Reproduction in organisms

  

Reproduction in organisms 

Chapter:-1

Reproduction in organisms

- Reproduction is the biological process by which organisms produce new individuals of the same species. It is essential for the survival and continuation of a species. Reproduction in organisms

-The periods from birth to the natural death of an organism represents its life span. Reproduction in organisms

-No individual is immortal, except unicellular organism .There is no natural death in unicellular organisms. Reproduction in organisms Class 12 NCERT  NOTES PDF.

Life Span of a few Organisms

Organism	Lifespan	organism	Lifespan
Elephant	50-70 yrs	Parrot	140 yrs
Crocodile	60 yrs	Dog	22 yrs
Butterfly	1-2 weeks	Fruits fly	2 week
Horse	40-50 yrs	Crow	15 yrs
Tortoise	100-150 yrs	Cow	22yrs

-Reproduction enables the continuity of the species generation.

-Reproduction is 2 types: 

Asexual reproduction & Sexual reproduction.

 Reproduction in organisms Class 12 NCERT  NOTES PDF.

#1.ASEXUAL REPRODUCTION

-It is the production of offspring by a single parent.

-The offspring are identical one another and to their parent. Such morphological and genetically similar individuals are known as clone.

-Asexual reproduction is found in unicellular organism and in simple plants and animals.

 Reproduction in organisms Class 12 NCERT  NOTES PDF.

 

Types of asexual reproduction

·         FISSION:

Fission is a type of asexual reproduction or nuclear reaction, depending on the context.

E.g.,  Portists and Monerans.

Fission Is 2Types:-

o   Binary Fission: It is the division of parents cell into two individuals. Reproduction in organisms

E.g., Amoeba, Paramecium.

o   Multiple Fission: It is the  division of parent cell into many individuals. E.g., Plasmodium, Amoeba.

·         Budding: In this, a small bud appears and grows in the parent body. After maturation, it is detached from the parent body to form new individual. E.g., Hydra , Sponge ,Yeast etc.

Other asexual reproduction structures: Zoospores (microscopic motile  structures in some algae and protists), conidia(penicillium) and gemmules(sponge). Reproduction in organisms Class 12 NCERT  NOTES PDF.

·         Vegetative propagation: In plants, vegetative propagules (the units of vegetative propagation such as runner, rhizome, sucker, tuber, offset and bulb) are capable of giving rise to new offspring’s.

Examples for vegetative propagation:-

-       Emergence of small plants from the buds (‘eyes‘) of the potato tuber from the nodes of modified stems. When the nodes come in contact with damp soil or water, they produced roots and new plants.

- Adventitious buds arise from the notches present at margins of leaves of Bryophyllum.

Asexual Reproductions is the common method in simple organisms like algae and fungi. 

Higher plants exhibits both asexual (vegetative) and sexual modes of reproduction. But most of reproduction .So most of the animal show only sexual reproduction. Reproduction in organisms Class 12 NCERT  NOTES PDF.

#2.SEXUAL REPRODUCTION

 Reproduction in organisms Class 12 NCERT  NOTES PDF.

Sexual reproduction is a biological process that involves the combination of genetic material from two parent organisms to produce offspring with genetic diversity. Here are the key points about sexual reproduction:

1. Genetic Variation: Sexual reproduction combines genetic material from two different gametes (sperm and egg in animals; pollen and ovule in plants), resulting in offspring with a unique combination of genes from both parents. This genetic variation enhances adaptability and survival. Reproduction in organisms

2. Gamete Formation: Gametes are specialized reproductive cells that are haploid, meaning they contain half the number of chromosomes of the parent organism. In animals, sperm and eggs are produced through a process called meiosis, which reduces the chromosome number by half.

3. Fertilization: Fertilization is the process where a sperm cell from a male parent and an egg cell from a female parent merge to form a zygote. This zygote is diploid, containing a full set of chromosomes (half from each parent). Reproduction in organisms

4. Development: The zygote undergoes multiple rounds of cell division and differentiation to develop into a new organism. This process includes growth, maturation, and specialization of cells and tissues.

