In this section we deals with reproduction in organism, simplified revision notes for NEET,board examnations
Reproduction is defined as a biological process in which an
organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death. Reproduction enables the continuity of the species, generation after generation.
The organism’s habitat, its internal physiology and several other factors
are collectively responsible for how it reproduces.
Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. When offspring is produced by a single parent with or without the involvement of gamete formation, the
reproduction is asexual. When two parents (opposite sex) participate in
the reproductive process and also involve fusion of male and female
gametes, it is called sexual reproduction.
1.1 ASEXUAL REPRODUCTION
In this method, a single individual (parent) is capable of producing
offspring. As a result, the offspring that are produced are not only
identical to one another but are also exact copies of their parent.
The term clone is used to describe such morphologically and
genetically similar individuals.
Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organisations. In Protists and Monerans, the organism or the parent cell divides by mitosis into two to give rise to new individuals Thus, in these organisms cell division is itself a mode of reproduction.
Many single-celled organisms reproduce by binary fission, where a
cell divides into two halves and each rapidly grows into an adult (e.g.,
Amoeba, Paramecium). In yeast, the division is unequal and small
buds are produced that remain attached initially to the parent cell
which, eventually gets separated and mature into new yeast
organisms (cells).
Under unfavourable condition the Amoeba withdraws
its pseudopodia and secretes a three-layered hard covering or cyst
around itself. This phenomenon is termed as encystation.
When favourable conditions return, the encysted Amoeba divides by multiple fission and produces many minute amoeba or pseudopodiospores;
the cyst wall bursts out, and the spores are liberated in the surrounding
medium to grow up into many amoebae. This phenomenon is known as sporulation.
Members of the Kingdom Fungi and simple plants such as algae
reproduce through special asexual reproductive structures
The most common of these structures are zoospores that usually are
microscopic motile structures. Other common asexual reproductive
structures are conidia (Penicillium). buds (Hydra) and gemmules (sponge).
While in animals and other simple organisms the term asexual is used
unambiguously, in plants, the term vegetative reproduction is frequently
used. In plants, the units of vegetative propagation such as runner,
rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new
offspring. These structures are called vegetative propagules.
the ‘terror of Bengal’, the aquatic plant ‘water hyacinth’
which is one of the most invasive weeds found growing wherever there is
standing water. It drains oxygen from the water, which leads to death of
fishes.
plants like potato, sugarcane, banana, ginger,from the nodes present in the modified stems of these plants. When the nodes come in contact with damp soil or water, they produce roots and new plants. Similarly, adventitious buds arise from the notches present at margins of leaves of Bryophyllum. This ability is fully exploited by gardeners and farmers for commercial propagation of such plants. It is interesting to note that asexual reproduction is the common method of reproduction in organisms that have a relatively simple organisation, like algae and fungi and that they shift to sexual method of reproduction just before the onset of adverse conditions.
1.2 SEXUAL REPRODUCTION
Sexual reproduction involves formation of the male and female gametes,
either by the same individual or by different individuals of the opposite
sex. These gametes fuse to form the zygote which develops to form the
new organism. It is an elaborate, complex and slow process as compared
to asexual reproduction. Because of the fusion of male and female gametes,
sexual reproduction results in offspring that are not identical to the parents
or amongst themselves.
A study of diverse organisms-plants, animals or fungi-show that
though they differ so greatly in external morphology, internal structure and physiology. when it comes to sexual mode of reproduction.
surprisingly. they share a similar pattern.
All organisms have to reach a certain stage of growth and maturity in
their life, before they can reproduce sexually. That period of growth is
called the juvenile phase. It is known as vegetative phase in plants.
This phase is of variable durations in different organisms.
The end of juvenile/vegetative phase which marks the beginning of
the reproductive phase can be seen easily in the higher plants when they
come to flower.
few plants exhibit unusual flowering phenomenon; some of them such as bamboo species flower only once in their life time, generally after 50-100 years, produce large number of fruits and die. Another plant, Strobilanthus kunthiana (neelakuranji), flowers once in 12 years. As many of you would know, this plant flowered during September-October 2006. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue stretches and attracted a large number of tourists. In animals, the juvenile phase is followed by morphological and physiological changes prior to active reproductive behaviour.
