REPRODUCTION
IN ORGANISMS
Reproduction:
defined as a biological process in
which an organism gives rise to young ones (offspring) similar to itself.
Asexual
reproduction:
·
Off
springs produced by single parents.
·
Without
involvement of gamete formation
·
Offsprings
are genetically identical to their parents.
Methods of
asexual reproduction:
·
Cell
division as a method of asexual reproduction as in Protista and monera.
·
Binary
fission e.g. Amoeba, Paramecium.
·
Budding:
e.g. yeast.
·
Asexual
reproductive structures:
o
Zoospores: aquatic fungi, Chlamydomonas.
o
Conidia: Penicillium.
o
Bud: Hydra
o
Gemmules: sponges.
·
Vegetative
propagation units in plant: (Vegetative propagules)
o
Runner,
rhizome, sucker, tuber, offset, bulb.
Sexual
reproduction:
·
Involvement
of single or two individual.
·
Production
of male and female gametes ( haploid)
·
Gametes
fused to form a diploid zygote.
·
Zygotes
developed into new organism.
·
The
offsprings are not genetically identical with their parents.
Features
of sexual reproduction:
·
Period
between birth and sexual maturity is called juvenile phase. It is known
as vegetative phase in plant.
·
Bamboo
species flower only once in their life time generally after 50-100 yr.
·
Strobilanthus
kunthiana (neelakranji) flowers once in 12
years.
·
Oestrus
cycle: cyclical changes during
reproduction in non-primate mammal like cows, sheep, rats, deers, dogs, tiger
etc.
·
Menstrual
cycle: cyclical changes during
reproduction in primate mammals like monkeys, ape, and humans.
·
Seasonal
breeders: reproductive cycle takes place in
favourable seasons as in wild animals.
·
Continuous
breeders: reproductively active throughout
their reproductive phase.
Pre-fertilization
events:
·
Process
of gamete formation is gametogenesis.
·
Two
gametes are similar in appearance are called homogametes (isogametes).
·
Gametes
produced are of two morphologically distinct types called heterogametes.
·
Male
gamete is called antherozoids or sperm and the female gamete is
called ovum or egg.
Sexuality
in organism:
·
Plant
having both male and female sex organ called homothallic or monoecious.
·
Plants
having only one sex organ is called heterothallic or dioecious.
·
In
flowering plants, the unisexual male flower is staminate, i.e. bearing
staments, while the female is pistillate or bearing pistils.
·
Animal
having one type of reproductive system, called unisexual.
·
Animal
having both male and female reproductive system, called hermaphrodite or
bisexual.
Cell
division during gamete formation:
·
Gametes
in all heterogametic species two types namely male and female.
·
Gametes
are always haploid irrespective of parent’s ploidy.
·
A
haploid parent produces gametes by mitotic division.
·
Diploid
parent produces gametes by meiotic division.
·
In
diploid organisms specialized cells called meiocytes (gamete mother
cell) undergo meiosis to produce haploid gametes.
Gamete
transfer:
·
Male
and female gamete must be physically brought together to facilitate fusion
called fertilization.
·
In
most cases male gametes are motile, female gametes are non-motile.
·
In
case of few fungi and algae, both male and female gametes are motile.
·
In
most cases water is the medium for gamete transfer.
·
Male
gametes are produced in several thousand times the number of female gametes
produced to compensate the loss during transfer.
Fertilization:
·
Fusion
of male and female gamete is called fertilization or syngamy.
·
The
female gamete undergoes development to form new organism without fertilization.
This phenomenon is called parthenogenesis.
·
Gametic
fusion takes place outside the body i.e. water is called external
fertilization.
·
Their
must be synchrony of gamete release, large number of gametes released to
enhance the chance of fertilization.
·
Enable
the individual to produce large number of offsprings.
·
A
major disadvantage is that the offsprings are extremely vulnerable to
predators.
·
Fertilization
takes place inside the body is called internal fertilization.
Zygote:
·
Formation
of zygote after fertilization is universal in all sexually reproducing
organisms.
·
Zygote
is formed usually in water in case of external fertilization.
·
Zygote
is formed inside the body of the organism in internal fertilization.
·
Zygote
of fungi and algae develops a thick wall that is resistant to dessication and
damage.
·
Organism
with haplontic life cycle, zygote undergoes meiosis to produce haploid spores.
Embryogenesis:
·
Development
of zygote into an embryo is called embryogenesis.
·
Zygote
undergoes cell division (mitosis) and cell differentiation.
·
Oviparous
animal which lays eggs and development takes place inside egg.
·
Viviparous
animal gives birth to the young. The development takes place inside the body of
the female.
·
In
plants:
o
Zygote developed into embryo.
o
Ovule developed into seed
o
Integument
of the ovule developed
into seed coat.
o
Ovary developed into fruit.
o
Ovary
wall developed into pericarp.