5. Reproductive Strategies: Sexual reproduction can involve different strategies, such as internal fertilization (e.g., in mammals) or external fertilization (e.g., in many fish and amphibians). The strategy often depends on the species' environment and evolutionary adaptations.

Sexual reproduction is a key mechanism for creating genetic diversity, which contributes to the evolutionary success and adaptability of species.

-Seasonal breeders: The mammals (living in natural conditions) exhibiting reproductive cycle during favourable seasons.

-Continuous breeders: The mammals those out their reproductive phase.

Senescence (old age):

 -It is the last phase .

-During this, concomitant changes in body (slowing of metabolism etc.) occur. It ultimatey, leads to death.

In plants & animals, hormones are responsible for transition between juvenile, reproductive & senescence phases.  Reproduction in organisms Class 12 NCERT  NOTES PDF.

Events in sexual reproduction

3 stages: Pre-fertilisation, Fertilisation & Post fertilisation events.

1.  Pre-fertilisation: These are all the events to the fusion of gametes .They include gametogenesis and gamete transfer. Reproduction in organisms Class 12 NCERT  NOTES PDF.

-It is the process of formation of male and female gametes (haploid cells).

-Iogametes are a type of gamete involved in sexual reproduction where both gametes are morphologically similar or identical in size and shape. Unlike the more common scenario where gametes are distinguished by size or function (e.g., sperm and eggs), isogametes do not have these differences. Reproduction in organisms

-In others, the male gametes are distinct types (heterogametes). Male gamete is called the antherozoid (sperm) and female gamete is called the egg (ovum).

Sexuality in organisms:

-Plants may be bisexual  or unisexual 

-In monoecious flowering plants, male & female flowers are presents on some individual.

E.g. cucurbits & coconuts.

-fungi may be homothallic (bisexual) or heterothallic (unisexual). Reproduction in organisms Class 12 NCERT  NOTES PDF.

-Bisexual  animals  (hermaphrodites): Earthworms, leech, sponge, tapeworm, etc.

 

-Unisexual animals: Cockroach, higher animals etc. Reproduction in organisms Class 12 NCERT  NOTES PDF.

 Cell division during gamete formation:

 

-Haploid parental body (many monera, fungi, algae and bryophytes) produces haploid gametes by meiosis of meiocytes (gamete mother cell).

-Diploid parental body (pteridophytes, gymnosperms, angiosperms & animals) produces haploid gametes by meiosis of meiocytes (gamete mother cell). Reproduction in organisms

Name of organisms	Chromosomes number
	In meiocytes (2n)	In gametes(n)
Human being	46	23
Housefly	12	24
Rat	42	21
Dog	78	39
Cat	38	19
Fruit fly	8	4
Ophioglossum	1260	630
Apple	34	17
Rice	24	12
Maize	20	10
Potato	48	24
butterfly	380	190
Onion	32	16

   Reproduction in organisms Class 12 NCERT  NOTES PDF.

b.  Gamete Transfer

Gamete transfer is a crucial process in sexual reproduction where gametes (sperm and eggs) are moved from one organism to another or within the same organism to facilitate fertilization. Here’s a breakdown of the key aspects of gamete transfer:

1. Mechanisms:

   • External Fertilization: Gamete transfer occurs outside the organism's body. In aquatic environments, many fish and amphibians release eggs and sperm into the water, where fertilization happens externally. Examples include many fish species and frogs. Reproduction in organisms
   • Internal Fertilization: Gametes are transferred directly into the female reproductive tract. This method is common in terrestrial animals, such as mammals, birds, and reptiles. Sperm is deposited inside the female’s body where fertilization occurs. This process involves specialized reproductive structures like the penis in mammals and the intromittent organ in some reptiles. Reproduction in organisms