In non-primate mammals like cows, sheep, rats, deers, dogs, tiger,
ete., such cyclical changes during reproduction are called oestrus cycle
where as in primates (monkeys, apes, and humans) it is called menstrual
cycle. Many mammals, especially those living in natural, wild conditions
exhibit such cycles only during favourable seasons in their reproductive phase and are therefore called seasonal breeders. Many other mammals
are reproductively active throughout their reproductive phase and hence
are called continuous breeders.
That we all grow old (if we live long enough), is something that we
recognise. But what is meant by growing old? The end of reproductive
phase can be considered as one of the parameters of senescence or old
age. There are concomitant changes in the body (like slowing of
metabolism, etc.) during this last phase of life span. Old age ultimately
leads to death.
In both plants and animals, hormones are responsible for the transitions between the three phases. Interaction between hormones and
certain environmental factors regulate the reproductive processes and
the associated behavioural expressions of organisms.
Events in sexual reproduction :
After attainment of maturity, all sexually reproducing organisms exhibit events and processes that have remarkable fundamental similarity, even though the structures associated with sexual reproduction are indeed very different. The events of sexual reproduction though elaborate and complex, follow a regular sequence. Sexual reproduction is characterised by the fusion (or fertilisation) of the male and
female gametes, the formation of zygote and embryogenesis. For convenience these sequential events may be grouped into three distinct stages namely, the pre-fertilisation, fertilisation and the post-fertilisation events.
1.2.1 Pre-fertilisation Events
These include all the events of sexual reproduction prior to the fusion of
gametes. The two main pre-fertilisation events are gametogenesis and
gamete transfer.
1.2.1.1 Gametogenesis
As you are already aware, gametogenesis refers to the process of formation
of the two types of gametes – male and female. Gametes are haploid cells.
In some algae the two gametes are so similar in appearance
that it is not possible to categorise them into male and female gametes.
They are hence called homogametes (isogametes).However, in a majority of sexually reproducing organisms the gametes produced are of two morphologically distinct types (heterogametes). In
such organisms the male gamete is called the antherozoid or sperm
and the female gamete is called the egg or ovum.
Sexuality in organisms: Sexual reproduction in organisms generally
involves the fusion of gametes from two different individuals. But this
is not always true.
Plants may have both male and female reproductive structures in the
same plant (bisexual) or on different plants (unisexual). In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual condition and heterothallic and dioecious are the terms used to describe unisexual condition. In flowering plants, the unisexual male flower is staminate, i.e., bearing stamens, while the female is pistillate or bearing pistils. In some flowering plants, both male and female flowers may be present on the same individual (monoecious) or on separate individuals (dioecious).
Some examples of monoecious plants are cucurbits and coconuts and of
dioecious plants are papaya and date palm.Earthworms,sponge,
tapeworm and leech, typical examples of bisexual animals that possess
both male and female reproductive organs, are hermaphrodites.
Cockroach is an example of a unisexual species
Cell division during gamete formation : Gametes in all heterogametic
species are of two types namely, male and female. Gametes are haploid
though the parent plant body from which they arise may be either haploid
or diploid. A haploid parent produces gametes by mitotic division.
Several organisms belonging to monera, fungi, algae and bryophytes
have haploid plant body, but in organisms belonging to pteridophytes,
gymnosperms, angiosperms and most of the animals including human
beings, the parental body is dipleid. It is obvious that meiosis, the reduction
division, has to occur if a diploid body has to produce haploid gametes.
In diploid organisms, specialised cells called meiocytes (gamete mother
cell) undergo meiosis. At the end of meiosis, only one set of chromosomes
gets incorporated into each gamete.
1.2.1.2 Gamete Transfer
After their formation. male and female gametes must be physically
brought together to facilitate fusion (fertilisation). In a majority of organisms. male gamete is motile and the female gamete is stationary. Exceptions are a
few fungi and algae in which both types of gametes are motile. There is a need for a medium through which the male
gametes move. In several simple plants like algae, bryophytes and
pteridophytes, water is the medium through which this gamete transfer
takes place. A large number of the male gametes, however. fail to reach
the female gametes, To compensate this loss of male gametes during
transport. the number of male gametes produced is several thousand
times the number of female gametes produced.
