NCERT Textbook - How do Organisms Reproduce Class 10 Notes | EduRev

Science & Technology for UPSC CSE

Class 10 : NCERT Textbook - How do Organisms Reproduce Class 10 Notes | EduRev

 Page 1


How do Organisms
Reproduce?
8 CHAPTER
B
efore we discuss the mechanisms by which organisms reproduce,
let us ask a more basic question – why do organisms reproduce?
After all, reproduction is not necessary to maintain the life of an individual
organism, unlike the essential life processes such as nutrition,
respiration, or excretion. On the other hand, if an individual organism is
going to create more individuals, a lot of its energy will be spent in the
process. So why should an individual organism waste energy on a process
it does not need to stay alive? It would be interesting to discuss the
possible answers in the classroom!
Whatever the answer to this question, it is obvious that we notice
organisms because they reproduce. If there were to be only one, non-
reproducing member of a particular kind, it is doubtful that we would
have noticed its existence. It is the large numbers of organisms belonging
to a single species that bring them to our notice. How do we know that
two different individual organisms belong to the same species? Usually,
we say this because they look similar to each other. Thus, reproducing
organisms create new individuals that look very much like themselves.
8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORGANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREATE EXA TE EXA TE EXA TE EXA TE EXACT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF
THEMSEL THEMSEL THEMSEL THEMSEL THEMSELVES? VES? VES? VES? VES?
Organisms look similar because their body designs are similar. If body
designs are to be similar, the blueprints for these designs should be
similar. Thus, reproduction at its most basic level will involve making
copies of the blueprints of body design. In Class IX, we learnt that the
chromosomes in the nucleus of a cell contain information for inheritance
of features from parents to next generation in the form of DNA (Deoxyribo
Nucleic Acid) molecules. The DNA in the cell nucleus is the information
source for making proteins. If the information is changed, different
proteins will be made. Different proteins will eventually lead to altered
body designs.
Therefore, a basic event in reproduction is the creation of a DNA
copy. Cells use chemical reactions to build copies of their DNA. This
creates two copies of the DNA in a reproducing cell, and they will need to
be separated from each other. However, keeping one copy of DNA in the
original cell and simply pushing the other one out would not work,
2020-21
Page 2


How do Organisms
Reproduce?
8 CHAPTER
B
efore we discuss the mechanisms by which organisms reproduce,
let us ask a more basic question – why do organisms reproduce?
After all, reproduction is not necessary to maintain the life of an individual
organism, unlike the essential life processes such as nutrition,
respiration, or excretion. On the other hand, if an individual organism is
going to create more individuals, a lot of its energy will be spent in the
process. So why should an individual organism waste energy on a process
it does not need to stay alive? It would be interesting to discuss the
possible answers in the classroom!
Whatever the answer to this question, it is obvious that we notice
organisms because they reproduce. If there were to be only one, non-
reproducing member of a particular kind, it is doubtful that we would
have noticed its existence. It is the large numbers of organisms belonging
to a single species that bring them to our notice. How do we know that
two different individual organisms belong to the same species? Usually,
we say this because they look similar to each other. Thus, reproducing
organisms create new individuals that look very much like themselves.
8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORGANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREATE EXA TE EXA TE EXA TE EXA TE EXACT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF
THEMSEL THEMSEL THEMSEL THEMSEL THEMSELVES? VES? VES? VES? VES?
Organisms look similar because their body designs are similar. If body
designs are to be similar, the blueprints for these designs should be
similar. Thus, reproduction at its most basic level will involve making
copies of the blueprints of body design. In Class IX, we learnt that the
chromosomes in the nucleus of a cell contain information for inheritance
of features from parents to next generation in the form of DNA (Deoxyribo
Nucleic Acid) molecules. The DNA in the cell nucleus is the information
source for making proteins. If the information is changed, different
proteins will be made. Different proteins will eventually lead to altered
body designs.
Therefore, a basic event in reproduction is the creation of a DNA
copy. Cells use chemical reactions to build copies of their DNA. This
creates two copies of the DNA in a reproducing cell, and they will need to
be separated from each other. However, keeping one copy of DNA in the
original cell and simply pushing the other one out would not work,
2020-21
Science
128
?
because the copy pushed out would not have any organised cellular
structure for maintaining life processes. Therefore, DNA copying is
accompanied by the creation of an additional cellular apparatus, and
then the DNA copies separate, each with its own cellular apparatus.