2. Reproductive Strategies:

   • Pollination: In plants, gamete transfer is often mediated by pollinators (e.g., bees, butterflies) or by wind. Pollen, which contains male gametes (sperm cells), is transferred from the male part (anther) of a flower to the female part (stigma) of another flower, leading to fertilization.
   • Sporulation and Gamete Release: In some fungi and algae, gametes are released into the environment and must encounter each other for fertilization to occur. For example, in many algae, both gametes are flagellated and swim towards each other. Reproduction in organisms

3. Adaptations:

   • Behavioral Adaptations: Many animals have developed complex mating behaviors to ensure successful gamete transfer. This can include courtship rituals, territory defense, or specific mating dances. Reproduction in organisms
   • Anatomical Adaptations: Various species have evolved specialized reproductive organs to facilitate internal fertilization. For example, mammals have a complex reproductive system that includes structures like the uterus and vagina for sperm deposition and embryo development.

4. Fertilization: After gametes are transferred, fertilization typically occurs either inside the female’s body (internal fertilization) or in the external environment (external fertilization). Successful fertilization results in the formation of a zygote, which will develop into a new organism.

5. Timing and Synchronization: In many species, timing and synchronization of gamete release are crucial for successful reproduction. This can involve seasonal breeding, environmental cues, or hormonal signals to ensure that gametes meet and fertilization occurs efficiently. Reproduction in organisms

Gamete transfer is essential for sexual reproduction and varies widely among different organisms, reflecting the diverse strategies evolved to maximize reproductive success. 

2.  Fertilisation (syngamy):

Fertilization, also known as syngamy, is the process by which two gametes (sperm and egg) merge to form a zygote, leading to the development of a new organism. Here’s a detailed look at the process:

1. Gamete Fusion:

   • Sperm and Egg: Fertilization begins with the union of male and female gametes. In animals, sperm (the male gamete) and egg (the female gamete) combine to form a zygote. In plants, pollen (containing male gametes) and ovule (containing female gametes) merge in a similar process. Reproduction in organisms
   • Gamete Compatibility: The gametes must be compatible, meaning they must have the correct molecular signals to recognize and bind to each other. This often involves specific proteins on the surface of the gametes.

2. Processes Involved:

   • Acrosome Reaction: In many animals, the sperm cell undergoes an acrosome reaction, where enzymes are released to help the sperm penetrate the egg’s outer layers. Reproduction in organisms
   • Penetration: The sperm enters the egg by merging its membrane with that of the egg, allowing the sperm’s nucleus to enter the egg’s cytoplasm.
   • Cortical Reaction: The egg’s membrane undergoes changes to prevent additional sperm from entering, ensuring that only one sperm fertilizes the egg. Reproduction in organisms

3. Formation of the Zygote:

   • Fusion of Nuclei: The nuclei of the sperm and egg combine, restoring the diploid number of chromosomes and creating a zygote with a complete set of genetic material from both parents.
   • Activation: The fertilized egg (zygote) undergoes activation, initiating cell division and development processes. Reproduction in organisms

4. Development:

   • Cleavage: The zygote begins to divide through a process called cleavage, leading to the formation of a multicellular embryo.
   • Embryo Formation: The embryo continues to develop, undergoing stages like gastrulation and organogenesis, eventually forming a fully developed organism. Reproduction in organisms

5. Types of Fertilization:  Fertilization can be categorized into different types based on where and how gametes meet and combine. Here’s an overview of the main types:

1. Internal Fertilization

Definition: Fertilization occurs inside the female's body. Reproduction in organisms

Process:

• Gamete Transfer: Sperm is deposited directly into the female reproductive tract.
• Fertilization: Sperm travels through the female reproductive system to meet the egg, usually in the oviduct or fallopian tube. Reproduction in organisms

Examples:

• Mammals: Humans, dogs, and whales.
• Birds: Chickens, eagles.
• Reptiles: Snakes, lizards.

Advantages:

• Increased likelihood of successful fertilization due to proximity of gametes.
• Protection of gametes and developing embryo within the female's body.

