Reproduction in Animals

Reproduction in Animals - Complete CBSE Class 8 Science Notes

Introduction to Reproduction in Animals

Reproduction is the fundamental biological process through which living organisms produce new individuals of their own kind. It stands as one of the most essential characteristics that distinguish living beings from non-living matter. Without reproduction, life would cease to exist on Earth, as species would have no means of continuing their lineage after individual organisms die.

The ability to reproduce ensures that when an organism dies, another individual of the same species takes its place, thereby guaranteeing the survival and continuity of species across generations. This process is not merely about creating copies but about maintaining the essence of life itself through group immortality.

Importance of Reproduction

Understanding why reproduction matters helps us appreciate its role in the biological world:

1. Essential Characteristic of Life: Reproduction is a defining feature that separates living organisms from non-living things
2. Species Survival: It ensures that species continue to exist despite individual mortality
3. Continuity of Race: Maintains genetic lineages and preserves species characteristics over time
4. Group Immortality: While individuals die, the species lives on through successive generations

Types of Reproduction in Animals

Animals reproduce through two fundamental methods, each with distinct characteristics and advantages:

1. Asexual Reproduction

Definition: Production of offspring by a single parent without the formation and fusion of gametes.

Key Characteristics:

• Uniparental: Only one parent is involved
• No gamete formation: No specialized sex cells are produced
• Mitosis-based: Cell division occurs through mitosis
• Genetic similarity: Offspring are genetically identical to the parent
• Rapid multiplication: Ensures quick increase in population numbers

Where It Occurs:

Asexual reproduction is found in:

• Protozoans (single-celled organisms)
• Sponges (Scypha)
• Coelenterates (Hydra, Tubularia)
• Certain worms
• Tunicates

Note: Asexual reproduction is absent in higher invertebrates and all vertebrates.

Modes of Asexual Reproduction

A. Fission

The simplest form of reproduction where a unicellular organism divides into two or more organisms.

Binary Fission (Division into Two):

• Nuclear division is followed by cytoplasmic division
• A constriction appears in the cell membrane
• The constriction deepens inward, dividing the cytoplasm
• Two daughter cells form, each with one nucleus
• Example: Amoeba

Multiple Fission (Division into Many):

• The nucleus divides several times into many daughter nuclei
• Daughter nuclei arrange at the periphery of the parent cell
• Each nucleus gets surrounded by a bit of cytoplasm
• The multinucleated body divides into many daughter cells
• Example: Plasmodium (malaria parasite)

B. Budding

Definition: Formation of a daughter individual from a small projection (bud) that arises on the parent body.

Exogenous Budding (External Budding):

• Bud develops from the outer surface of the parent body
• In Hydra, one or more bulges appear on the body
• These bulges develop into new individuals
• The bud initially remains attached to derive nutrition
• Upon maturation, it detaches and becomes independent
• Example: Hydra

Endogenous Budding (Internal Budding):

• Bud arises inside or within the parent body
• Develops internally before being released
• Example: Sponges

Important Note: During budding, the bud stays attached to the parent body for nutritional support. As it matures and develops its own systems, it detaches to lead an independent life.

C. Fragmentation

Definition: A type of reproduction where an entire new organism can grow from certain pieces or cells of the parent organism.

This demonstrates the remarkable regeneration ability of organisms to:

• Replace lost body parts
• Grow complete new individuals from fragments
• Example: Flatworms can regenerate entire organisms from body fragments

D. Spore Formation

Definition: A reproductive process commonly found in fungi and some bacteria where specialized structures produce spores.

Process:

1. A structure called sporangium forms
2. Inside the sporangium, the nucleus divides multiple times
3. Each nucleus, along with a small amount of cytoplasm, forms a spore
4. Spores are liberated into the environment
5. Each spore develops into a new organism (hyphae in fungi)

• Example: Rhizopus (bread mold)

Sexual Reproduction in Animals

Definition: The type of reproduction that begins with the fusion of male and female gametes to form a zygote, which then develops into a new individual.