Effectively, a cell divides to give rise to two cells.
These two cells are of course similar, but are they likely to be
absolutely identical? The answer to this question will depend on how
accurately the copying reactions involved occur. No bio-chemical reaction
is absolutely reliable. Therefore, it is only to be expected that the process
of copying the DNA will have some variations each time. As a result, the
DNA copies generated will be similar, but may not be identical to the
original. Some of these variations might be so drastic that the new DNA
copy cannot work with the cellular apparatus it inherits. Such a newborn
cell will simply die. On the other hand, there could still be many other
variations in the DNA copies that would not lead to such a drastic
outcome. Thus, the surviving cells are similar to, but subtly different
from each other. This inbuilt tendency for variation during reproduction
is the basis for evolution, as we will discuss in the next chapter.
8.1.1 The Importance of Variation
Populations of organisms fill well-defined places, or niches, in the
ecosystem, using their ability to reproduce. The consistency of DNA
copying during reproduction is important for the maintenance of body
design features that allow the organism to use that particular niche.
Reproduction is therefore linked to the stability of populations of species.
However, niches can change because of reasons beyond the control
of the organisms. Temperatures on earth can go up or down, water levels
can vary, or there could be meteorite hits, to think of a few examples. If
a population of reproducing organisms were suited to a particular niche
and if the niche were drastically altered, the population could be wiped
out. However, if some variations were to be present in a few individuals
in these populations, there would be some chance for them to survive.
Thus, if there were a population of bacteria living in temperate waters,
and if the water temperature were to be increased by global warming,
most of these bacteria would die, but the few variants resistant to heat
would survive and grow further. Variation is thus useful for the survival
of species over time.
QUESTIONS
1. What is the importance of DNA copying in reproduction?
2. Why is variation beneficial to the species but not necessarily
for the individual?
2020-21
Page 3


How do Organisms
Reproduce?
8 CHAPTER
B
efore we discuss the mechanisms by which organisms reproduce,
let us ask a more basic question – why do organisms reproduce?
After all, reproduction is not necessary to maintain the life of an individual
organism, unlike the essential life processes such as nutrition,
respiration, or excretion. On the other hand, if an individual organism is
going to create more individuals, a lot of its energy will be spent in the
process. So why should an individual organism waste energy on a process
it does not need to stay alive? It would be interesting to discuss the
possible answers in the classroom!
Whatever the answer to this question, it is obvious that we notice
organisms because they reproduce. If there were to be only one, non-
reproducing member of a particular kind, it is doubtful that we would
have noticed its existence. It is the large numbers of organisms belonging
to a single species that bring them to our notice. How do we know that
two different individual organisms belong to the same species? Usually,
we say this because they look similar to each other. Thus, reproducing
organisms create new individuals that look very much like themselves.
8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORGANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREATE EXA TE EXA TE EXA TE EXA TE EXACT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF
THEMSEL THEMSEL THEMSEL THEMSEL THEMSELVES? VES? VES? VES? VES?
Organisms look similar because their body designs are similar. If body
designs are to be similar, the blueprints for these designs should be
similar. Thus, reproduction at its most basic level will involve making
copies of the blueprints of body design. In Class IX, we learnt that the
chromosomes in the nucleus of a cell contain information for inheritance
of features from parents to next generation in the form of DNA (Deoxyribo
Nucleic Acid) molecules. The DNA in the cell nucleus is the information
source for making proteins. If the information is changed, different
proteins will be made. Different proteins will eventually lead to altered
body designs.
Therefore, a basic event in reproduction is the creation of a DNA
copy. Cells use chemical reactions to build copies of their DNA. This
creates two copies of the DNA in a reproducing cell, and they will need to
be separated from each other. However, keeping one copy of DNA in the
original cell and simply pushing the other one out would not work,
2020-21
Science
128
?
because the copy pushed out would not have any organised cellular
structure for maintaining life processes. Therefore, DNA copying is
accompanied by the creation of an additional cellular apparatus, and
then the DNA copies separate, each with its own cellular apparatus.
Effectively, a cell divides to give rise to two cells.
