Science: Class 8 — Chapter 6: Reproduction in Animals
Introduction to Reproduction
Reproduction: Essential process for the continuation of a species, ensuring the continuation of similar kinds of individuals, generation after generation.
Previous Learning: Students have studied reproduction in plants and other essential processes like digestion, circulation, and respiration.
Chapter Focus: To learn how reproduction takes place in animals.
6.1 Modes of Reproduction6.1.1 Observing Young Ones
Observation Task: Recall and name the young ones of various animals (e.g., Human - Baby, Cat - Kitten, Dog - Puppy, Butterfly - Caterpillar/Larva, Hen - Chick, Cow - Calf, Frog - Tadpole).
Birth: Animals are born in various ways; some young ones appear different from adults.
6.1.2 Two Modes of Reproduction
Sexual Reproduction: Involves two parents and the fusion of gametes.
Asexual Reproduction: Involves a single parent.
6.2 Sexual Reproduction6.2.1 Overview of Sexual Reproduction
Recall Plants: Like plants, animals reproduce sexually with distinct male and female reproductive parts/organs.
Gametes: Reproductive parts produce male and female gametes.
Zygote Formation: Gametes fuse to form a zygote, which develops into a new individual.
Definition: Reproduction beginning from the fusion of male and female gametes.
6.2.2 Male Reproductive Organs
Components: Include a pair of testes, two sperm ducts, and a penis.
Testes Function: Produce male gametes called sperms (millions are produced).
Sperm Structure: Each sperm is a single cell with a head, a middle piece, and a tail.
Tail Purpose: Helps the sperm move towards the egg.
6.2.3 Female Reproductive Organs
Components: Include a pair of ovaries, oviducts (fallopian tubes), and the uterus.
Ovary Function: Produces female gametes called ova (eggs).
Egg Release: In humans, a single matured egg is released into the oviduct by one ovary monthly.
Uterus Function: The part where the development of the baby takes place.
Egg Structure: Like sperm, an egg is also a single cell, containing a nucleus.
Egg Size Variation: Animal egg sizes vary greatly (e.g., human eggs are small, hen eggs are larger, ostrich eggs are the largest).
6.2.4 Fertilisation6.2.4.1 Process of Fertilisation
First Step: The fusion of a sperm and an ovum.
Fusion: When sperms contact an egg, one sperm fuses with it.
Result: The nuclei of the sperm and egg fuse to form a single nucleus.
Zygote Formation: This forms a fertilised egg or zygote, the beginning of a new individual.
Inheritance: The new individual inherits characteristics from both mother and father.
6.2.4.2 Internal Fertilisation
Definition: Fertilisation that takes place inside the female body.
Examples: Occurs in many animals, including humans, cows, dogs, and hens.
Test-Tube Babies (IVF): A technique where fertilisation (in vitro fertilisation) occurs outside the body.
IVF Process: Doctors collect eggs and sperms, allow fertilisation externally, develop zygote for a week, then implant it into the mother's uterus for complete development.
Misleading Term: "Test-tube babies" is misleading as development happens in the uterus, not test tubes.
6.2.4.3 External Fertilisation
Definition: Fertilisation that takes place outside the body of the female, typically in water.
Commonality: Very common in aquatic animals.
Examples: Fish, starfish, and frogs.
Frog Reproduction: During spring/rainy season, frogs/toads move to water, female lays hundreds of delicate eggs (not shelled), male deposits sperms over them.
Egg Protection: A layer of jelly holds frog eggs together and provides protection.
Sperm Movement: Each sperm swims randomly with its long tail, contacting the eggs.
Survival Strategy: Fish and frogs lay hundreds of eggs and release millions of sperms to ensure at least a few get fertilised, due to exposure to water movement, wind, rainfall, and predators.
6.2.5 Development of Embryo6.2.5.1 Zygote to Foetus in Viviparous Animals
Zygote Development: Fertilisation forms a zygote, which divides repeatedly to form a ball of cells.
Embryo Formation: Cells group to form different tissues and organs, developing into an embryo.
Embedding: The embryo gets embedded in the wall of the uterus for further development.
Foetus Stage: When all body parts (hands, legs, head, eyes, ears) can be identified, the embryo is called a foetus.
Birth: Once foetal development is complete, the mother gives birth to the baby.
6.2.5.2 Development in Oviparous Animals (e.g., Hen)
Internal Fertilisation: Occurs in hens, but they lay eggs.
Zygote Journey: Zygote divides repeatedly and travels down the oviduct.
