NCERT Textbook - Reproduction in Organisms NEET Notes | EduRev

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NEET : NCERT Textbook - Reproduction in Organisms NEET Notes | EduRev

 Page 1


Each and every organism can live only for a certain period
of time. The period from birth to the natural death of an
organism represents its life span. Life spans of a few
organisms are given in Figure 1.1. Several other organisms
are drawn for which you should find out their life spans
and write in the spaces provided.  Examine the life spans
of organisms represented in the Figure 1.1. Isn’t it both
interesting and intriguing to note that it may be as short
as a few days or as long as a few thousand years?  Between
these two extremes are the life spans of most other living
organisms. You may note that life spans of organisms are
not necessarily correlated with their sizes; the sizes of
crows and parrots are not very different yet their life spans
show a wide difference. Similarly, a mango tree has a much
shorter life span as compared to a peepal tree. Whatever
be the life span, death of every individual organism is a
certainty, i.e., no individual is immortal, except single-celled
organisms. Why do we say there is no natural death in
single-celled organisms? Given this reality, have you ever
wondered how vast number of plant and animal species
have existed on earth for several thousands of years?  There
must be some processes in living organisms that ensure
this continuity.  Yes, we are talking about reproduction,
something that we take for granted.
CHAPTER 1
REPRODUCTION IN ORGANISMS
1.1 Asexual
Reproduction
1.2 Sexual
Reproduction
Page 2


Each and every organism can live only for a certain period
of time. The period from birth to the natural death of an
organism represents its life span. Life spans of a few
organisms are given in Figure 1.1. Several other organisms
are drawn for which you should find out their life spans
and write in the spaces provided.  Examine the life spans
of organisms represented in the Figure 1.1. Isn’t it both
interesting and intriguing to note that it may be as short
as a few days or as long as a few thousand years?  Between
these two extremes are the life spans of most other living
organisms. You may note that life spans of organisms are
not necessarily correlated with their sizes; the sizes of
crows and parrots are not very different yet their life spans
show a wide difference. Similarly, a mango tree has a much
shorter life span as compared to a peepal tree. Whatever
be the life span, death of every individual organism is a
certainty, i.e., no individual is immortal, except single-celled
organisms. Why do we say there is no natural death in
single-celled organisms? Given this reality, have you ever
wondered how vast number of plant and animal species
have existed on earth for several thousands of years?  There
must be some processes in living organisms that ensure
this continuity.  Yes, we are talking about reproduction,
something that we take for granted.
CHAPTER 1
REPRODUCTION IN ORGANISMS
1.1 Asexual
Reproduction
1.2 Sexual
Reproduction
4
BIOLOGY
Figure 1.1 Approximate life spans of some organisms
4
Page 3


Each and every organism can live only for a certain period
of time. The period from birth to the natural death of an
organism represents its life span. Life spans of a few
organisms are given in Figure 1.1. Several other organisms
are drawn for which you should find out their life spans
and write in the spaces provided.  Examine the life spans
of organisms represented in the Figure 1.1. Isn’t it both
interesting and intriguing to note that it may be as short
as a few days or as long as a few thousand years?  Between
these two extremes are the life spans of most other living
organisms. You may note that life spans of organisms are
not necessarily correlated with their sizes; the sizes of
crows and parrots are not very different yet their life spans
show a wide difference. Similarly, a mango tree has a much
shorter life span as compared to a peepal tree. Whatever
be the life span, death of every individual organism is a
certainty, i.e., no individual is immortal, except single-celled
organisms. Why do we say there is no natural death in
single-celled organisms? Given this reality, have you ever
wondered how vast number of plant and animal species
have existed on earth for several thousands of years?  There
must be some processes in living organisms that ensure
this continuity.  Yes, we are talking about reproduction,
something that we take for granted.
CHAPTER 1
REPRODUCTION IN ORGANISMS
1.1 Asexual
Reproduction
1.2 Sexual
Reproduction
4
BIOLOGY
Figure 1.1 Approximate life spans of some organisms
4 5
REPRODUCTION IN ORGANISMS
Reproduction is defined as a biological process in which an
organism gives rise to young ones (offspring) similar to itself. The offspring
grow, mature and in turn produce new offspring. Thus, there is a cycle
of birth, growth and death. Reproduction enables the continuity of the
species, generation after generation. You will study later in Chapter 5
(Principles of Inheritance and Variation) how genetic variation is created
and inherited during reproduction.
There is a large diversity in the biological world and each organism
has evolved its own mechanism to multiply and produce offspring.
The organism’s habitat, its internal physiology and several other factors
are collectively responsible for how it reproduces.  Based on whether
there is participation of one organism or two in the process of
reproduction, it is of two types. When offspring is produced by a single
parent with or without the involvement of gamete formation, the
reproduction is asexual. When two parents (opposite sex) participate in
the reproductive process and also involve fusion of male and female
gametes, it is called sexual reproduction.
1.1  ASEXUAL REPRODUCTION
In this method, a single individual (parent) is capable of producing
offspring. As a result, the offspring that are produced are not only
identical to one another but are also exact copies of their parent.
Are these offspring likely to be genetically identical or different?
The term clone is used to describe such morphologically and
genetically similar individuals.
Figure 1.2 Cell division in unicellular organism: (a) Budding in
yeast; (b) Binary fission in Amoeba
(b)
(a)
Let us see how widespread asexual reproduction is, among different
groups of organisms. Asexual reproduction is common among
single-celled organisms, and in plants and animals with relatively simple
organisations. In Protists and Monerans, the organism or the parent
cell divides into two to give rise to new individuals (Figure1.2). Thus,
Page 4


