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 Reproduction
6.1.1 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 Reproduction
6.2.1 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 Fertilisation
6.2.4.1 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 Embryo
6.2.5.1 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 Reproduction
6.3.1 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).
- Oviparous Animals: Animals that lay eggs (e.g., hen, frog, lizard, butterfly).
- Metamorphosis: The transformation of the larva into an adult through drastic changes.
- Asexual Reproduction Definition: Only a single parent is involved.
- Budding in Hydra: New individuals develop from buds.
- Binary Fission in Amoeba: Reproduces by dividing itself into two.