Std10 Ch12-Life-cycle
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Transcript of Std10 Ch12-Life-cycle
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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