How do Organisms Reproduce?
Before 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 2 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.
DO ORGANISMS CREATE EXACT COPIES OF THEMSELVES?
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 2
copies of the DNA in a reproducing cell, & they will need to be separated
from each other. However, keeping one copy of DNA in the original cell &
simply pushing the other one out would not work, 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
1
,additional cellular apparatus, & then the DNA copies separate, each with
its own cellular apparatus. Effectively, a cell divides to give rise to 2 cells.
These 2 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.
2
,● 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.
3
, 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 &
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, & 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 & grow further. Variation is thus
useful for the survival of species over time.
4
Before 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 2 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.
DO ORGANISMS CREATE EXACT COPIES OF THEMSELVES?
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 2
copies of the DNA in a reproducing cell, & they will need to be separated
from each other. However, keeping one copy of DNA in the original cell &
simply pushing the other one out would not work, 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
1
,additional cellular apparatus, & then the DNA copies separate, each with
its own cellular apparatus. Effectively, a cell divides to give rise to 2 cells.
These 2 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.
2
,● 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.
3
, 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 &
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, & 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 & grow further. Variation is thus
useful for the survival of species over time.
4