These two cells are of course similar, but are they likely to be
absolutely identical? The answer to this question will depend on how
accurately the copying reactions involved occur. No bio-chemical reaction
is absolutely reliable. Therefore, it is only to be expected that the process
of copying the DNA will have some variations each time. As a result, the
DNA copies generated will be similar, but may not be identical to the
original. Some of these variations might be so drastic that the new DNA
copy cannot work with the cellular apparatus it inherits. Such a newborn
cell will simply die. On the other hand, there could still be many other
variations in the DNA copies that would not lead to such a drastic
outcome. Thus, the surviving cells are similar to, but subtly different
from each other. This inbuilt tendency for variation during reproduction
is the basis for evolution, as we will discuss in the next chapter.
8.1.1 The Importance of Variation
Populations of organisms fill well-defined places, or niches, in the
ecosystem, using their ability to reproduce. The consistency of DNA
copying during reproduction is important for the maintenance of body
design features that allow the organism to use that particular niche.
Reproduction is therefore linked to the stability of populations of species.
However, niches can change because of reasons beyond the control
of the organisms. Temperatures on earth can go up or down, water levels
can vary, or there could be meteorite hits, to think of a few examples. If
a population of reproducing organisms were suited to a particular niche
and if the niche were drastically altered, the population could be wiped
out. However, if some variations were to be present in a few individuals
in these populations, there would be some chance for them to survive.
Thus, if there were a population of bacteria living in temperate waters,
and if the water temperature were to be increased by global warming,
most of these bacteria would die, but the few variants resistant to heat
would survive and grow further. Variation is thus useful for the survival
of species over time.
QUESTIONS
1. What is the importance of DNA copying in reproduction?
2. Why is variation beneficial to the species but not necessarily
for the individual?
2020-21
How do Organisms Reproduce? 129
Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Binary fission in Amoeba
Activity 8.2 Activity 8.2 Activity 8.2 Activity 8.2 Activity 8.2
8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE
ORGANISMS ORGANISMS ORGANISMS ORGANISMS ORGANISMS
Activity 8.1 Activity 8.1 Activity 8.1 Activity 8.1 Activity 8.1
n Dissolve about 10 gm of sugar in 100 mL of water.
n Take 20 mL of this solution in a test tube and add a pinch of yeast
granules to it.
n Put a cotton plug on the mouth of the test tube and keep it in a
warm place.
n After 1 or 2 hours, put a small drop of yeast culture from the test
tube on a slide and cover it with a coverslip.
n Observe the slide under a microscope.
n Wet a slice of bread, and keep it in a cool, moist and dark place.
n Observe the surface of the slice with a magnifying glass.
n Record your observations for a week.
Compare and contrast the ways in which yeast grows in the first
case, and how mould grows in the second.
Having discussed the context in which reproductive processes work,
let us now examine how different organisms actually reproduce. The
modes by which various organisms reproduce depend on the body
design of the organisms.
8.2.1 Fission
For unicellular organisms, cell division, or fission, leads to the creation
of new individuals. Many different patterns of fission have been observed.
Many bacteria and protozoa simply split into two equal halves during
cell division. In organisms such as Amoeba, the splitting of the two cells
during division can take place in any plane.
Activity 8.3 Activity 8.3 Activity 8.3 Activity 8.3 Activity 8.3
n Observe a permanent slide of
Amoeba under a microscope.
n Similarly observe another
permanent slide of Amoeba
showing binary fission.
n Now, compare the observations of
both the slides.
However, some unicellular organisms
show somewhat more organisation of their
bodies, such as is seen in Leishmania (which
cause kala-azar), which have a whip-like
structure at one end of the cell. In such
organisms, binary fission occurs in a definite orientation in relation to
Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Binary fission in Leishmania
  (a)         (b)          (c)          (d)            (e)                        (f)
2020-21
Page 4


How do Organisms
Reproduce?
8 CHAPTER
B
efore we discuss the mechanisms by which organisms reproduce,
let us ask a more basic question – why do organisms reproduce?
After all, reproduction is not necessary to maintain the life of an individual
organism, unlike the essential life processes such as nutrition,
respiration, or excretion. On the other hand, if an individual organism is
going to create more individuals, a lot of its energy will be spent in the
process. So why should an individual organism waste energy on a process
it does not need to stay alive? It would be interesting to discuss the
possible answers in the classroom!
Whatever the answer to this question, it is obvious that we notice
organisms because they reproduce. If there were to be only one, non-
reproducing member of a particular kind, it is doubtful that we would
have noticed its existence. It is the large numbers of organisms belonging
to a single species that bring them to our notice. How do we know that
two different individual organisms belong to the same species? Usually,
we say this because they look similar to each other. Thus, reproducing
organisms create new individuals that look very much like themselves.