Characteristics:

• Biparental: Involves two parents (male and female)
• Gamete formation and fusion: Specialized sex cells are produced and unite
• Meiosis and mitosis: Meiosis occurs during gamete formation; mitosis occurs after zygote formation
• Genetic variation: Offspring inherit characteristics from both parents
• Examples: Humans, birds, mammals, most insects

Differences Between Sexual and Asexual Reproduction

Aspect	Asexual Reproduction	Sexual Reproduction
Number of parents	Single parent (uniparental)	Two parents (biparental)
Gamete involvement	No gametes formed or fused	Gametes formed and fused
Cell division type	Only mitosis	Meiosis (gamete formation) + mitosis (development)
Genetic variation	Offspring identical to parent	Offspring show genetic variation
Speed	Rapid multiplication	Relatively slower process
Occurrence	Lower organisms, some invertebrates	Most animals, all vertebrates
Examples	Amoeba, Hydra, Plasmodium	Humans, fish, frogs, birds

Classification Based on Sex Organs

Unisexual Animals (Dioecious)

• Male and female sexes remain separate
• Each individual has either male or female reproductive organs
• Example: Human beings, fish, birds

Bisexual Animals (Monoecious)

• Both male and female sex organs present in a single individual
• Also called hermaphrodites
• Example: Earthworm, leech, some snails

Viviparous vs Oviparous Animals

Understanding how animals bring forth new life:

Viviparous Animals

Definition: Animals that give birth to live young ones (babies)

Characteristics:

• Embryo develops inside the mother's body
• Young ones are born alive and relatively well-developed
• Mother provides nourishment through placenta
• Examples: Humans, dogs, cats, cows, elephants, whales

Oviparous Animals

Definition: Animals that lay eggs from which young ones later develop

Characteristics:

• Fertilized eggs are laid outside the mother's body
• Embryo develops within the protective egg covering
• Young ones hatch from eggs after development is complete
• Examples: Frogs, fish, birds, reptiles, most insects, butterflies

Types of Fertilization

Fertilization is the fusion of male gamete (sperm) with female gamete (ovum) to form a zygote.

External Fertilization

Definition: Fusion of gametes occurs outside the female body, typically in water.

Process in Frogs and Fish:

1. Female lays hundreds of eggs in water
2. Eggs are not covered by a hard shell (delicate)
3. A layer of jelly holds eggs together and provides protection
4. Male deposits sperms over the eggs
5. Sperms swim randomly in water using their tails
6. Sperms come in contact with eggs
7. Fertilization occurs in the aquatic environment

Characteristics:

• Common in aquatic animals (fish, frogs, starfish)
• Requires water medium
• Large number of eggs produced (many may not get fertilized)
• No parental care after egg-laying typically

Why External Fertilization Works for Aquatic Animals:

• Water provides medium for sperm movement
• Water prevents eggs from drying out
• Ensures sperm can reach eggs without internal structures

Internal Fertilization

Definition: Fusion of gametes occurs inside the female body.

Process:

1. During mating, male deposits sperms inside female's body
2. Sperms swim toward the eggs within female reproductive tract
3. One sperm fertilizes one egg
4. Fertilized egg (zygote) develops within female body or in laid egg

Characteristics:

• Common in land animals (mammals, birds, reptiles, many insects)
• Does not require water medium
• Higher success rate of fertilization
• Often involves parental care
• Fewer eggs needed as fertilization success is high

Examples: Humans, cows, dogs, hens, lizards, snakes

Basic Processes of Sexual Reproduction

Sexual reproduction involves four fundamental stages:

A. Gametogenesis

Definition: The process of formation of haploid gametes.

• Male gametes (sperms): Produced in testes through spermatogenesis
• Female gametes (ova): Produced in ovaries through oogenesis
• Gametes are haploid (contain half the number of chromosomes)

B. Fertilization

Definition: The fusion of haploid male and female gametes to form a diploid zygote.

• Sperm nucleus fuses with egg nucleus
• Restores diploid chromosome number
• Marks the beginning of a new individual

C. Embryogenesis

Definition: Development of zygote into an embryo and then a foetus.