Each and every organism can live only for a certain period
of time. The period from birth to the natural death of an
organism represents its life span. Life spans of a few
organisms are given in Figure 1.1. Several other organisms
are drawn for which you should find out their life spans
and write in the spaces provided.  Examine the life spans
of organisms represented in the Figure 1.1. Isn’t it both
interesting and intriguing to note that it may be as short
as a few days or as long as a few thousand years?  Between
these two extremes are the life spans of most other living
organisms. You may note that life spans of organisms are
not necessarily correlated with their sizes; the sizes of
crows and parrots are not very different yet their life spans
show a wide difference. Similarly, a mango tree has a much
shorter life span as compared to a peepal tree. Whatever
be the life span, death of every individual organism is a
certainty, i.e., no individual is immortal, except single-celled
organisms. Why do we say there is no natural death in
single-celled organisms? Given this reality, have you ever
wondered how vast number of plant and animal species
have existed on earth for several thousands of years?  There
must be some processes in living organisms that ensure
this continuity.  Yes, we are talking about reproduction,
something that we take for granted.
CHAPTER 1
REPRODUCTION IN ORGANISMS
1.1 Asexual
Reproduction
1.2 Sexual
Reproduction
4
BIOLOGY
Figure 1.1 Approximate life spans of some organisms
4 5
REPRODUCTION IN ORGANISMS
Reproduction is defined as a biological process in which an
organism gives rise to young ones (offspring) similar to itself. The offspring
grow, mature and in turn produce new offspring. Thus, there is a cycle
of birth, growth and death. Reproduction enables the continuity of the
species, generation after generation. You will study later in Chapter 5
(Principles of Inheritance and Variation) how genetic variation is created
and inherited during reproduction.
There is a large diversity in the biological world and each organism
has evolved its own mechanism to multiply and produce offspring.
The organism’s habitat, its internal physiology and several other factors
are collectively responsible for how it reproduces.  Based on whether
there is participation of one organism or two in the process of
reproduction, it is of two types. When offspring is produced by a single
parent with or without the involvement of gamete formation, the
reproduction is asexual. When two parents (opposite sex) participate in
the reproductive process and also involve fusion of male and female
gametes, it is called sexual reproduction.
1.1  ASEXUAL REPRODUCTION
In this method, a single individual (parent) is capable of producing
offspring. As a result, the offspring that are produced are not only
identical to one another but are also exact copies of their parent.
Are these offspring likely to be genetically identical or different?
The term clone is used to describe such morphologically and
genetically similar individuals.
Figure 1.2 Cell division in unicellular organism: (a) Budding in
yeast; (b) Binary fission in Amoeba
(b)
(a)
Let us see how widespread asexual reproduction is, among different
groups of organisms. Asexual reproduction is common among
single-celled organisms, and in plants and animals with relatively simple
organisations. In Protists and Monerans, the organism or the parent
cell divides into two to give rise to new individuals (Figure1.2). Thus,
6
BIOLOGY
in these organisms cell division is itself a mode of reproduction. Many
single-celled organisms reproduce by binary fission, where a cell divides
into two halves and each rapidly grows into an adult (e.g., Amoeba,
Paramecium). In yeast, the division is unequal and small buds are
produced that remain attached initially to the parent cell which,
eventually gets separated and  mature into new yeast organisms (cells).
Members of the Kingdom Fungi and simple plants such as algae
reproduce through special asexual reproductive structures (Figure 1.3).
The most common of these structures are zoospores that usually are
microscopic motile structures.  Other common asexual reproductive
structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).
Figure1.3 Asexual reproductive structures: (a) Zoospores of chlamydomonas; (b) Conidia of
Penicillium; (c) Buds in Hydra; (d) Gemmules in sponge
(d)
(a)
(b)
(c)
Page 5