8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORGANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREATE EXA TE EXA TE EXA TE EXA TE EXACT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF
THEMSEL THEMSEL THEMSEL THEMSEL THEMSELVES? VES? VES? VES? VES?
Organisms look similar because their body designs are similar. If body
designs are to be similar, the blueprints for these designs should be
similar. Thus, reproduction at its most basic level will involve making
copies of the blueprints of body design. In Class IX, we learnt that the
chromosomes in the nucleus of a cell contain information for inheritance
of features from parents to next generation in the form of DNA (Deoxyribo
Nucleic Acid) molecules. The DNA in the cell nucleus is the information
source for making proteins. If the information is changed, different
proteins will be made. Different proteins will eventually lead to altered
body designs.
Therefore, a basic event in reproduction is the creation of a DNA
copy. Cells use chemical reactions to build copies of their DNA. This
creates two copies of the DNA in a reproducing cell, and they will need to
be separated from each other. However, keeping one copy of DNA in the
original cell and simply pushing the other one out would not work,
2020-21
Science
128
?
because the copy pushed out would not have any organised cellular
structure for maintaining life processes. Therefore, DNA copying is
accompanied by the creation of an additional cellular apparatus, and
then the DNA copies separate, each with its own cellular apparatus.
Effectively, a cell divides to give rise to two cells.
These two cells are of course similar, but are they likely to be
absolutely identical? The answer to this question will depend on how
accurately the copying reactions involved occur. No bio-chemical reaction
is absolutely reliable. Therefore, it is only to be expected that the process
of copying the DNA will have some variations each time. As a result, the
DNA copies generated will be similar, but may not be identical to the
original. Some of these variations might be so drastic that the new DNA
copy cannot work with the cellular apparatus it inherits. Such a newborn
cell will simply die. On the other hand, there could still be many other
variations in the DNA copies that would not lead to such a drastic
outcome. Thus, the surviving cells are similar to, but subtly different
from each other. This inbuilt tendency for variation during reproduction
is the basis for evolution, as we will discuss in the next chapter.
8.1.1 The Importance of Variation
Populations of organisms fill well-defined places, or niches, in the
ecosystem, using their ability to reproduce. The consistency of DNA
copying during reproduction is important for the maintenance of body
design features that allow the organism to use that particular niche.
Reproduction is therefore linked to the stability of populations of species.
However, niches can change because of reasons beyond the control
of the organisms. Temperatures on earth can go up or down, water levels
can vary, or there could be meteorite hits, to think of a few examples. If
a population of reproducing organisms were suited to a particular niche
and if the niche were drastically altered, the population could be wiped
out. However, if some variations were to be present in a few individuals
in these populations, there would be some chance for them to survive.
Thus, if there were a population of bacteria living in temperate waters,
and if the water temperature were to be increased by global warming,
most of these bacteria would die, but the few variants resistant to heat
would survive and grow further. Variation is thus useful for the survival
of species over time.
QUESTIONS
1. What is the importance of DNA copying in reproduction?
2. Why is variation beneficial to the species but not necessarily
for the individual?
2020-21
How do Organisms Reproduce? 129
Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Binary fission in Amoeba
Activity 8.2 Activity 8.2 Activity 8.2 Activity 8.2 Activity 8.2
8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE
ORGANISMS ORGANISMS ORGANISMS ORGANISMS ORGANISMS
Activity 8.1 Activity 8.1 Activity 8.1 Activity 8.1 Activity 8.1
n Dissolve about 10 gm of sugar in 100 mL of water.
n Take 20 mL of this solution in a test tube and add a pinch of yeast
granules to it.
n Put a cotton plug on the mouth of the test tube and keep it in a
warm place.
n After 1 or 2 hours, put a small drop of yeast culture from the test
tube on a slide and cover it with a coverslip.
n Observe the slide under a microscope.
n Wet a slice of bread, and keep it in a cool, moist and dark place.
n Observe the surface of the slice with a magnifying glass.
n Record your observations for a week.
Compare and contrast the ways in which yeast grows in the first
case, and how mould grows in the second.
Having discussed the context in which reproductive processes work,
let us now examine how different organisms actually reproduce. The
modes by which various organisms reproduce depend on the body
design of the organisms.
8.2.1 Fission
For unicellular organisms, cell division, or fission, leads to the creation
of new individuals. Many different patterns of fission have been observed.
