NCERT Book Class 10 Science Chapter 7 How do Organisms Reproduce?

CHAPTER 7
How do Organisms
Reproduce?

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.

7.1 DO ORG ANISMS CREA
ORGANISMS TE EXA
CREATE CT COPIES OF
EXACT
THEMSELVES?
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 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,