Read Chapter 1st Of Class 12th Reproduction in Organism

Read and Learn Note in Simple way
Of Biology Subject

Chapter
Reproduction in Organisms

Asexual Reproduction
● The period through which a certain organism lives is known
as its life span.
● Reproduction is the process by which every organism
ensures its continuity.
● It is the process through which organisms produce young
ones, which in turn mature to give rise to their young ones.
● Reproduction can be:
○ Asexual − Only one individual is involved
○ Sexual − Two individuals (male and female) are
involved
Asexual Reproduction
● In this type, a single parent can produce offspring.
● The produced offspring are clones of each other (i.e.,
identical to each other and to the parent).
● It is commonly seen in unicellular organisms belonging to
protista and monera.
● Here, the cell division itself is the mode of reproduction.
Means of Asexual Reproduction
● Binary Fission​ − In this process, the cell divides into
halves, and each half develops into an adult (example:
Amoeba, Paramecium).

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● Budding​ − In this process, the cell divides unequally to
form buds, which remain attached to the parent initially, and
then detach and develop into a mature cell (example:
yeast).
● Formation of specialized structures
○ Conidia − (Example: Penicillium)
○ Gemmules − (Example: Sponges)
○ Buds − (Example: Hydra)
○ Zoospores − Microscopic, motile spores (Example:
Algae)
● Vegetative propagation​ − It means of asexual
reproduction in plants. Different structures are capable ofgiving rise to new plants.
○ Runner − (Example: Gladiolus)
○ Rhizome − (Example: Ginger)
○ Sucker
○ Tuber − (Example: Potato)
○ Offset
○ Bulb − (Example: Onion)
Sexual Reproduction: Pre-Fertilisation Events
● Sexual reproduction involves the formation of the male and
female gametes in either the same individual or two
individuals. These gametes fuse to form a zygote, which
develops into a new individual.
● Offspring are not identical to each other or to the parents.
So, sexual reproduction gives rise to diversity among living organisms.
● All organisms pass through two stages.
○ Juvenile phase − Period of growth; non reproductive
○ Vegetative phase or reproductive phase
● In non-primate mammals like rats, sheep, dogs, cows and
tigers, the cyclic change in the activities of the ovaries and
the oviduct is called the oestrus cycle​; in primates like
monkeys, apes and humans, it is called the menstrual
cycle​.
● Certain mammals are called continuous breeders​ since
they can reproduce throughout their reproductive phase,
while some are called seasonal breeders​ since they can
reproduce only in the favourable seasons.
Events in Sexual Reproduction
● Organisms reproducing sexually exhibit certain events.
These are:
○ Pre-fertilisation events
○ Fertilisation events
○ Post-fertilisation events
Pre-Fertilisation Events
● Events taking place before the fusion of the gametes
● Consist of:
○ Gametogenesis
○ Gamete transfer
Gametogenesis
● Process of formation of gametes (male and female)
● Gametes are haploid
● In some organisms (like algae), they are almost similar
(homo or isogametes), and cannot be categorised as male
and female gametes.
● In others, the two gametes are morphologically and
physiologically different (heterogametes), and are of two
types—antherozoid or sperm (male gamete) and egg or
ovum (female gamete).
● In some organisms both the sexes are present in the same
individual (monoecious or homothallic), and in others, they
are present in two individuals (dioecious or heterothallic).
In a unisexual flower, the male flower is called staminate and
the female flower is called pistillate.
● Gamete formation takes place by cell division.
In haploid parents, it is by mitosis; in diploid parents, it is by
meiosis, with specialised cells called meiocytes undergoing
meiosis.
.Gamete Transfer
● For their fusion to take place, the gametes need to be
transferred.
● In most organisms, the male gametes are motile, while the
female gametes are non-motile, and the male gametes need
a medium for their movement. A large number of male
gametes do not make it to the female gamete, and hence,
several thousands of male gametes are produced to
overcome this loss.
● In angiosperms, the pollen grain carries the male gamete
and the ovule carries the female gamete.
● Pollen grains are produced in the anther and need to be
transferred to the stigma for fertilisation to occur. This is
easy in monoecious plants as both the anther and the
stigma are present close by; in dioecious plants, it takes
place by pollination.

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Sexual Reproduction: Fertilisation Events
● Fertilisation is the most important event in sexual
reproduction.
● This process is also called syngamy​ and leads to the
formation of the zygote.
● However, in some organisms, zygote formation takes place
without fertilisation, and is known as parthenogenesis
(occurs in rotifers, honeybees and some lizards).
● In most aquatic organisms and amphibians, fertilisation
takes place outside their body (in the water), and is termed
as external fertilization​.Their eggs and offspring are
highly vulnerable to predators and this threatens their
survival up to adulthood.
● In most terrestrial organisms, fertilisation is internal, i.e., it
takes place inside the female body. In this process, the male
gamete is motile and reaches the female gamete to fuse
with it, thereby forming zygote. Male gametes are produced
in large numbers.

Sexual Reproduction: Post-Fertilisation Events
● Events taking place after fertilisation are called
post-fertilisation events.
Zygote
● The haploid gametes fuse to form a diploid zygote in all
organisms.
● In external fertilisation, a zygote is formed in an external
medium, and in internal fertilisation, a zygote is formed
inside the individual.
● The development of a zygote depends upon the life cycle of
an organism and its surroundings. In some organisms, the
zygote does not develop immediately, and develops a thick
wall around itself. This wall is resistant to damage and
desiccation.
Embryogenesis
● It is the process of development of the embryo from the
zygote.
● The zygote undergoes cell division and differentiation.
● Cell division increases the number of cells of the embryo,
and cell differentiation helps the cells undergo modifications
to form specialised tissues and organs.
● Animals can be grouped into two categories based on how
and where the development of the zygote takes place. These
categories are:
○ Oviparous − The fert ​ ilised egg is covered by a
calcareous shell and is released into the outside
environment. The development takes place inside the
egg and the young one hatches out (example: birds
and reptiles).

○ Viviparous − The develo ​ pment of the zygote takes
place inside the female body, and the developed young
one is delivered outside (example: mammals, including
humans).
● In flowering plants, the zygote is formed inside the ovule.
○ Zygote → Develops into → Embryo
○ Ovule → Develops into → Seed
○ Ovary → Develops into → Fruit → Contains → Seeds →Disperse and germinate to form new plants

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