Many bacteria and protozoa simply split into two equal halves during
cell division. In organisms such as Amoeba, the splitting of the two cells
during division can take place in any plane.
Activity 8.3 Activity 8.3 Activity 8.3 Activity 8.3 Activity 8.3
n Observe a permanent slide of
Amoeba under a microscope.
n Similarly observe another
permanent slide of Amoeba
showing binary fission.
n Now, compare the observations of
both the slides.
However, some unicellular organisms
show somewhat more organisation of their
bodies, such as is seen in Leishmania (which
cause kala-azar), which have a whip-like
structure at one end of the cell. In such
organisms, binary fission occurs in a definite orientation in relation to
Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Binary fission in Leishmania
  (a)         (b)          (c)          (d)            (e)                        (f)
2020-21
Science
130
these structures. Other single-celled organisms, such as the malarial
parasite, Plasmodium, divide into many daughter cells simultaneously
by multiple fission.
Yeast, on the other hand, can put out small buds that separate and
grow further, as we saw in Activity 8.1.
8.2.2 Fragmentation
Figure 8.2 Figure 8.2 Figure 8.2 Figure 8.2 Figure 8.2
Multiple fission in
Plasmodium
Activity 8.4 Activity 8.4 Activity 8.4 Activity 8.4 Activity 8.4
n Collect water from a lake or pond that appears dark green and
contains filamentous structures.
n Put one or two filaments on a slide.
n Put a drop of glycerine on these filaments and cover it with a coverslip.
n Observe the slide under a microscope.
n Can you identify different tissues in the Spirogyra filaments?
In multi-cellular organisms with relatively simple body organisation,
simple reproductive methods can still work. Spirogyra, for example,
simply breaks up into smaller pieces upon maturation. These pieces or
fragments grow into new individuals. Can we work out the reason for
this, based on what we saw in Activity 8.4?
This is not true for all multi-cellular organisms. They cannot simply
divide cell-by-cell. The reason is that many multi-cellular organisms, as
we have seen, are not simply a random collection of cells. Specialised
cells are organised as tissues, and tissues are organised into organs,
which then have to be placed at definite positions in the body. In such a
carefully organised situation, cell-by-cell division would be impractical.
Multi-cellular organisms, therefore, need to use more complex ways of
reproduction.
A basic strategy used in multi-cellular organisms is that different
cell types perform different specialised functions. Following this general
pattern, reproduction in such organisms is also the function of a specific
cell type. How is reproduction to be achieved from a single cell type, if
the organism itself consists of many cell types? The answer is that there
must be a single cell type in the organism that is capable of growing,
proliferating and making other cell types under the right circumstances.
8.2.3 Regeneration
Many fully differentiated organisms have the ability to give rise to new
individual organisms from their body parts. That is, if the individual is
somehow cut or broken up into many pieces, many of these pieces grow
into separate individuals. For example, simple animals like Hydra and
Planaria can be cut into any number of pieces and each piece grows
into a complete organism. This is known as regeneration (see Fig. 8.3).
Regeneration is carried out by specialised cells. These cells proliferate
and make large numbers of cells. From this mass of cells, different cells
undergo changes to become various cell types and tissues. These changes
2020-21
Page 5


How do Organisms
Reproduce?
8 CHAPTER
B
efore we discuss the mechanisms by which organisms reproduce,
let us ask a more basic question – why do organisms reproduce?
After all, reproduction is not necessary to maintain the life of an individual
organism, unlike the essential life processes such as nutrition,
respiration, or excretion. On the other hand, if an individual organism is
going to create more individuals, a lot of its energy will be spent in the
process. So why should an individual organism waste energy on a process
it does not need to stay alive? It would be interesting to discuss the
possible answers in the classroom!
Whatever the answer to this question, it is obvious that we notice
organisms because they reproduce. If there were to be only one, non-
reproducing member of a particular kind, it is doubtful that we would
have noticed its existence. It is the large numbers of organisms belonging
to a single species that bring them to our notice. How do we know that
two different individual organisms belong to the same species? Usually,
we say this because they look similar to each other. Thus, reproducing
organisms create new individuals that look very much like themselves.
8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORG 8.1 DO ORGANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREA ANISMS CREATE EXA TE EXA TE EXA TE EXA TE EXACT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF CT COPIES OF
THEMSEL THEMSEL THEMSEL THEMSEL THEMSELVES? VES? VES? VES? VES?