2. External Fertilization

Definition: Fertilization occurs outside the body, typically in the external environment. Reproduction in organisms

Process:

• Gamete Release: Both sperm and eggs are released into the environment (e.g., water).
• Fertilization: Gametes meet and fuse in the external environment.

Examples:

• Fish: Many species, such as salmon and trout.
• Amphibians: Frogs and salamanders.
• Plants: Pollination often involves external fertilization where pollen (male gametes) meets ovules (female gametes) in flowers. Reproduction in organisms

Advantages:

• Can produce large numbers of offspring.
• Often associated with species that live in aquatic environments or have external developmental stages. Reproduction in organisms

3. Self-Fertilization (Autogamy)

Definition: A single organism produces offspring by using its own gametes.

Process:

• Self-compatibility: The organism’s gametes fuse with each other, leading to the formation of a zygote. Reproduction in organisms 

Examples:

• Plants: Many flowering plants can self-pollinate.
• Hermaphroditic Animals: Some invertebrates, such as earthworms and certain types of flatworms.

Advantages:

• Ensures reproduction when mates are scarce.
• Can be advantageous in stable environments where genetic variation is less critical.

4. Cross-Fertilization (Allogamy)

Definition: Fertilization occurs between gametes from different individuals. Reproduction in organisms

Process:

• Gamete Exchange: Gametes are exchanged between different organisms.
• Fertilization: The fusion of gametes from different individuals leads to genetic diversity in offspring.

Examples:

• Humans: Fertilization occurs between sperm and egg from different individuals.
• Many Animals: Most sexually reproducing animals.
• Plants: Many rely on cross-pollination facilitated by pollinators.

Advantages:

• Increases genetic diversity, which can enhance adaptability and survival. Reproduction in organisms

5. Hybrid Fertilization

Definition: Involves the fertilization between gametes from different species, resulting in hybrid offspring. Reproduction in organisms

Process: 

• Interbreeding: Gametes from different but related species fuse to form hybrids.

Examples:

• Plants: Hybrid crops like certain types of wheat or corn.
• Animals: Hybrids such as ligers (lion-tiger cross) and mules (horse-donkey cross). Reproduction in organisms

Advantages:

• Can combine desirable traits from both parent species.

Each type of fertilization has evolved to suit the reproductive strategies and environmental conditions of different organisms, contributing to the diversity of life on Earth

3.  Post fertilisation events

The Zygote

The zygote is the initial cell formed when two gametes (sperm and egg) fuse during fertilization. It marks the beginning of the development of a new organism. Here’s a detailed look at the zygote:

1. Formation

• Fertilization: The zygote is created when a sperm cell from a male parent and an egg cell from a female parent merge. This fusion combines their genetic material.
• Genetic Material: The zygote is diploid, meaning it contains a complete set of chromosomes—half from each parent. This restoration of the diploid number is crucial for the development of a new organism. Reproduction in organisms

2. Structure

• Single Cell: Initially, the zygote is a single, undifferentiated cell.
• Nucleus: The zygote’s nucleus contains a full set of chromosomes, including all the genetic instructions necessary for development. Reproduction in organisms
• Cytoplasm: The cytoplasm of the zygote contains nutrients and organelles that will support initial cell divisions and development. Reproduction in organisms

3. Development Stages

• Cleavage: The zygote undergoes a series of rapid cell divisions called cleavage, resulting in a multicellular embryo. These divisions do not increase the size of the embryo but rather increase the number of cells. Reproduction in organisms
• Blastula Formation: The cells continue to divide and eventually form a blastula, a hollow sphere of cells surrounding a fluid-filled cavity.
• Gastrulation: The blastula undergoes gastrulation, where cells begin to differentiate into various tissue types and layers, forming the basic body plan of the embryo.
• Organogenesis: As development progresses, organs and tissues start to form through a process called organogenesis. Reproduction in organisms

