Page 119 - NCERT Science Class 10 English Medium

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.

7.1.1 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.

Q U E S T I O N S

?
1. What is the importance of DNA copying in reproduction?
2. Why is variation beneficial to the species but not necessarily
for the individual?

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