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CLASS : X CHAPTER 8HOW DO ORGANISMS REPRODUCE?
NOTES
Introduction
All living organisms multiply or reproduce and produce offspring of a similar kind. Reproduction is an
essential process for the existence of a species and the continuation of life.
Reproduction
The production of new organisms from the existing organisms of the same species is
known as reproduction.
● Reproduction ensured continuity of life on earth.
● It is a bridge to hereditary transmission.
● It involves a continuation of characters from the parents to daughter cells
by copying of DNA (Deoxyribose Nucleic Acid) molecules present in the
chromosomes of the cell.
● Copying of DNAs is also not a foolproof exercise, even minute changes
bring about variation in the blue print of the offsprings.
● The useful variations are retained while the harmful ones do not go beyond.
● Actually, variations help the species to withstand drastic environmental
changes, thus save the species from becoming extinct and promotes its
survival for a longer time.
● This inbuilt tendency of variation is the “basis” for Evolution.
Asexual Reproduction
● It involves only one parent.
● There is no formation and fusion of gametes.
● The young ones formed are almost identical to each other as well as to the
parent cell.
● Asexual reproduction generally occurs during favourable environmental
conditions and when there is an abundance of food.
● It is a faster method of reproduction.
Types of Asexual Reproduction is Unicellular Organism
, (i) Binary Fission: Seen in bacteria, protozoa like Amoeba, Paramecium. (In these first
pseudopodia withdrawn (karyokinesis) the nucleus of the parent cell divides and then
the cytoplasm divides (cytokinesis) resulting in the formation of two daughter cells). It
occurs during highly favourable conditions. The cell division can occur in any plane as in
case of Amoeba. However, organisms like Leishmania. (cause Kala-azar), which have a
whip like flagella at one end, binary fission occurs in a definite orientation in relation to
the flagellum.
Cytokinesis: Division of cytoplasm.
Karyokinesis: Division of Nucleus.
(ii) Multiple Fission: Seen in Plasmodium, (a malarial parasite). In this during
unfavourable conditions, the parent cell develops a thick resistant wall around itself
forming a cyst. Within the wall, the cytoplasm divides many times to form many
plasmodia. When conditions become favourable, the cyst wall breaks and the
Plasmodium are released.
(iii) Budding: Seen in Yeast (a fungus). The parent yeast cell develops a protrusion or an
outgrowth at its upper end. The nucleus of the parent cell divides and one of them
moves into the outgrowth which grows bigger and finally separates from the parent cell
to lead an independent existence. Very often if the conditions are highly favourable, a
chain of buds is formed.
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