Stages:

1. Cleavage: Zygote undergoes rapid mitotic divisions
2. Blastula formation: Forms a hollow sphere of cells
3. Gastrulation: Cells migrate to form three germ layers
4. Morphogenesis: Organs and body structures begin to form
5. Differentiation: Cells specialize into different tissue types

D. Parturition

Definition: Birth of a fully developed young one from the mother's uterus.

• Triggered by hormones from the developing fetus
• Involves labor contractions
• Umbilical cord is cut after birth
• Leaves a scar called the navel

Human Reproductive System

Male Reproductive System

The male reproductive system is designed for sperm production and delivery.

Components:

1. Testes (Singular: Testis)

• Location: Present in scrotal sacs outside the body cavity
• Function: Primary sex organs that produce sperms
• Temperature: Maintained at 2°C lower than body temperature (optimal for sperm production)
• Internal structure: Contains seminiferous tubules where sperm formation occurs
• Production: Millions of sperms produced continuously

Sperm Structure:

• Head: Contains nucleus with genetic material
• Middle piece: Contains mitochondria for energy
• Tail (Flagellum): Provides motility for swimming
• Single-celled structure

2. Epididymis

• Elongated, flattened structure
• Formed of approximately 6-meter-long highly coiled tube
• Function: Storage and maturation of sperms

3. Vas Deferentia (Sperm Ducts)

• Long (about 30 cm), narrow, muscular tubes
• Connects epididymis to ejaculatory duct
• Merges with ducts from accessory glands:
  • Seminal vesicles: Form 60% of semen
  • Prostate gland: Forms 30% of semen
  • Cowper's glands: Add lubricating fluid
• Unites with urinary duct to form urinogenital canal

4. Penis

• Cylindrical, erectile organ
• Functions:
  • Copulatory organ (for mating)
  • External genitalia
  • Urination

Female Reproductive System

The female reproductive system is designed for egg production, fertilization, and fetal development.

Components:

1. Ovaries

• Number: One pair
• Characteristics: Small, almond-shaped structures
• Function: Primary female sex organs that produce ova (eggs)
• Production: Typically one mature egg released per month (in humans)

2. Fallopian Tubes (Oviducts)

• Length: About 12 cm each
• Structure: Long tubes with ciliated and muscular walls
• Function:
  • Transport ovum from ovary to uterus
  • Site of fertilization (typically in the upper third)
  • Cilia help move egg along the tube

3. Uterus

• Shape: Large, hollow, muscular, pear-shaped organ
• Structure: Broad main body and narrow cervix
• Functions:
  • Site of fetal growth during pregnancy
  • Participates in placenta formation
  • Provides nourishment and protection to developing baby

4. Vagina

• Length: About 7.5 cm
• Structure: Long muscular tube extending from cervix to external opening (vestibule)
• Location: Extends backward in front of rectum
• Functions:
  • Acts as copulation canal (receives penis during mating)
  • Serves as birth canal during delivery

5. Vulva

• Forms the external genitalia of females
• Includes structures protecting the vaginal opening

Fertilization and Development in Humans

The Fertilization Process

1. Ovulation: Mature egg released from ovary into fallopian tube
2. Sperm journey: Millions of sperms travel through vagina → uterus → fallopian tubes
3. Meeting point: Sperm and egg typically meet in the upper fallopian tube
4. Fusion: One sperm penetrates the egg, nuclei fuse
5. Zygote formation: Fertilized egg (zygote) with diploid chromosome number forms

Result: The new individual inherits genetic characteristics from both mother (egg) and father (sperm).

Development of Embryo

Stage 1: Cleavage (Days 1-5)

• Zygote undergoes rapid mitotic divisions
• Cell number increases but overall size remains similar
• Forms a ball of cells

Stage 2: Blastula Formation (Day 5-6)

• Cells form a hollow, single-layered spherical structure
• Called blastula or blastocyst in mammals

Stage 3: Gastrulation (Week 2-3)

• Cells from blastula surface migrate to predetermined positions
• Forms three primary germ layers:
  • Ectoderm (outer layer)
  • Mesoderm (middle layer)
  • Endoderm (inner layer)
• Stage called gastrula

Stage 4: Morphogenesis and Differentiation (Week 3 onwards)

• Three germ layers develop into specific organ systems:
  • Ectoderm: Forms skin, nervous system, sense organs
  • Mesoderm: Forms muscles, bones, circulatory system, reproductive organs
  • Endoderm: Forms digestive system, respiratory organs, liver
• Embryo develops recognizable body features

Stage 5: Foetal Development

• Once all body parts are identifiable, it's called a foetus
• Continued growth and refinement of organs
• Development of hands, legs, head, eyes, ears, etc.