Organisms look similar because their body designs are similar. If body
designs are to be similar, the blueprints for these designs should be
similar. Thus, reproduction at its most basic level will involve making
copies of the blueprints of body design. In Class IX, we learnt that the
chromosomes in the nucleus of a cell contain information for inheritance
of features from parents to next generation in the form of DNA (Deoxyribo
Nucleic Acid) molecules. The DNA in the cell nucleus is the information
source for making proteins. If the information is changed, different
proteins will be made. Different proteins will eventually lead to altered
body designs.
Therefore, a basic event in reproduction is the creation of a DNA
copy. Cells use chemical reactions to build copies of their DNA. This
creates two copies of the DNA in a reproducing cell, and they will need to
be separated from each other. However, keeping one copy of DNA in the
original cell and simply pushing the other one out would not work,
2020-21
Science
128
?
because the copy pushed out would not have any organised cellular
structure for maintaining life processes. Therefore, DNA copying is
accompanied by the creation of an additional cellular apparatus, and
then the DNA copies separate, each with its own cellular apparatus.
Effectively, a cell divides to give rise to two cells.
These two cells are of course similar, but are they likely to be
absolutely identical? The answer to this question will depend on how
accurately the copying reactions involved occur. No bio-chemical reaction
is absolutely reliable. Therefore, it is only to be expected that the process
of copying the DNA will have some variations each time. As a result, the
DNA copies generated will be similar, but may not be identical to the
original. Some of these variations might be so drastic that the new DNA
copy cannot work with the cellular apparatus it inherits. Such a newborn
cell will simply die. On the other hand, there could still be many other
variations in the DNA copies that would not lead to such a drastic
outcome. Thus, the surviving cells are similar to, but subtly different
from each other. This inbuilt tendency for variation during reproduction
is the basis for evolution, as we will discuss in the next chapter.
8.1.1 The Importance of Variation
Populations of organisms fill well-defined places, or niches, in the
ecosystem, using their ability to reproduce. The consistency of DNA
copying during reproduction is important for the maintenance of body
design features that allow the organism to use that particular niche.
Reproduction is therefore linked to the stability of populations of species.
However, niches can change because of reasons beyond the control
of the organisms. Temperatures on earth can go up or down, water levels
can vary, or there could be meteorite hits, to think of a few examples. If
a population of reproducing organisms were suited to a particular niche
and if the niche were drastically altered, the population could be wiped
out. However, if some variations were to be present in a few individuals
in these populations, there would be some chance for them to survive.
Thus, if there were a population of bacteria living in temperate waters,
and if the water temperature were to be increased by global warming,
most of these bacteria would die, but the few variants resistant to heat
would survive and grow further. Variation is thus useful for the survival
of species over time.
QUESTIONS
1. What is the importance of DNA copying in reproduction?
2. Why is variation beneficial to the species but not necessarily
for the individual?
2020-21
How do Organisms Reproduce? 129
Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Figure 8.1(a) Binary fission in Amoeba
Activity 8.2 Activity 8.2 Activity 8.2 Activity 8.2 Activity 8.2
8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE 8.2 MODES OF REPRODUCTION USED BY SINGLE
ORGANISMS ORGANISMS ORGANISMS ORGANISMS ORGANISMS
Activity 8.1 Activity 8.1 Activity 8.1 Activity 8.1 Activity 8.1
n Dissolve about 10 gm of sugar in 100 mL of water.
n Take 20 mL of this solution in a test tube and add a pinch of yeast
granules to it.
n Put a cotton plug on the mouth of the test tube and keep it in a
warm place.
n After 1 or 2 hours, put a small drop of yeast culture from the test
tube on a slide and cover it with a coverslip.
n Observe the slide under a microscope.
n Wet a slice of bread, and keep it in a cool, moist and dark place.
n Observe the surface of the slice with a magnifying glass.
n Record your observations for a week.
Compare and contrast the ways in which yeast grows in the first
case, and how mould grows in the second.
Having discussed the context in which reproductive processes work,
let us now examine how different organisms actually reproduce. The
modes by which various organisms reproduce depend on the body
design of the organisms.
8.2.1 Fission
For unicellular organisms, cell division, or fission, leads to the creation
of new individuals. Many different patterns of fission have been observed.
Many bacteria and protozoa simply split into two equal halves during
cell division. In organisms such as Amoeba, the splitting of the two cells
during division can take place in any plane.
