Chapter 9: Reproduction in Animals
जंतुओं में जनन (आंतरिक व बाह्य निषेचन, जरायुज व अंडप्रजक, कायांतरण, मुकुलन एवं द्विखंडन)
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- Modes of Reproduction: Sexual (involves fusion of male and female gametes) vs Asexual (single parent produces offspring without gamete fusion).
- Fertilization Architecture:
- - Internal Fertilization (आंतरिक निषेचन): Takes place inside female body. Examples: Humans, cows, dogs, hens, cats, birds.
- - External Fertilization (बाह्य निषेचन): Occurs outside female body in aquatic medium (water). Female lays hundreds of delicate jelly-coated eggs; male discharges sperms over them. Examples: Frogs, toads, fish, starfish.
- Viviparous vs Oviparous Organisms (CTET Trap Focus):
- - Viviparous Animals (जरायुज): Give birth directly to developed young ones (Humans, cows, dogs, whales, bats).
- - Oviparous Animals (अंडप्रजक): Lay eggs; embryo matures outside parent's body inside protective egg shell (Birds, frogs, lizards, snakes, butterflies).
- Metamorphosis (कायांतरण): Drastic biological transformation of a larval form into an adult through distinct stages. Examples: Silk moth (Egg -> Silkworm larva -> Pupa inside cocoon -> Adult moth); Frog (Egg -> Tadpole larva with tail and gills -> Adult frog with lungs and webbed legs). Metamorphosis in frogs is controlled by THYROXINE hormone produced by thyroid gland; THYROXINE PRODUCTION REQUIRES IODINE IN WATER. If pond water lacks iodine, tadpoles CANNOT metamorphose into adult frogs!
- Asexual Reproduction Paradigms:
- - Budding in Hydra (मुकुलन): New individuals arise from repeated cell division at one specific site forming a bulge called a 'bud', which detaches when mature.
- - Binary Fission in Amoeba (द्विखंडन): Single-celled parent replicates nucleus into two, followed by division of cytoplasm into two identical daughter amoebae.
- Trap: Hen has internal fertilization, but is it viviparous or oviparous? Correct: OVIPAROUS. Fertilization is internal, but hen lays eggs.
- Trap: Why do tadpoles fail to develop into adult frogs in iodine-deficient pond water? Correct: Thyroxine hormone controls metamorphosis, and thyroxine synthesis requires IODINE.
- Trap: Binary fission occurs in Hydra. FALSE: Binary fission occurs in AMOEBA; Budding occurs in HYDRA and YEAST.
NCERT Core Facts: Embryology, Hormonal Metamorphosis & Asexual Modes
Reproductive mechanisms, amphibian metamorphosis, and budding vs fission appear every session:
1. Reproductive Modalities Matrix
| Category | Biological Event Location | Animal Examples | CTET Exam Trap |
|---|---|---|---|
| Internal Fertilization | Inside female oviduct/fallopian tube | Humans, cows, dogs, Hens | Hens have internal fertilization but are oviparous! |
| External Fertilization | Outside body in water pools | Frogs, Toads, Bony Fish | Requires water medium; millions of gametes released |
| Viviparous (जरायुज) | Development in uterus; live birth | Mammals (Cows, Whales, Humans) | Bats and Whales are viviparous mammals |
| Oviparous (अंडप्रजक) | External egg hatching | Birds, reptiles, insects, frogs | Calcareous or gelatinous protective coat |
2. Hormonal Regulation of Metamorphosis in Frogs
- Critical Exam Point: If a tadpole is reared in water lacking dissolved Iodine, thyroxine hormone cannot be synthesized, and the tadpole remains a giant tadpole permanently, failing to metamorphose into an adult frog!
3. Asexual Reproduction Mechanics
- Budding (मुकुलन): Typical in Hydra and Yeast. Mitotic protrusion (bud) grows and separates.
- Binary Fission (द्विखंडन): Typical in Amoeba. Nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis) into two identical daughter cells.
Test Your Mastery: Reproduction in Animals
Authentic CBSE exam questions tested directly from this NCERT chapter.
A biology student discovers that tadpoles living in a specific experimental tank fail to metamorphose into adult frogs and remain as tadpoles. Which mineral was likely deficient in the tank water?
जीव विज्ञान का एक छात्र पाता है कि एक विशिष्ट प्रायोगिक टैंक में रहने वाले टैडपोल वयस्क मेंढक में रूपांतरित होने में विफल रहते हैं और टैडपोल ही बने रहते हैं। टैंक के पानी में संभवतः किस खनिज की कमी थी?
Match the mode of reproduction in List I with the respective organism in List II: (a) Binary Fission - (i) Hydra (b) Budding - (ii) Amoeba (c) Internal fertilization with Oviparous birth - (iii) Hen (d) External fertilization with Oviparous birth - (iv) Frog
सूची I में जनन के प्रकार को सूची II में संबंधित जीव से सुमेलित कीजिए: (a) द्विखंडन - (i) हाइड्रा (b) मुकुलन - (ii) अमीबा (c) आंतरिक निषेचन और अंडप्रजक जन्म - (iii) मुर्गी (d) बाह्य निषेचन और अंडप्रजक जन्म - (iv) मेंढक
Why does a frog produce hundreds of eggs at a time, whereas a cow produces only one egg at a time?
Frogs undergo external fertilization in open water where eggs are vulnerable to predators and currents. Producing hundreds ensures at least a few survive. Cows have protected internal fertilization.
What hormone regulates metamorphosis in tadpoles, and what mineral is indispensable for it?
Thyroxine hormone regulates metamorphosis, and its synthesis by the thyroid gland requires dissolved iodine in the pond water.