Standard/ Grade/ Class 10 Life Cycle (Reproduction)

Gurudatta K Wagh

Reproduction

Fundamental characteristic of living things

Necessary to maintain the number of individuals of a species and to prevent their extinction

All living organisms produce new individuals (new generation) of the same species

Creation of a DNA copy

• Basic process in reproduction

• Some variations take place in this process each time

• DNA copies generated are similar but not identical to the original

Importance of variationsChanges in ecological system can wipe out a species

But, if variations in some individuals are suitable for new environment, there is a chance of survival of that species

Modes of reproduction: (A) Asexual, and (B) Sexual

(A) Asexual reproduction

• Only one parent is involved

• Cells divide mitotically

• There is no fusion of two different cells

• The daughter cells produced are genetically identical

to the parent cells

• Disadvantage - Lack of genetic variations

• Advantage - rapid reproduction

Mitosis: A type of cell division leading to

growth and development

It occurs in somatic cells (any cell of a living

organism other than the reproductive cells)

Modes of asexual reproduction in unicellular organisms

Binary Fission • Employed by most prokaryotes, some protozoa and some

organelles within eukaryotic cells• Living cell divides into two equal or nearly equal parts which

have potential to grow to the size of the original one• The plane of cytoplasmic division is through any plane in

amoeba (simple binary fission)

Amoeba - simple binary fission

Paramoecium - transverse binary fission

Euglena - longitudinal binary fission

In some organisms fission occurs through a specific axis, viz. transverse, longitudinal

Multiple fission • During unfavourable condition, the amoeba withdraws its

psuedopodia, becomes almost round and secretes a hard covering called cyst

• Inside the cyst nucleus divides into many nuclei by repeated division, followed by division of cytoplasm

• Many daughter cells are formed• The cyst bursts to release the daughter cells during

favourable condition

Amoeba - Multiple fission

Budding

• In yeast a small outgrowth is formed on the parental cell • The nucleus of the parental cell divides• One daughter nucleus migrates into the bud• The bud increases in size, separates and grows further

Modes of asexual reproduction in multicellular organisms

They have different cell types which perform different functions

They have special organs, placed at a definite place in the body which are formed by specific tissues

In such organised organisms method of cell-by-cell division to reproduce is impractical

They show complex ways of reproduction

The complexity in reproduction increases with the complexity in structure of organisms

Fragmentation

• When water and nutrients are available, spirogyra grow and multiply rapidly

• The filament of spirogyra undergoes fragmentation resulting in numerous filaments

• With cell enlargement and subsequent mitosis, each fragment grows and develops into a mature filament

Spirogyra

Regeneration

• Some animals can reconstruct the entire body from the isolated body cells

• Regeneration is carried out by specialised 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

• When Planaria is cut into many pieces, each piece develops into a whole Planaria

Planaria

• Regeneration occurs only if the planarial body gets cut into pieces

• But animals cannot

wait to be cut to reproduce

• So regeneration is

not the same as reproduction

Budding

• Hydra uses regenerative cells for reproduction in the process of budding

• When Hydra reaches maturity and is well fed, its body wall begins to form a rounded growth (bud) from the stalk of the adult

• The bud develops into a miniature hydra

Hydra

• The body layers, body cavity and the digestive cavity of the young hydra are continuous with that of the parent hydra

• 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 individual to break off from the parent hydra

Vegetative Propagation

New plants are produced from the vegetative parts (roots, stems, leaves and buds)

e.g. potato plant develops 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.

Features of vegetative propagation• Plants are similar to the parent, as they are

produced from a single parent • Plants take less time to grow and bear flowers and

fruits earlier than those produced from seeds

Spore formation The hyphae of bread mould (Mucor) are thread like structures

The mould forms spores inside a sporangium

When the spores are ready to leave the sporangium, it breaks open. If they land in a moist place, they germinate to form new mould

CloningAnimal 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. It has the DNA which is identical to parent DNA

e.g. Dolly sheep was the first mammal to be cloned from an adult cell rather than an embryo

(B) Sexual reproduction

• Two parents are involved in sexual reproduction,

one male and the other female

• The two main processes involved in sexual

reproduction are 1) Meiosis, and 2) Fertilization

(1) Meiosis: Halving the number of chromosomes - from 2n to n - takes place resulting in the formation of haploid gametes

(2) Fertilization: It is a process in which the male gamete fuses with the female gamete resulting in the formation of diploid zygote. It restores the number of chromosomes (2n)

Features of sexual reproduction

• 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 a functional unit concerned with sexual

reproduction

Carpel: Present in the centre of flower. Made up of three parts - stigma, style and ovary

Stigma: Sticky terminal part of the style. Receptive organ on which pollen germinates

Female reproductive part of a flower

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 (female germ cell)

Stamen: Made up of two parts anther and filament Anther: Usually bilobed and produces pollen grains.Male germ cells are produced by pollen grains

Filament: Stalk of anther

Male reproductive part of a flower

PollinationInvolves the transfer of pollen grains from anther to stigma

Fertilization• 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 • Each pollen tube contains two male

gametes and releases them near the egg

• 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 endosperm • This is called as double fertilization • 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

Fertilization

•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 conditions•This process is known as germination

Germination

•After fertilization, the zygote divides several times to form an embryo within the ovule

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