Protective Layers: Many protective layers, including the hard shell, form around the developing embryo.
Egg Laying: The hen lays the egg after the hard shell forms.
Incubation: The hen sits on the eggs to provide warmth; development of the chick occurs inside the egg shell over ~3 weeks.
Hatching: After full development, the chick bursts open the egg shell.
6.2.5.3 Development in External Fertilisation (e.g., Frog)
External Development: Embryo development takes place outside the female body.
Growth: Embryos grow within their egg coverings in water.
Hatching: Eggs hatch after embryos develop, producing tadpoles.
6.2.6 Viviparous and Oviparous Animals
Viviparous Animals: Animals that give birth to young ones.
Examples of Viviparous: Humans, cows, dogs, cats.
Oviparous Animals: Animals that lay eggs which later develop into young ones.
Examples of Oviparous: Hen, frog, lizard, butterfly, moth, crow, most birds.
Observation Ease: Eggs of oviparous animals are often easy to observe as they are laid outside the body.
6.2.7 Young Ones to Adults (Metamorphosis)
Growth: New individuals grow from birth/hatching until they become adults.
Appearance Variation: In some animals (e.g., frog), young ones look very different from adults.
Frog Life Cycle: Involves three distinct stages: egg → tadpole (larva) → adult.
Metamorphosis Definition: The transformation of the larva into an adult through drastic changes.
Human Development: Humans do not undergo metamorphosis; body parts similar to adults are present from birth.
6.3 Asexual Reproduction6.3.1 Overview of Asexual Reproduction
Involvement: Only a single parent is involved.
Target Organisms: Common in very small animals like hydra and microscopic organisms like amoeba.
6.3.2 Budding in Hydra
Observation: Hydra may show one or more bulges on the parent body.
Buds: These bulges are developing new individuals called buds (similar to buds in yeast).
Process: New individuals develop as outgrowths from a single parent.
Definition of Budding: A type of asexual reproduction where new individuals develop from buds.
6.3.3 Binary Fission in Amoeba
Organism Type: Amoeba is a single-celled organism.
First Step: The nucleus divides into two nuclei.
Second Step: The body then divides into two, with each part receiving a nucleus.
Result: Two daughter amoebae are produced from one parent amoeba.
Definition of Binary Fission: A type of asexual reproduction where an animal reproduces by dividing itself into two individuals.
Story of Dolly, the Clone
Cloning Definition: The production of an exact genetic copy of a cell, any living part, or a complete organism.
First Mammal Clone: Dolly the sheep, born July 5, 1996, at Roslin Institute, Scotland, by Ian Wilmut and colleagues.
Cloning Process (Dolly):
A cell was collected from the mammary gland of a Finn Dorsett sheep (donor of nucleus).
An egg was obtained from a Scottish blackface ewe (donor of enucleated egg).
The nucleus was removed from the Scottish blackface ewe's egg.
The nucleus from the Finn Dorsett sheep's mammary gland cell was inserted into the enucleated egg.
This reconstructed egg was implanted into the uterus of a Scottish blackface ewe (surrogate mother).
The egg developed normally, and Dolly was born.
Genetic Identity: Dolly was genetically identical to the Finn Dorsett sheep (nucleus donor), showing no traits of the surrogate mother.
Dolly's Life: Dolly was a healthy clone, produced offspring through normal sexual means, but died on February 14, 2003, due to lung disease.
Cloning Challenges: Many cloned animals die before or soon after birth and are often born with severe abnormalities.
Summary: What You Have Learnt
Modes of Reproduction: Animals reproduce by (i) Sexual reproduction and (ii) Asexual reproduction.
Sexual Reproduction Definition: Results from the fusion of male and female gametes.
Female Reproductive Organs: Include ovaries, oviducts, and uterus.
Male Reproductive Organs: Include testes, sperm ducts, and penis.
Gamete Production: Ovaries produce ova (female gametes), testes produce sperms (male gametes).
Fertilisation Definition: The fusion of ovum and sperm.
Zygote: The fertilised egg is called a zygote.
Internal Fertilisation: Occurs inside the female body (e.g., humans, hens, cows, dogs).
External Fertilisation: Occurs outside the female body (e.g., frogs, fish, starfish).
Embryo Development: Zygote divides repeatedly to give rise to an embryo.
Embryo Embedding: The embryo gets embedded in the wall of the uterus for further development.
Foetus: The stage where all body parts of the embryo are identifiable.
Viviparous Animals: Animals that give birth to young ones (e.g., humans, cows, dogs).