4. Key Processes

• Genetic Combination: The zygote’s genetic material combines the DNA from both parents, leading to genetic variation in the offspring. Reproduction in organisms
• Cell Division: The zygote’s initial cell division stages are crucial for setting the stage for all subsequent development.
• Environmental Interaction: The zygote's development is influenced by the surrounding environment and maternal factors, including nutrient availability and hormonal signals. Reproduction in organisms

5. Importance

• Foundation of Development: The zygote is the starting point for the development of all multicellular organisms, including humans, plants, and animals.
• Genetic Inheritance: It is the first cell to contain a complete set of genetic instructions for the organism, which will be passed on to all subsequent cells. Reproduction in organisms
• Biological Continuity: The zygote represents the continuity of genetic material from one generation to the next, ensuring the transfer of traits and characteristics. Reproduction in organisms

The zygote’s journey from a single cell to a fully developed organism involves complex and tightly regulated processes, underscoring its fundamental role in reproduction and development. Reproduction in organisms Class 12 NCERT  NOTES PDF.

-Embryogenesis is the process by which a fertilized egg (zygote) develops into a fully formed embryo, eventually becoming a mature organism. This process involves a series of complex and highly regulated stages that lead to the formation of tissues and organs. Here’s a detailed overview of embryogenesis:

1. Cleavage

• Definition: Cleavage is the series of rapid mitotic cell divisions that follow fertilization. Reproduction in organisms
• Process: The zygote divides into smaller cells called blastomeres without increasing the overall size of the embryo. These divisions lead to the formation of a solid ball of cells known as the morula.
• Outcome: The morula then develops into a blastocyst (in mammals) or blastula (in other animals), which is a hollow sphere of cells surrounding a fluid-filled cavity. Reproduction in organisms

2. Gastrulation

• Definition: Gastrulation is the process where the blastula transforms into a three-layered structure called the gastrula. Reproduction in organisms
• Process: Cells undergo extensive movement and rearrangement to form three primary germ layers:
  • Ectoderm: The outer layer, which will develop into the skin and nervous system.
  • Mesoderm: The middle layer, which will give rise to muscles, bones, and the circulatory system.
  • Endoderm: The inner layer, which will form the digestive tract and associated organs.
• Outcome: This process establishes the basic body plan and the primary tissue layers. Reproduction in organisms

3. Neurulation

• Definition: Neurulation is the formation of the nervous system.
• Process: The ectoderm forms the neural plate, which then folds to create the neural tube. The neural tube will develop into the brain and spinal cord.
• Outcome: Formation of the neural tube is critical for the development of the central nervous system. Reproduction in organisms

4. Organogenesis

• Definition: Organogenesis is the formation and development of organs from the three germ layers.
• Process: Cells continue to differentiate and organize into tissues and organs. This includes the development of specific structures such as the heart, lungs, and limbs. Reproduction in organisms
• Outcome: By the end of organogenesis, the embryo has recognizable organs and body systems.

5. Fetal Development

• Definition: Fetal development follows embryogenesis and involves the maturation of the organs and systems formed during embryogenesis. Reproduction in organisms
• Process: The embryo is now called a fetus and continues to grow and develop. This stage involves further differentiation of tissues, growth in size, and functional development of organs.
• Outcome: The fetus develops into a fully formed organism ready for birth or hatching. Reproduction in organisms

Additional Points

• Morphogenesis: Throughout embryogenesis, morphogenetic processes shape the embryo and involve cell movements, changes in cell shape, and tissue folding. Reproduction in organisms
• Regulation: Embryogenesis is tightly regulated by genetic and environmental factors. Proper timing and coordination of developmental processes are crucial for normal development.
• Model Organisms: Studies in model organisms (e.g., fruit flies, frogs, mice) provide insights into the mechanisms of embryogenesis and help in understanding human development. Reproduction in organisms

Embryogenesis is a fundamental aspect of developmental biology and is essential for the growth and formation of all multicellular organisms. The complexity and precision of this process highlight the intricate nature of life’s development from a single cell to a fully formed organism. Reproduction in organisms