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The Life Cycle
Reproduction
All living organisms produce new individuals of the
same species that is a new generation of the species
from
an
existing
individual.
This
fundamental
characteristic of living things is known as reproduction.
∙
Plants belonging to same species look similar.
A basic process in reproduction is the creation of a
DNA copy.
In this process of copying, some variations take
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place each time. Thus DNA copies generated are
similar but not identical to the original.
Importance of Variations
Variations provide a chance of survival for the
species
under
unfavorable
environmental
conditions.
Plant and animals have different modes of
reproduction
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Types of asexual reproduction
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Binary fission
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Multiple fission
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Budding
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Fragmentation
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Regeneration
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Vegetative propagation
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Spore formation
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Asexual reproduction
Only one parent is involved in asexual reproduction
in which cells divide mitotically.
Since there is no fusion of two different cells, the
daughter cells produced by asexual
reproduction is genetically identical to the parent cells.
Lack of genetic variations is the disadvantage while
rapid reproduction is the advantage
Of asexual reproduction.
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Binary fission
In this method, the living cell is divided into two nearly
equal or unequal parts, which have potential to grow to
the size of original one.
E.g. Binary fission in amoeba, Paramecium.
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In paramecium transverse binary fission is observed
(fig. a) while in Euglena longitudinal binary fission is
observed. (Fig. b)
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Multiple fission
Multiple fission is observed in amoeba.
During unfavorable condition secretes a hard
covering called cyst.
Inside the cyst nucleus divides into many nuclei
by repeated division, followed by division by
cytoplasm forming many daughter cells.
The cyst bursts to release the daughter cells
during favorable conditions.
Budding
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Budding is observed in yeast cells.
1. Yeast reproduces by budding in which a small
outgrowth is formed on the parental cell.
2. The nucleus of the parental cell gets divided.
3. One daughter nucleus migrates into the bud.
4. The bud increases in size, separated and grows
further.
In unicellular organism, all the life processes
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occur
in
single
cell,
while
multicellular
organisms have special tissues, organs, organ
system to carry out life processes.
In unicellular organism, all the life processes
occur
in
single
cell,
while
multicellular
organisms have special tissues, organs, organ
system to carry out life processes.
∙
The complexity in reproduction increases with
the complexity in structure of organism.
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Fragmentation
1. When water and nutrients are available, spirogyra
grow and multiply rapidly.
2. The filament of spirogyra undergoes fragmentation
resulting in numerous filaments.
3. With cell enlargement and subsequent mitosis,
each fragment grows and develops into a mature
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filament.
Regeneration
∙
Some animals can reconstruct the entire body from
the isolated body cells, is called regeneration.
∙
Regeneration is not reproduction.
∙
Regeneration is carried out by specialized cells.
These cells proliferate and make large number of
cells which later develop into various cell types and
tissues and help in production of new organism.
E.g. Regeneration in Planaria
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1. When Planaria is cut into many pieces, each piece
develops into a whole Planaria as shown in above
figure.
2. This process occurs only if the planarial body gets
cut into pieces
3. But animals cannot wait to be cut to reproduce. So
regeneration is not the same as reproduction.
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Budding
In Hydra, it uses regenerative cells for reproduction in
the process of budding.
1. When Hydra reaches maturity and is well fed, its
body wall begins to form a rounded growth from the
stalk of the adult.
2. This growth is called a bud which develops in time
into a miniature hydra.
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4. The young hydra gets nourishment from the parent.
When the young hydra is sufficiently developed to
take up an independent existence, the base of the
new hydra seals off and thus allows the new
individuals to break off from the parent hydra.
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Vegetative Propagation
When the plants are produced from the vegetative
parts (roots, stems, leaves and buds), it is known as
vegetative propagation.
For example, new plants of potato develop from
eyes (buds on potato), Bryophyllum reproduces from
the buds on the leaf margin, the roots of sweet potato
give rise to new plants,etc…..,
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a. In a sweet potato plant vegetative propagation is
with the help of roots.
b. In figure, vegetative propagation with the help of
adventitious buds present in the notches of leaf of
Bryophyllum.
c. Fig. shows vegetative propagation of potato with
the help of stem (underground stem of potato). The
adventitious buds present on the potato develop
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into new plant.
Spore formation
In the mucor vegetative propagation is done with
the help of spores.
1. The hyphae of bread mould (Mucor) are thread like
structures.
2. The mould forms spores inside a sporangium.
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3. When
the
spores
are
ready
to
leave
the
sporangium, it breaks open.
4. If they land in a moist place, they germinate to form
new mould.
Cloning
∙
Animal cloning is the process by which an entire
organism is reproduced in a genetically identical
manner, from a single cell taken from the parent
organism
∙
This means the cloned animal is an exact duplicate
of its parent in every way.
Sexual reproduction
Two parents are involved in sexual reproduction,
one male and the other female. The main two
processes involved in sexual reproduction are,
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1. Meiosis
2. Fertilization
1. Meiosis: It is a process in which halving the
number of chromosomes (2n to n) takes place
resulting in the formation of haploid gametes
∙
It is a type of cell division leading to production
of gametes.
∙
∙
It occurs in reproductive cells.
Diagrammatically it can be shown as,
2. Fertilization: It is a process in which the male
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gamete fuses with the female gamete resulting in
the formation of diploid zygote. It restores the
number of chromosomes (2n).
∙
Since there is fusion of two germ cells;
offsprings produced by sexual reproduction are
different from parents.
∙
Variations give rise to variety and diversity.