Implantation and Pregnancy

Implantation (Day 6-7):

• Developing embryo embeds itself in the uterine wall
• Establishes connection for nourishment

Placenta Formation:

• Special tissue that connects developing baby to mother
• Functions:
  • Acts as physiological barrier between maternal and fetal blood
  • Works as an ultrafilter allowing exchange of:
    • Nutrients (from mother to fetus)
    • Oxygen (from mother to fetus)
    • Waste products (from fetus to mother)
  • Does not allow direct blood mixing

First Trimester (Months 1-3):

• Basic body structure forms
• Critical developmental period
• Embryo becomes foetus

Hormonal Maintenance:

• Progesterone: Called "pregnancy hormone"
• Maintains uterine lining and prevents menstruation
• Essential for continuing pregnancy

Gestation Period:

• Definition: Period from fertilization to birth
• Human gestation: Approximately 280 days (9 months or 40 weeks)

Parturition (Birth Process)

Initiation:

• Developing fetus secretes hormones from adrenal gland
• These hormones stimulate mother's pituitary gland
• Pituitary releases "birth hormones" (oxytocin)

Labor:

• Powerful uterine contractions begin
• Cervix dilates
• Baby is pushed through birth canal (vagina)
• Umbilical cord is tied and cut after birth
• Cut cord shrinks and forms the navel (belly button)

After Birth Care

Newborn Needs:

• Human babies are unable to care for themselves initially
• Completely dependent on parents for survival

Nutrition:

• First few days: Mother's milk (contains antibodies for immunity)
• Later: Continued breastfeeding or formula milk
• Proper nutrition is critical for growth and development

Immune Development:

• Immune system strengthens during early months
• Mother's milk provides passive immunity
• Vaccinations build active immunity

Health Monitoring:

• Regular visits to health centers essential
• Vaccination schedule must be followed properly
• Protects against diseases like:
  • Polio
  • Measles
  • Diphtheria
  • Tetanus
  • Tuberculosis
  • Hepatitis B

Metamorphosis: Young Ones to Adults

Definition: The transformation of a larva into an adult through drastic physical changes.

Examples:

Silkworm Life Cycle:

1. Egg: Laid by adult moth
2. Larva (Caterpillar): Hatches from egg, feeds voraciously
3. Pupa: Non-feeding stage, transformation occurs inside cocoon
4. Adult: Emerges as a moth

Frog Life Cycle:

1. Egg: Laid in water in jelly-like mass
2. Tadpole Larva: Aquatic stage with gills and tail
3. Metamorphosis: Gradual development of legs, absorption of tail, development of lungs
4. Adult Frog: Terrestrial/semi-aquatic with lungs

Characteristics of Metamorphosis:

• Drastic changes in body form and structure
• Often involves change in habitat (water to land)
• Change in feeding habits
• Development of new organs (e.g., lungs replacing gills)

Advanced Reproductive Technologies

Test Tube Baby (In Vitro Fertilization - IVF)

Definition: A biotechnology technique where fertilization occurs outside the female body in laboratory conditions.

Process:

1. Ovum collection: Mature eggs obtained from the female's ovary
2. Sperm collection: Sperms obtained from the male
3. Fertilization: Egg and sperm united in laboratory dish ("in vitro" = in glass)
4. Cleavage monitoring: Fertilized egg begins dividing
5. Embryo transfer: Developing embryo transplanted into female's uterus
6. Normal development: Embryo implants and develops normally in uterus

Significance:

• Helps women unable to conceive naturally
• Overcomes various fertility issues:
  • Blocked fallopian tubes
  • Low sperm count
  • Unexplained infertility

Historical Milestones:

• First test tube baby in the world: Louise Joy Brown (July 25, 1978, Britain)
• First test tube baby in India: "Harsha" (1986)

Cloning: The Story of Dolly

Definition: The production of an exact genetic copy of a cell, tissue, or complete organism.

