asexual reproduction.
Occurrence: Sexual reproduction occurs almost in all higher plants and animals.
Important note:
(a) All organisms have to reach a certain stage of growth and maturity in their life, before they
can reproduce sexually.
(i) Period of growth: The period of growth is called the juvenile phase. It is known as
vegetative phases in plants.
(ii) Reproductive period: The end of juvenile/vegetative phase marks the beginning of the
reproductive phase.
(iii) Period of Ageing: The end of reproductive phase can be considered as senescence or old
age. Old age ultimately leads to death.
(b) Sexual reproduction may occur all through the year or seasonally. Based on it, organisms are
categorized as:
(i) Seasonal breeders: Organisms which are reproductively active only during favourable
seasons, are called as seasonal breeders.
(ii) Continuous breeders: Organisms which are reproductively active throughout their
reproductive phase are called as continuous breeders.
(c) The females of placental mammals exhibit cyclical changes in the activities of ovaries and
accessory ducts as well as hormones during the reproductive phase. In non-primate mammals
like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during reproduction are
called oestrus cycle whereas in primates (monkeys, apes, and humans) it is called menstrual
cycle.
Events in sexual reproduction
Events in sexual reproduction: After attainment of maturity, all sexually reproducing organisms
exhibit events and processes that have remarkable fundamental similarity and follow a regular
sequence. These sequential events may be grouped into three distinct stages namely, the prefertilisation, fertilisation and the post-fertilisation events.
1.
Pre-fertilisation Events: These include all the events of sexual reproduction prior to the fusion
of gametes. The two main pre-fertilisation events are gametogenesis and gamete transfer.
A. Gametogenesis: refers to the process of formation of the two types of gametes—male and
female. Gametes are haploid cells. Gametes can be of 2 types based on appearance:
(a) Homogametes (Isogametes): In some algae the two gametes are so similar in appearance
that it is not possible to categorise them into male and female gametes.
Figure 5 (a) Isogametes
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(b) Heterogametes: In a majority of sexually reproducing organisms the gametes produced are
of two morphologically distinct types (heterogametes). In such organisms the male gamete
is called the antherozoid or sperm and the female gamete is called the egg or ovum.
Figure 5 (b) Heterogametes
Sexuality in organisms: Sexual reproduction in organisms generally involves the fusion of
gametes from two different individuals.
Exceptions: There exist species of plants and animals in which both male & female
reproductive structures are present in the same individual. Based on this there are 2 types of
plants and animals.
(a) Plants – 2 types
(i) Bisexual: The plants in which both male & female reproductive structures are present
in the same plant. They are also called as – Homothallic or Monoecious. Ex. some
fungi, plants – cucurbits & coconuts
(ii) Unisexual: The plants in which male and female reproductive structures are present on
different plants. The unisexual condition is also referred as – Heterothallic or
Dioecious. Eg., of dioecious plants – papaya and date palm
Figure 6(a) Monoecious plant (chara)
Figure 6 (b) Dioecious plant (Marchantia)
(b)Animals – 2 types
(i) Unisexual: The organisms in which male & female reproductive organs are present in
separate individuals. Ex. Cockroach, dogs, apes, human beings.
(ii) Bisexual: The organisms which possess both the reproductive organs are called as –
bisexual. Eg., Earthworm, sponge, tapeworm, leech.
These are also called as – Hermaphrodite animals.
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Figure 7 (a) Bisexual Animal
Figure 7 (b) Unisexual Animal
Cell division during gamete Formation
 Gametes in all heterogametic species are of two types namely, male and female. Gametes are
haploid though the parent plant body from which they arise may be either haploid or diploid.
 Several organisms belonging to monera, fungi, algae and bryophytes have haploid plant body. A
haploid parent produces gametes by mitotic division.
 Organisms belonging to pteridophytes, gymnosperms, angiosperms and most of the animals
including human beings, the parental body is diploid. Meiosis, the reduction division, occurs if a
diploid body has to produce haploid gametes. In diploid organisms, specialized cells called
meiocytes (gamete mother cell) undergo meiosis. At the end of meiosis, only one set of
chromosomes gets incorporated into each gamete.
Significance of Meiosis
(i) Maintenance of Chromosome Number: The gametes are usually formed by meiotic divisions.
Therefore, they are haploid, i.e., have halved or reduced (n) number of chromosomes. In sexual
reproduction, the male and female gametes fuse to form a single cell, the zygote. This process is
called fertilization. The zygote formed by the fusion of two haploid gametes is naturally diploid,
i.e., has double or normal number (2n) of chromosomes.
(ii) Genetic Recombinations
Meiosis has another advantage. During this process, random segregation of chromosomes
(independent Assortment) and exchange of genetic material between homologus chromosomes
(crossing over) result in new combinations of genes in the gametes, and this reshuffling increases
genetic diversity.
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Figure 8 Spermatogenesis
Table: 3 Chromosome Numbers in Meiocytes (diploid, 2n) and Gametes (haploid, n) of some
organisms
Name of organism
Chromosome number in meiocyte
Chromosome number in
(2n)
gamete (n)
Human beings
46
23
House fly
12
6
Maize
20
10
Potato
48
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B. Gamete Transfer: After their formation, male and female gametes must be physically brought
together to facilitate fusion (fertilisation). In a majority of organisms, male gamete is motile and
the female gamete is stationary.
