Commerce students can refer to the Reproduction in Organisms Notes Class 12 Biology given below which is an important chapter in class 12 Biology book. These notes and important questions and answers have been prepared based on the latest CBSE and NCERT syllabus and books issued for the current academic year. Our team of Biology teachers have prepared these notes for the benefit of students so that you can read these revision notes and understand each topic carefully.

Reproduction in Organisms Notes Class 12 Biology

Refer to the notes and important questions given below for Reproduction in Organisms which are really useful and have been recommended by Class 12 Biology teachers. Understanding the concepts in detail and then solving questions by yourself will help you to learn all topics given in your NCERT Books for Class 12 Biology. 

– Reproduction is a process in which an organism produces young ones (offspring) similar to itself.
– The period from birth to the natural death of an organism is known as its lifespan.
– No individual is immortal, except unicellular organisms. There is no natural death in unicellular organisms. 

– Based on the number of participants, reproduction is 2 types: Asexual reproduction & Sexual reproduction.

ASEXUAL REPRODUCTION

– It is the production of offspring by a single parent.
– It is seen in unicellular organisms, simple plants & animals.
– The offspring are identical to one another and to their parent. Such morphologically and genetically similar individuals are known as clone.

Types of asexual reproduction

a. Fission : In this, the parent cell divides (cell division) into two or more individuals. E.g. Protists and Monerans. Fission is 2 types:
Binary fission : It is the division of parent cell into two individuals. E.g., Amoeba, Paramecium.
Multiple fission : It is the division of parent cell into many individuals. E.g. Plasmodium, Amoeba. 

Under unfavourable condition, Amoeba withdraws its pseudopodia and secretes a 3-layered hard covering (cyst) around itself. It is called encystation. Under favourable conditions, encysted Amoeba undergoes multiple fission to give many minute amoeba or pseudopodiospores. The cyst wall bursts out and spores are liberated to grow up into many amoebae. This is called sporulation.

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

c. Fragmentation : In this, the body breaks into distinct pieces (fragments) and each fragment grows into an adult capable of producing offspring. E.g. Hydra.
d. Vegetative propagation : It is the production of offspring from vegetative propagules in plants.
Vegetative propagules are units of vegetative propagation.
Examples for vegetative propagules :
• Buds (‘eyes’) of the potato tuber.
• Rhizomes of banana & ginger.
Buds & Rhizomes arise from the nodes of modified stems. The nodes come in contact with damp soil or water and produce roots and new plants.
• Adventitious buds of Bryophyllum. They arise from the notches at margins of leaves.
• Bulbil of Agave.
• Offset of water hyacinth.
• Runner, sucker, tuber, bulb etc.  

Other asexual reproductive structures : E.g. zoospores (microscopic motile structures in some algae and protists),
conidia (Penicillium) and gemmules (sponge).  

Asexual reproduction is the common method in simple organisms like algae and fungi. During adverse conditions, they can shift to sexual method.
Higher plants reproduce asexually (vegetative) & sexually. But most of the animals show only sexual reproduction.

– It is the reproduction that involves formation of male and female gametes, either by the same individual or by different individuals of the opposite sex.
– It results in offspring that are not identical to the parents or amongst themselves.
– It is an elaborate, complex and slow process as compared to asexual reproduction.
– The period of growth to reach in maturity for sexual reproduction is called the juvenile phase. In plants, it is known as vegetative phase.
– In higher plants, the flowering indicates the end of vegetative phase (beginning of reproductive phase).
– Annual & biennial plants show clear cut vegetative, reproductive & senescent phases. In perennial plants, these phases are very difficult to identify.
– Some plants exhibit unusual flowering. E.g.
• Bamboo species flower only once in their lifetime (after 50-100 years), produce large number of fruits and die.
• Strobilanthus kunthiana flowers once in 12 years.
– In animals, juvenile phase is followed by morphological & physiological changes prior to reproductive behaviour.
– Birds living in nature lay eggs only seasonally. However, birds in captivity (e.g. poultry) can be made to lay eggs throughout the year.
– The females of placental mammals exhibit cyclical changes in the ovaries, accessory ducts and hormones during the reproductive phase. It is called oestrus cycle in non-primates (cows, sheep, rat, deer, dog, tiger etc.) and menstrual cycle in primates (monkeys, apes & humans).

Based on breeding season, mammals are 2 types:
a. Seasonal breeders : The mammals (living in natural conditions) exhibiting reproductive cycles only during favourable seasons.
b. Continuous breeders : They are reproductively active throughout their reproductive phase.
Senescence (old age):
– It is the last phase of lifespan and end of reproductive phase.
– During this, concomitant changes occur in the body. E.g. slowing of metabolism etc. It ultimately leads to death.
In plants & animals, hormones cause transition between juvenile, reproductive & senescence phases. Interaction between hormones and environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms.