Dolly the Sheep:

• Born: July 5, 1996
• Location: Roslin Institute, Edinburgh, Scotland
• Scientists: Ian Wilmut and colleagues
• Significance: First mammal cloned from an adult somatic cell

Cloning Process Used for Dolly:

1. Donor cell preparation:
   • Completely differentiated cell taken from Finn Dorsett sheep's udder
   • Nucleus with diploid (2N) chromosome set isolated
2. Recipient egg preparation:
   • Unfertilized egg taken from Scottish Blackface ewe
   • Nucleus removed (denucleation), leaving cytoplasm
3. Nuclear transfer:
   • Finn Dorsett nucleus inserted into denucleated Scottish Blackface egg
   • Fusion achieved through electroporation
4. Development:
   • Reconstructed egg underwent cleavage
   • Developed into blastula stage embryo
5. Surrogate pregnancy:
   • Blastula implanted in surrogate Scottish Blackface ewe
   • Developed normally through gestation
6. Result:
   • Dolly born as genetic clone of Finn Dorsett sheep
   • Showed no characteristics of Scottish Blackface (egg donor or surrogate)
   • Proved adult cells could be reprogrammed

Dolly's Life:

• Lived normally and produced several offspring through natural sexual reproduction
• Demonstrated that clones could reproduce normally
• Death: February 14, 2003 (due to lung disease)
• Lived for about 7 years

Significance of Cloning:

• Proves genetic information in adult cells can create entire organism
• Opens possibilities for:
  • Preserving endangered species
  • Medical research
  • Therapeutic applications
• Raises ethical questions about cloning

Summary of Key Concepts

Reproduction Essentials:

• Two main modes: Sexual and Asexual reproduction
• Sexual reproduction involves fusion of male and female gametes
• Asexual reproduction involves single parent without gamete fusion

Reproductive Organs:

• Male: Testes, sperm ducts, penis
• Female: Ovaries, fallopian tubes, uterus, vagina
• Gametes: Testes produce sperms; ovaries produce ova

Fertilization:

• Fusion of sperm and ovum forms zygote
• Internal fertilization: Inside female body (humans, hens, cows, dogs)
• External fertilization: Outside female body (frogs, fish, starfish)

Development:

• Zygote divides repeatedly to form embryo
• Embryo embeds in uterine wall for development
• Stage with identifiable body parts is called foetus
• Viviparous animals give birth to live young (humans, cows, dogs)
• Oviparous animals lay eggs (hens, frogs, lizards, butterflies)

Asexual Reproduction Methods:

• Budding: New individuals develop from buds (Hydra)
• Binary fission: Division into two (Amoeba)
• Multiple fission: Division into many (Plasmodium)
• Fragmentation: Growth from body pieces (Flatworms)
• Spore formation: Through spores (Rhizopus)

Special Processes:

• Metamorphosis: Larva transforms into adult through drastic changes
• Test tube baby: IVF technique for assisted reproduction
• Cloning: Creating genetic copies (Dolly the sheep)

Conclusion

Reproduction in animals represents one of nature's most fascinating processes, ensuring the continuity of life through diverse and adaptive mechanisms. From simple asexual reproduction in microscopic organisms to complex sexual reproduction in mammals, each method serves the fundamental purpose of species survival.

Understanding these reproductive processes helps us appreciate:

• The remarkable diversity of life strategies
• The intricate balance between genetic continuity and variation
• The sophisticated systems that have evolved to protect and nurture developing life
• The technological advances that help overcome reproductive challenges

Whether through the rapid multiplication of amoeba through binary fission, the remarkable metamorphosis of a tadpole into a frog, or the complex nine-month journey of human development, reproduction stands as testimony to the power and persistence of life itself.