CHAPTER 01 > Reproduction in Organisms 1
CHAPTER > 01
Reproduction
in Organisms
NEET KEY NOTES
Å The period from birth to the natural death of an organism represents its lifespan. Whatever be the lifespan, death of every
individual organism is a certainty, i.e. no individual is immortal, except single-celled organisms.
Å Reproduction is a vital biological process by which living organisms produce new individuals of their own species.
It enables continuity of species. Reproduction is of two types asexual and sexual.
Asexual Reproduction
Å It involves the participation of a single parent to produce an offspring. As a result, the offspring produced are morphologically
and genetically similar to one another and also to their parents and can be referred to as clones.
Å The unit of reproduction is commonly formed from the somatic cells of the parent. Meiosis does not occur in asexual reproduction.
Å Asexual reproduction is common among single-celled organisms and in plants and animals with relatively simpler body
organisation.
Å Cell division itself is a mode of reproduction in protists, e.g. Amoeba and monerans. In bacteria, the parent cell divides into two
to give rise to new individuals.
Å It may take place by following methods
Asexual Reproduction
Fission Budding Spore formation Fragmentation Regeneration
Binary Parent body divides External An outgrowth
Spores are Breaking of Formation of the whole
into two daughter individuals, (bud) formed on the
propagules which parent body body of an organism from
e.g. Amoeba. parent body constricts
germinate to produce into fragments and a small fragment or by
and separates to form a
Multiple Parent body new individuals, each fragment replacement of the lost
new organism, e.g. Hydra,
divides into many e.g. fungi. develops into a new body part, e.g. sponge
yeast.
daughter individuals, organism, and Planaria.
e.g. Plasmodium. Internal Small, gemmules e.g. Spirogyra.
(internal buds) form new
individuals, e.g. Spongilla
(sponge).
Å During unfavourable conditions, the products of multiple fission become individually surrounded by resistant coats, i.e. cyst
(encystation) and gets released, once favourable condition arises sporulation occurs. It is seen in Amoeba.
, In Monera, Protista, Fungi and Algae, spores formed can
Sexual Reproduction
Å
be of following main types
n
Pseudopodiospore (with fine pseudopodia) – Amoeba Å In sexual reproduction, there is formation of male and
n
Zoospores (motile and flagellated) – Chlamydomonas female gametes either by same individual or by different
and Ulothrix individuals of opposite sex. These gametes fuse to form a
n
Conidia (non-motile) – Penicillium new cell called zygote, e.g. occurs in all flowering plants,
humans, etc.
n
Sporangiospores (non-motile) – Rhizopus
Å The period of growth from birth up to the reproductive
Å While in animals and other simple organisms, the term maturity in an organism is called as juvenile phase. In
asexual is used unambiguously, in plants, the term plants, this period of growth starting from seed
vegetative reproduction is frequently used. germination up to initiation of flowering is called
Å In plants, units of vegetative propagation such as runner, vegetative phase.
rhizome, sucker, tuber, offset, bulb, etc., have capability to Å The later phase, when the organisms start reproducing
give rise to new offspring. These structures are known as sexually, is called reproductive phase. The end of
vegetative propagules. reproductive phase is marked by the onset of another
Types of Vegetative Propagation in Plants phase called senescent phase which is the last phase in life
and their Description cycle after which the organism dies.
Types of Vegetative
Description
Å A few plants exhibit unusual flowering phenomenon;
Propagation in Plants some of them such as bamboo species flower only once in
1. Natural Vegetative Propagation their lifetime, generally after 50-100 years, produce large
Root (Root tubers) Buds on the roots grow into leafy shoots above number of fruits and die. Another plant, Strobilanthes
the ground and adventitious roots at the bases, kunthiana (Neelakurinji), flowers once in 12 years.
e.g. sweet potato, guava, etc. Å In non-primate mammals like cows, sheep, rats, deers,
Underground Modified stems possess buds dogs, tigers, etc., the cyclical changes during reproduction
which grow into new plants. Few types are
are called oestrous cycle whereas in primates (monkeys,
Suckers, e.g. mint, Chrysanthemum apes and humans) it is called as menstrual cycle.
Rhizomes, e.g. ginger, turmeric
Stem Å Many mammals exhibit such cycles only during
Bulbs, e.g. onion, garlic
Tubers, e.g. potato favourable seasons in their reproductive phase and are
Corms, e.g. Colocasia, Gladiolus. called seasonal breeders. While those are reproductively
Creeping Few types include active throughout their reproductive phase are called
Runners, e.g. Oxalis, water hyacinth continuous breeders.
Offsets, e.g. Pistia.
Aerial modified stems develop new plants when Events in Sexual Reproduction
stem segments fall on the ground, e.g. cacti,
Opuntia.
The events of sexual reproduction may be grouped into three
distinct stages namely, the pre-fertilisation, fertilisation and
Leaves Adventitious buds develop on leaves, detach
and form new plants, e.g. Bryophyllum. the post-fertilisation events.