EVENTS IN SEXUAL REPRODUCTION

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

1. Pre-fertilisation Events

These are the events prior to the fusion of gametes. They include gametogenesis and gamete transfer.
a. Gametogenesis
It is the formation of male and female gametes Gametes (haploid cells) are 2 types:
a. Homogametes (isogametes) : Similar gametes. They cannot categorize into male & female gametes. E.g. Some algae like Cladophora.
b. Heterogametes : The male and female gametes are distinct types. Male gamete is called antherozoid (sperm) and female gamete is called egg (ovum). E.g. Fucus (an alga), Human beings etc.   

Sexuality (bisexual or unisexual) in organisms :
a. Bisexual: Male & female reproductive structures present in the same individual.
Bisexual plants: E.g. Hibiscus, Pisum.
In flowering plants, male flower is staminate (bears stamens) and female flower is pistillate (bears pistils).
If male & female flowers are present on same plant, it is called monoecious. E.g. Cucurbits, coconuts, Chara.      

Bisexual animals (hermaphrodites): E.g. Earthworms, leech, sponge, tapeworm, etc.
b. Unisexual: Male and female reproductive structures are present on different individuals.
If male & female flowers are present on different plants, it is called dioecious. E.g. papaya, date palm, Marchantia.    

Unisexual animals: E.g. Cockroach, higher animals etc. Fungi may be homothallic (bisexual) or heterothallic (unisexual).
Cell division during gamete formation:
– Many monerans, fungi, algae & bryophytes have haploid
parental body. They produce haploid gametes by mitosis.
– Pteridophytes, gymnosperms, angiosperms & animals have diploid parental body. They produce haploid gametes by meiosis of meiocytes (gamete mother cell).     

b. Gamete Transfer

– Male gametes need a medium to move towards female gametes for fertilisation.
– In most organisms, male gamete is motile and the female gamete is stationary.
In some fungi and algae, both types of gametes are motile.        

– In simple plants (algae, bryophytes & pteridophytes), gamete transfer takes
place through water medium. To compensate the loss of male gametes during transport, large number of male gametes is produced.
– In seed plants, pollen grains (in anthers) carry male gametes and ovule carries the egg. Pollen grains are transferred to the stigma.
– In bisexual self-fertilizing plants (e.g. peas), anthers & stigma are closely located for easy transfer of pollen grains.
– In cross pollinating plants (including dioecious plants), pollination helps in transfer of pollen grains. Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.
– In dioecious animals, the fertilisation helps for successful transfer and coming together of gametes.

2. Fertilisation (syngamy)

– It is the fusion of gametes to form a diploid zygote.
– In rotifers, honeybees, some lizards, birds (turkey) etc., female gamete develops to new organisms without fertilisation. This is called parthenogenesis.
Types of fertilization:
a. External fertilisation: Syngamy occurs in the external medium (water), i.e. zygote is formed outside the body.
E.g. most aquatic organisms (many algae, bony fishes etc.) and amphibians.
Such organisms show synchrony between the sexes and release large number of gametes into the surrounding medium to ensure syngamy.
Disadvantage: The offspring are extremely vulnerable to predators threatening their survival up to adulthood.
b. Internal fertilisation: Syngamy occurs inside the body of the organism. E.g. terrestrial organisms, belonging to fungi, animals (reptiles, birds, mammals) & plants (bryophytes, pteridophytes, gymnosperms & angiosperms). In this, non-motile egg is formed inside the female body to where motile male gamete reaches and fuses.
In seed plants, the non-motile male gametes are carried to female gamete by pollen tubes.
There is large number of sperms produced but the number of eggs is very low.

3. Post-fertilisation Events

These are the events after the formation of zygote.
Zygote

– Development of the zygote depends on the type of life cycle of the organism and the nature of environment.
– In fungi and algae, zygote develops a thick wall that is resistant to desiccation and damage. It undergoes a period of rest before germination.
– In organisms with haplontic life cycle, zygote divides by meiosis into haploid spores that grow into haploid individuals.
– Sexually reproducing organisms begin life as a zygote.
– Zygote is the vital link between organisms of one generation and the next.

Embryogenesis
– It is the development of embryo from the zygote.
– During embryogenesis, zygote undergoes cell division
(mitosis) and cell differentiation.
– Cell divisions increase the number of cells in the embryo. Cell differentiation causes the modifications of groups of cells into various tissues and organs to form an organism.
Based on place of zygote development, animals are 2 types:
a. Oviparous : Here, animals lay fertilized/unfertilized eggs.
E.g. Reptiles & birds lay fertilized eggs covered by hard
calcareous shell. After incubation, young ones hatch out.
b. Viviparous : Here, zygote develops into a young one inside the female body. Later, the young ones are delivered out of the body. E.g. most of mammals.
It shows proper care and protection. So the chances of survival of young ones are greater.
Embryogenesis in flowering plants (see next chapter)