Activity 8.3 Activity 8.3 Activity 8.3 Activity 8.3 Activity 8.3
n Observe a permanent slide of
Amoeba under a microscope.
n Similarly observe another
permanent slide of Amoeba
showing binary fission.
n Now, compare the observations of
both the slides.
However, some unicellular organisms
show somewhat more organisation of their
bodies, such as is seen in Leishmania (which
cause kala-azar), which have a whip-like
structure at one end of the cell. In such
organisms, binary fission occurs in a definite orientation in relation to
Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Figure 8.1(b) Binary fission in Leishmania
  (a)         (b)          (c)          (d)            (e)                        (f)
2020-21
Science
130
these structures. Other single-celled organisms, such as the malarial
parasite, Plasmodium, divide into many daughter cells simultaneously
by multiple fission.
Yeast, on the other hand, can put out small buds that separate and
grow further, as we saw in Activity 8.1.
8.2.2 Fragmentation
Figure 8.2 Figure 8.2 Figure 8.2 Figure 8.2 Figure 8.2
Multiple fission in
Plasmodium
Activity 8.4 Activity 8.4 Activity 8.4 Activity 8.4 Activity 8.4
n Collect water from a lake or pond that appears dark green and
contains filamentous structures.
n Put one or two filaments on a slide.
n Put a drop of glycerine on these filaments and cover it with a coverslip.
n Observe the slide under a microscope.
n Can you identify different tissues in the Spirogyra filaments?
In multi-cellular organisms with relatively simple body organisation,
simple reproductive methods can still work. Spirogyra, for example,
simply breaks up into smaller pieces upon maturation. These pieces or
fragments grow into new individuals. Can we work out the reason for
this, based on what we saw in Activity 8.4?
This is not true for all multi-cellular organisms. They cannot simply
divide cell-by-cell. The reason is that many multi-cellular organisms, as
we have seen, are not simply a random collection of cells. Specialised
cells are organised as tissues, and tissues are organised into organs,
which then have to be placed at definite positions in the body. In such a
carefully organised situation, cell-by-cell division would be impractical.
Multi-cellular organisms, therefore, need to use more complex ways of
reproduction.
A basic strategy used in multi-cellular organisms is that different
cell types perform different specialised functions. Following this general
pattern, reproduction in such organisms is also the function of a specific
cell type. How is reproduction to be achieved from a single cell type, if
the organism itself consists of many cell types? The answer is that there
must be a single cell type in the organism that is capable of growing,
proliferating and making other cell types under the right circumstances.
8.2.3 Regeneration
Many fully differentiated organisms have the ability to give rise to new
individual organisms from their body parts. That is, if the individual is
somehow cut or broken up into many pieces, many of these pieces grow
into separate individuals. For example, simple animals like Hydra and
Planaria can be cut into any number of pieces and each piece grows
into a complete organism. This is known as regeneration (see Fig. 8.3).
Regeneration is carried out by specialised cells. These cells proliferate
and make large numbers of cells. From this mass of cells, different cells
undergo changes to become various cell types and tissues. These changes
2020-21
How do Organisms Reproduce? 131
Figure 8.3 Figure 8.3 Figure 8.3 Figure 8.3 Figure 8.3 Regeneration in Planaria
take place in an organised
sequence referred to as
development. However,
regeneration is not the same
as reproduction, since most
organisms would not
normally depend on being cut
up to be able to reproduce.
8.2.4 Budding
Organisms such as Hydra
use regenerative cells for
reproduction in the process of
budding. In Hydra, a bud
develops as an outgrowth due
to repeated cell division at one
specific site (Fig. 8.4). These buds develop into tiny individuals and when
fully mature, detach from the parent body and become new independent
individuals.
Figure 8.4 Figure 8.4 Figure 8.4 Figure 8.4 Figure 8.4 Budding in Hydra
8.2.5 Vegetative Propagation
There are many plants in which parts like the root, stem and leaves
develop into new plants under appropriate conditions. Unlike in most
animals, plants can indeed use such a mode for reproduction. This
property of vegetative propagation is used in methods such as layering
or grafting to grow many plants like sugarcane, roses, or grapes for
agricultural purposes. Plants raised by vegetative propagation can bear
flowers and fruits earlier than those produced from seeds. Such methods
also make possible the propagation of plants such as banana, orange,
rose and jasmine that have lost the capacity to produce seeds. Another
advantage of vegetative propagation is that all plants produced are
genetically similar enough to the parent plant to have all its
characteristics.
2020-21
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