2. Artificial Vegetative Propagation 1. Pre-fertilisation Events
Cutting A small piece of any plant organ is used for Å These are the events of sexual reproduction which take
propagation, e.g. leaves (Bryophyllum), roots
(tamarind), stems (sugarcane), etc. place before the fusion of gametes. These include
gametogenesis and gamete transfer.
Layering Roots are artificially induced on stem branches
before these detach from the parent plant, e.g.
jasmine, litchi, etc.
Gametogenesis
Å The process of formation of gametes is known as
Grafting Two plant parts, root system (stock) and shoot
system (scion) from two different plants are gametogenesis.
joined to form a new plant, e.g. rose, apple, etc. Å The reproductive units in sexual reproduction are
Micropropagation Involves culturing of cells, tissues or organs to specialised cells called gametes.
form callus which later differentiates to form a
large number of plantlets.
Å Depending upon the morphological similarity, gametes
can be classified into homogametes or isogametes (in
Å Water hyacinth (scourage of the water bodies or Terror of Cladophora) and heterogametes (in human). The gametes
Bengal) propagates very quickly by vegetative mode and are generally of two kinds, i.e. male (antherozoid or
drains out dissolved O2 from water bodies. sperm) and female (egg or ovum).
NEET KEY NOTES
, Å Sexuality in organisms On the basis of location of Å When the female gamete undergoes development to form
reproductive structure, sexuality in organisms can be new organisms without fertilisation, the process is called
classified as follows as parthenogenesis, e.g. rotifers, honeybees, some lizards
n
Homothallic and monoecious Both male and female and birds (turkey).
reproductive structures are present on same individual Å Depending upon, where does the syngamy occur,
(bisexual condition), e.g. in several fungi like Mucor and fertilisation may be of two types
plants like maize, cucurbits and coconuts. n
External fertilisation It is the fusion of gametes that
n
Heterothallic and dioecious Male and female take place outside the body of an organism in external
reproductive structures are present on separate medium such as water, e.g. in bony fishes, frogs, etc.
individuals (unisexual condition), e.g. papaya and date A large number of gametes are released in the
palm. In flowering plants, the unisexual male flower is surrounding medium by such animals.
staminate, while female is pistillate. n
Internal fertilisation It is the fusion of gametes that
n
Bisexual animals have both male and female take place inside the body, e.g. in fungi, higher
reproductive organs in single individual, e.g. animals such as birds, mammals and majority of
earthworm, sponge, leech, tapeworm, etc. These are also plants. The number of ova produced is less, but a large
called as hermaphrodites. number of male gametes are formed, as many of them
n
Unisexual animals have male and female reproductive fail to reach the ova.
organs in different individuals, e.g. cockroach, human,
dog, etc. 3. Post-fertilisation Events
After the fusion of male and female gametes, following
Å Cell division during gamete formation Gametes are
processes occur
always haploid, but they divide either by mitotic or meiotic
division. The organisms belonging to Monera, Fungi, Algae Zygote
and Bryophyta have haploid plant body. These organisms Å It is the beginning of new life. It is always diploid and
produce gametes by mitotic division. ensures the continuity of race from generation to
Å In pteridophytes, gymnosperms, angiosperms and most of generation.
the animals including humans, the parental body is Å Organism such as fungi, develops a thick wall around it
diploid. In these organisms, specialised cells called that is resistant to external damaging factors.
meiocytes or gamete mother cells undergo meiosis that
results in the formation of haploid gametes.
Å Zygote divides by meiosis to form haploid spores that
grow into haploid individuals in case of organisms which
Gamete Transfer lead haplontic life cycle, e.g. Volvox, Spirogyra, etc.
Å It occurs in various ways in different organisms. Å In organisms with diplontic life cycle, zygote undergoes
n
In most organisms, male gamete is motile and female mitotic division and develops into an embryo, e.g. all
gamete is non-motile. flowering plants.
n
Both gametes are motile in few fungi and algae. Embryogenesis
n
In algae, bryophytes and pteridophytes, gamete transfer Å It is the process of development of embryo from zygote.
takes place through water by producing many male Å The events which occur in animals during embryogenesis
gametes to ensure fertilisation because during gamete are
transfer, large number of male gametes are lost. In n
Mitosis (cell division or cleavage) occurs leading to
bryophytes and pteridophytes, male gametes are known
the growth of embryo.
as antherozoid.
n
Cells of embryo undergo differentiation and attain
n
In self-fertilising or bisexual plants, e.g. peas, gamete
specific shape, size and function.
transfer is easy as the anthers and stigma are closely
located. In cross-fertilising plants, gamete transfer occurs
n
Cell differentiation leads to the production of different
by pollination. It is the process of transfer of pollen tissues, organs and organ systems.
grains from anther to stigma, e.g. dioecious plants. Å Depending on the development of embryo the animals
can be
2. Fertilisation (i) Oviparous animals, e.g. reptiles and birds. Here,
Å It is the complete and permanent fusion of two gametes embryo develops outside the body of female (eggs).
from different parents or from the same parent to form a In these animals, the fertilised eggs are covered by
diploid zygote. This process is also called as syngamy. hard calcareous shell.
NEET KEY NOTES