In seed plants. pollen grains are the carriers of male
gametes and ovule have the egg. Pollen grains
produced in anthers therefore, have to be transferred
to the stigma before it can lead to fertilisation . In bisexual, self-fertilising plants, e.g.. peas,transfer of pollen grains to the stigma is relatively easy
as anthers and stigma are located close to each other;
pollen grains soon after they are shed, come in contact
with the stigma. But in cross pollinating plants
(including dioecious plants), a specialised event called
pollination facilitates transfer of pollen grains to the
stigma. Pollen grains germinate on the stigma and the
pollen tubes carrying the male gametes reach the ovule
and discharge male gametes near the egg. In dioecious
animals, since male and female gametes are formed in
different individuals, the organism must evolve a
special mechanism for gamete transfer. Successful
transfer and coming together of gametes is essential
for the most critical event in sexual reproduction, the fertilisation.
1.2.2 Fertilisation
The most vital event of sexual reproduction is perhaps
the fusion of gametes. This process called syngamy
results in the formation of a diploid zygote. The term
fertilisation is also often used for this process. The
terms syngamy and fertilisation are frequently used
though , interchangeably.However. it has to be mentioned here that in some
organisms like rotifers, honeybees and even some lizards
and birds (turkey). the female gamete undergoes development to form new organisms without fertilisation. This phenomenon is called parthenogenesis.
In most aquatic organisms, such as amajority of algae and fishes as well as amphibians, syngamy occurs in
the external medium (water), i.c.. outside the body of the organism. This
type of gametic fusion is called external fertilisation. Organisms
exhibiting external fertilisation show great synchrony between the sexes
and release a large number of gametes into the surrounding medium
(water) in order to enhance the chances of syngamy. This happens in the
bony fishes and frogs where a large number of offspring are produced. A
major disadvantage is that the offspring are extremely vulnerable to
predators threatening their survival up to adulthood.
In many terrestrial organisms. belonging to fungi. higher animals such
as reptiles. birds, mammals and in a majority of plants (bryophytes,
pteridophytes, gymnosperms and angiosperms). syngamy occurs inside
the body of the organism. hence the process is called internal fertilisation.
In all these organisms, egg is formed inside the female body where they
fuse with the male gamete. In organisms exhibiting internal fertilisation,
the male gamete is motile and has to reach the egg in order to fuse with it.
In these even though the number of sperms produced is very large, there
is a significant reduction in the number of eggs produced. In seed plants,
however, the non-motile male gametes are carried to female gamete by
pollen tubes.
1.2.3 Post-fertilisation Events
Events in sexual reproduction after the formation of zygote are called
post-fertilisation events.
The Zygote Formation of the diploid zygote is universal in all sexually reproducing organisms. In organisms with external fertilisation, zygote is formed in the external medium (usually water), whereas in those exhibiting internal fertilisation, zygote is formed inside the body of the organism.
Further development of the zygote depends on the type of life cycle
the organism has and the environment it is exposed to. In organisms
belonging to fungi and algae, zygote develops a thick wall that is resistant
to dessication and damage. It undergoes a period of rest before
germination. In organisms with haplontic life cycle (As you have read
in Class XI), zygote divides by meiosis to form haploid spores that grow
into haploid individuals. Consult your Class XI book and find out what
kind of development takes place in the zygote in organisms with diplontic
and haplo-diplontic life cycles. Zygote is the vital link that ensures continuity of species between organisms of one generation and the next. Every sexually reproducing organism, including human beings begin life as a single cell–the zygote
1.2.3.2 Embryogenesis
Embryogenesis refers to the process of development of embryo from the
zygote. During embryogenesis, zygote undergoces cell division (mitosis)
and cell differentiation. While cell divisions increase the number of cells
in the developing embryo: cell differentiation helps groups of cells to
undergo certain modifications to form specialised tissues and organs to
form an organism.
Animals are categorised into oviparous and viviparous based on
whether the development of the zygote takes place outside the body of
the female parent or inside. i.e., whether they lay fertilised /unfertilised
eggs or give birth to young ones. In oviparous animals like reptiles and
birds. the fertilised eggs covered by hard calcareous shell are laid in a
safe place in the environment; after a period of incubation young ones
hatch out. On the other hand. in viviparous animals (majority of mammals
including human beings). the zygote develops into a young one inside
the body of the female organism. After attaining a certain stage of growth,
the young ones are delivered out of the body of the female organism.
Because of proper embryonic care and protection, the chances of survival
of young ones is greater in viviparous organisms.
In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall
off. The zygote develops into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function. After dispersal, seeds germinate under favourable conditions to produce new plants.
