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,
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,
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 QUESTIONS QUESTIONS QUESTIONS 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?