Exceptions:
 Few fungi and algae in which both types of gametes are motile.
 In several simple plants like algae, bryophytes and pteridophytes, water is the medium
through which this gamete transfer takes place. A large number of the male gametes,
however, fail to reach the female gametes, to compensate this loss of male gametes during
transport, the number of male gametes produced is several thousand times then number of
female gametes produced.
 In seed plants, pollen grains are the carriers of male gametes and ovule have the egg. A
specialized event called pollination facilitates transfer of pollen grains to the stigma. Pollen
grains germinate on the stigma and the pollen tubes carrying the male gametes reach the
ovule and discharge male gametes near the egg. In bisexual, self-fertilising plants, e.g., peas,
transfer of pollen grains to the stigma is relatively easy as anthers and stigma are located
close to each other.
 In dioecious animals, since male and female gametes are formed in different individuals, the
organism evolve a special mechanism for gamete transfer.
 Successful transfer and coming together of gametes is essential for the most critical event in
sexual reproduction, the fertilisation.
2.
Fertilisation
Definition: The fusion of male and female gametes is called as – syngamy or fertilisation. It
results in the formation of a diploid zygote (2n). It is the most vital event of sexual reproduction.
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Figure 9 Homogametic contact in alga
Exception – Parthenogenesis: In some organisms like rotifers, honeybees and even some
lizards and birds (turkey), the female gamete undergoes development to form new organisms
without fertilisation. This phenomenon is called parthenogenesis.
Fertilisation can be of 2 types:
(a) External Fertilisation (b) Internal Fertilisation
Table 4
External Fertilisation
Internal Fertilisation
(i) When fusion of gametes (syngamy) occurs (i) When fusion of gametes (syngamy)
outside the body of the organism, it is
occurs inside the body of the organism, it
called external fertilisation.
is called internal fertilisation.
(ii) A large number of gametes are released in (ii) The number of ova produced is less, but a
the surrounding medium by such animals.
large number of male gametes are
e.g. bony fishes, amphibians, etc.
formed, as they have to travel towards the
ovum. E.g. birds, mammals, earthworm,
etc.
(iii) The offsprings are extremely vulnerable to (iii)The offsprings are safe as they are
predators threatening their survival upto
protected inside mother’s body.
adulthood.
3.
Post-fertilisation events: Events in sexual reproduction after the formation of zygote are called
post-fertilisation events.
(A) Zygote:
 Formation of the diploid zygote is universal in all sexually reproducing organisms.
 Zygote is the vital link that ensures continuity of species between organisms of one
generation and the next.
 Every sexually reproducing organism, including human beings begin life as a single cell-the
zygote.
 In organisms with external fertilisation, zygote is formed in the external medium (usually
water), whereas in those exhibiting internal fertilisation, zygote is formed inside the body of
the organism.
 Development of zygote depends on:
(a) the type of life cycle of the organism - In organisms showing haplontic life cycle, zygote
undergoes meiosis, while in organisms showing diplontic life-cycle, zygote undergoes
mitosis.
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
(b) the environment it is exposed to - In many algae and fungi, the zygote develops a thick
wall around it and undergoes a period of rest before germination.
The zygote develops into an embryo (embryogenesis).
(B) Embryogenesis: Embryogenesis refers to the process of development of embryo from the
zygote.
 Embryogenesis involves:
(i) cell division, to increase the number of cells.
(ii) cell enlargement or growth, to increases the volume/weight of living matter.
(iii) cell differentiation, for the formation of different kinds of tissues.
 Depending on whether the development of zygote occurs inside or outside the body of the
female organism (parent), animals are categorized as oviparous and viviparous.
Table 5
Oviparous animals
Viviparous animals
(i) These are the animals which lay (i) These are the animals which give birth to
fertilized or unfertilized eggs.
young individuals.
(ii) The fertilized eggs have a calcareous (ii) The eggs have no calcareous shell, as they
shell to protect them from the harsh
are protected inside the mother’s body.
environment.
(iii) Chances of survival of young ones is (iii) Chances of survival is greater of young
low
ones.
(iv) Eg.-reptiles & birds
(iv) Eg.-mammals, human beings
Flowering plants
 In flowering plants, the zygote is formed inside the ovule, where it develops into the embryo;
simultaneously the ovule becomes the seed and the ovary becomes the fruit.
 The seeds germinate after they are dispersed from the fruit and produce new plants.
Figure 10 Fruits showing pericarp, placenta and seeds
Important Note:
 Under unfavourable condition the Amoeba withdraws its pseudopodia and secretes a threelayered hard covering or cyst around itself. This phenomenon is termed as encystation. When
favourable conditions return, the encysted Amoeba divides by multiple fission and produces
many minute amoeba or pseudopodiospores; the cyst wall bursts out, and the spores are liberated
in the surrounding medium to grow up into many amoebae. This phenomenon is known as
sporulation. In some organisms, if the body breaks into distinct pieces (fragments) each
fragment grows into an adult capable of producing offspring (e.g., Hydra). This is also a mode of
asexual reproduction called fragmentation.
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
Each organism has an invariably defined range of conditions that it can tolerate, diversity in the
resources it utilises and a distinct functional role in the ecological system, all these together
comprise its niche.