Page 124 - NCERT Science Class 10 English Medium

All the modes of reproduction that we have discussed so far allow
new generations to be created from a single individual. This is known as
asexual reproduction.

Q U E S T I O N S

1. How does binary fission differ from multiple fission?
2. How will an organism be benefited if it reproduces through spores?
3. Can you think of reasons why more complex organisms cannot give ?
rise to new individuals through regeneration?
4. Why is vegetative propagation practised for growing some types of
plants?
5. Why is DNA copying an essential part of the process of reproduction?

SEXUAL REPRODUCTION
REPRODUCTION
7.3 SEXUAL REPRODUCTIONSEXUAL REPRODUCTION
7.3 SEXUAL REPRODUCTION
7.3
7.3 SEXUAL
7.3
We are also familiar with modes of reproduction that depend on the
involvement of two individuals before a new generation can be created.
Bulls alone cannot produce new calves, nor can hens alone produce
new chicks. In such cases, both sexes, males and females, are needed to
produce new generations. What is the significance of this sexual mode
of reproduction? Are there any limitations of the asexual mode of
reproduction, which we have been discussing above?
7.3.1 Why the Sexual Mode of Reproduction?

The creation of two new cells from one involves copying of the DNA as
well as of the cellular apparatus. The DNA copying mechanism, as we
have noted, cannot be absolutely accurate, and the resultant errors are
a source of variations in populations of organisms. Every individual
organism cannot be protected by variations, but in a population,
variations are useful for ensuring the survival of the species. It would
therefore make sense if organisms came up with reproductive modes
that allowed more and more variation to be generated.
While DNA-copying mechanisms are not absolutely accurate, they
are precise enough to make the generation of variation a fairly slow
process. If the DNA copying mechanisms were to be less accurate, many
of the resultant DNA copies would not be able to work with the cellular
apparatus, and would die. So how can the process of making variants
be speeded up? Each new variation is made in a DNA copy that already
has variations accumulated from previous generations. Thus, two
different individuals in a population would have quite different patterns
of accumulated variations. Since all of these variations are in living
individuals, it is assured that they do not have any really bad effects.
Combining variations from two or more individuals would thus create
new combinations of variants. Each combination would be novel, since
it would involve two different individuals. The sexual mode of

How do Organisms Reproduce? 119

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