∙
Variation enables organisms to adapt and
survive in the changing environment.
∙
It helps to prevent the complete extinction of
animal and plant species.
Sexual Reproduction in Plants
Flower is functional unit concerned with sexual
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reproduction.
sepals, petals, stamens and carpels are
different parts of a flower.
Carpel :
Female reproductive part of a flower
Present in the centre of flower.
Made up of three parts stigma, style and
ovary.
Stigma :
Sticky terminal part of the style.
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Receptive
organ
on
which
pollen
germinates.
Style
:
Elongated part of carpel bearing stigma at
its tip.
Ovary :
The swollen lower part of carpel containing
one or more ovules.
Each ovule has an egg cell
Stamen : Male reproductive part of a flower.
Made up of two parts anther and filament.
Anther :
Usually bilobed and produces pollen grains.
Male germ cells are produced by pollen
grains.
Filaments: Stalk of anther .
Pollination
1. The processes of transfer of pollen grains from
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anther to stigma is called as pollination.
2. If this transfer of pollen occurs in the same flower or
another flower of the same plant, it is known as self
pollination.
3. On the other hand, if pollen is transferred from one
flower to the flower of another plant of the same
species, it is known as cross pollination.
The agents of cross pollination are wind, water or
animals.
Some flowers have attractive colours while
some have fragrance, to attract insects and
pollination takes place. Like this insects help in
pollination.
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Fertilization
Formation of zygote by fusion of egg and male
gametes is termed as fertilization
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1. After the pollen grain lands on the stigma, it
germinates. The pollen tube grows out from a
pollen grain. It travels through the style to reach the
ovary.
2. Each pollen tube contains two male gametes and
releases them near the egg.
3. One male gamete fuses with the egg cell to form
zygote. The second male gamete fuses with the
secondary nucleus in the embryo sac to form
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endosperm. This is called as double fertilization.
4. The
zygote
develops
into
embryo
and
the
endosperm serves as nutritive tissue for the
growing embryo. This embryo is capable of growing
into a new plant.
Germination
After fertilization, the zygote divides several times
to form an embryo within the ovule.
The ovule develops into a seed and ovary develops
into the fruit.
The seed contains the future plant. It develops into
the seedling under appropriate condition. This process
is known as germination.
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Reproduction in human beings
Changes in human girls and boys according to their
ages are shown in the table given below:
The Male Reproductive System
Testis and penis are the main reproductive organs
of the male reproductive system.
Testis : Produces sperms (male germ cells) As
formation of sperms require temperature lower than the
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normal body temperature, testis are located outside the
abdominal cavity in the scrotum.
Testis secretes the hormone testosterone which brings
about changes in boys during puberty.
Epididymis: Immature sperm travels to the epididymis
for development and storage.
Vas deferens: It is a passage through which the sperm
travels the urethra.
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Seminal vesicle and Prostate glands: They produce
ejaculatory fluid which helps the sperm in transport and
provides nutrition.
Penis: It is the portion of the reproductive system that
delivers the sperms to the site of fertilization.
Sperms: A sperm cell is composed of a head which
carries the genetic information, a middle part which
carries mitochondria required for energy production
and a tail which is like a flagellum to help in movement
towards the female germ cells.
The female reproductive system
Vagina, uterus, oviduct and ovaries are the main
reproductive organs of the female reproductive system.
Vagina:
It is muscular tube that extends from the vaginal
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opening to uterus.
It provides the route for the menstrual blood to
leave the body during menstruation. It is a pathway
through which sperms enter into the woman’s body and
is a pathway through which a baby comes out of the
woman’s body during childbirth.
Uterus:
Uterus is a muscular organ. Due to strong muscles
and ability to expand and contract, the uterus can
accommodate a growing foetus and can push the baby
during labour.
Oviducts:
Connect uterus to the ovary.
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Ovaries:
Two oval shaped organs lie to the upper right and
left of the uterus. They develop and release eggs into
the oviduct. They secrete hormone estrogen which
brings about changes in girls during puberty.
When a baby girl is born, the ovaries already
contain thousands of immature eggs which remain
inactive till maturity.
On reaching puberty, women’s ovaries usually
release one egg each month.
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Menstruation:
When the egg is not fertilized, this unfertilized egg
along with blood and mucous form a flow which leaves
the uterus. This is called menstruation which lasts from
three to five days.
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Fertilization, development and birth
The sperms enter through the vaginal passage,
travel upwards and reach the oviduct where they may
encounter the egg. As soon as it is fertilized, the egg
cell (zygote) begins to divide until becomes a ball of
cells called a blastocyst.
This ball then implants itself in the wall of the
uterus. The development of the embryo takes place
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inside the uterus where the embryo gets all the
nutrients and oxygen from its mother’s blood in the
oviduct supplied through the umbilical cord. From nine
months onwards, the baby is ready to take birth. It
moves down. During birth, the cervix gradually opens
and the baby is released through the vagina.
Need for and method of family planning
As the population density increases, decrease in
per capita income and natural resources takes place.
General health also goes down. It creates an
economical burden on the nation.
Economic pressure, mother’s poor health, children
neglected
at
home,
poor
housing,
malnutrition,
insufficient medical care, lack of better education are
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some of the disadvantages of large family size.
Another problem related to human reproduction
is the spread of sexually transmitted diseases (STDs).
Preventive measures
Sex education
Use of condom
Contraception (to avoid pregnancy) and to keep
sexual health by the use of the condoms, oral pills,
copper – T etc.
The WHO has prepared guidelines and considered
reproductive health as fundamental human right.
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