Each and every organism can live only for a certain period
of time. The period from birth to the natural death of an
organism represents its life span. Life spans of a few
organisms are given in Figure 1.1. Several other organisms
are drawn for which you should find out their life spans
and write in the spaces provided.  Examine the life spans
of organisms represented in the Figure 1.1. Isn’t it both
interesting and intriguing to note that it may be as short
as a few days or as long as a few thousand years?  Between
these two extremes are the life spans of most other living
organisms. You may note that life spans of organisms are
not necessarily correlated with their sizes; the sizes of
crows and parrots are not very different yet their life spans
show a wide difference. Similarly, a mango tree has a much
shorter life span as compared to a peepal tree. Whatever
be the life span, death of every individual organism is a
certainty, i.e., no individual is immortal, except single-celled
organisms. Why do we say there is no natural death in
single-celled organisms? Given this reality, have you ever
wondered how vast number of plant and animal species
have existed on earth for several thousands of years?  There
must be some processes in living organisms that ensure
this continuity.  Yes, we are talking about reproduction,
something that we take for granted.
CHAPTER 1
REPRODUCTION IN ORGANISMS
1.1 Asexual
Reproduction
1.2 Sexual
Reproduction
4
BIOLOGY
Figure 1.1 Approximate life spans of some organisms
4 5
REPRODUCTION IN ORGANISMS
Reproduction is defined as a biological process in which an
organism gives rise to young ones (offspring) similar to itself. The offspring
grow, mature and in turn produce new offspring. Thus, there is a cycle
of birth, growth and death. Reproduction enables the continuity of the
species, generation after generation. You will study later in Chapter 5
(Principles of Inheritance and Variation) how genetic variation is created
and inherited during reproduction.
There is a large diversity in the biological world and each organism
has evolved its own mechanism to multiply and produce offspring.
The organism’s habitat, its internal physiology and several other factors
are collectively responsible for how it reproduces.  Based on whether
there is participation of one organism or two in the process of
reproduction, it is of two types. When offspring is produced by a single
parent with or without the involvement of gamete formation, the
reproduction is asexual. When two parents (opposite sex) participate in
the reproductive process and also involve fusion of male and female
gametes, it is called sexual reproduction.
1.1  ASEXUAL REPRODUCTION
In this method, a single individual (parent) is capable of producing
offspring. As a result, the offspring that are produced are not only
identical to one another but are also exact copies of their parent.
Are these offspring likely to be genetically identical or different?
The term clone is used to describe such morphologically and
genetically similar individuals.
Figure 1.2 Cell division in unicellular organism: (a) Budding in
yeast; (b) Binary fission in Amoeba
(b)
(a)
Let us see how widespread asexual reproduction is, among different
groups of organisms. Asexual reproduction is common among
single-celled organisms, and in plants and animals with relatively simple
organisations. In Protists and Monerans, the organism or the parent
cell divides into two to give rise to new individuals (Figure1.2). Thus,
6
BIOLOGY
in these organisms cell division is itself a mode of reproduction. Many
single-celled organisms reproduce by binary fission, where a cell divides
into two halves and each rapidly grows into an adult (e.g., Amoeba,
Paramecium). In yeast, the division is unequal and small buds are
produced that remain attached initially to the parent cell which,
eventually gets separated and  mature into new yeast organisms (cells).
Members of the Kingdom Fungi and simple plants such as algae
reproduce through special asexual reproductive structures (Figure 1.3).
The most common of these structures are zoospores that usually are
microscopic motile structures.  Other common asexual reproductive
structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).
Figure1.3 Asexual reproductive structures: (a) Zoospores of chlamydomonas; (b) Conidia of
Penicillium; (c) Buds in Hydra; (d) Gemmules in sponge
(d)
(a)
(b)
(c)
7
REPRODUCTION IN ORGANISMS
You have learnt about vegetative reproduction in plants in Class XI.
What do you think – Is vegetative reproduction also a type of asexual
reproduction? Why do you say so? Is the term clone applicable to the
offspring formed by vegetative reproduction?
While in animals and other simple organisms the term asexual is used
unambiguously, in plants, the term vegetative reproduction is frequently
used. In plants, the units of vegetative propagation such as runner,
rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new
offspring (Figure1.4). These structures are called vegetative propagules.
Obviously, since the formation of these structures does not involve two
parents, the process involved is asexual.
Figure 1.4 Vegetative propagules in angiosperms: (a) Eyes of potato; (b) Rhizome of ginger;
(c) Bulbil of Agave; (d) Leaf buds of Bryophyllum; (e) Offset of water hyacinth
Adventitious
Buds
(a)
(c)
(d)
(e)
(b)
Buds
Nodes
Adventitious
Root
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