CHAPTER – 1
Reproduction
in Organism
Asexual Reproduction
·
The
period through which a certain organism lives is known as life span ( from
birth to natural death)
·
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 o
· Sexual -
Two individuals (male and female) are
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 organism.
All
organisms pass through two stages
1.
Juvenile phase - Period of
growth; non reproductive ( all organism
have to reach a certain stage of growth and maturity in their life , before
they can reproduce sexually)
2.
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 –
1.
Pre-fertilisation events
2.
Fertilisation events
3. Post-fertilisation
1. Pre-Fertilisation
Events –
·
Events taking place before the fusion of the gametes
Consist
of:
ü Gametogenesis
ü 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 organism
, 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 take place by pollination.
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 , fertilization takes place outside their body ( in
the water) , and its 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 – fertilization event.
Zygote -
Viviparous - The development 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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