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NEET NCERT based class 12th biology
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NEET NCERT BASED CLASS 12TH biology notes includes short notes Question answers(etc) , for the whole syllabus.
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- 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.
Organism Lifespan Organism Lifespan Organism Lifespan
Rose 5-7 years Butterfly 1-2 weeks Tortoise 100-150 yrs
Life spans Rice plant 3-7 months Fruit fly 2 weeks Crow 15 yrs
of some Banyan tree 400+ yrs Parrot 140 yrs Cow 22 yrs
organisms Banana tree 2-3 yrs Crocodile 60 yrs Elephant 50-70 yrs
Dog 22 yrs Horse 40-50 yrs
- 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. Examples for vegetative propagules:
- It is seen in unicellular organisms, simple plants & animals. Buds (‘eyes’) of the potato tuber.
- The offspring are identical to one another and to their Rhizomes of banana & ginger.
parent. Such morphologically and genetically similar Buds & Rhizomes arise from the nodes of modified stems.
individuals are known as clone. The nodes come in contact with damp soil or water and
Types of asexual reproduction produce roots and new plants.
Adventitious buds of Bryophyllum. They arise from the
a. Fission: In this, the parent cell divides (cell division) into
notches at margins of leaves.
two or more individuals. E.g. Protists and Monerans.
Bulbil of Agave.
Fission is 2 types:
Offset of water hyacinth.
Binary fission: It is the division of parent cell into two
Runner, sucker, tuber, bulb etc.
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 Other asexual reproductive structures: E.g. zoospores
form new individual. E.g. Hydra, Sponge, Yeast etc. (microscopic motile structures in some algae and protists),
conidia (Penicillium) and gemmules (sponge).
c. Fragmentation: In this, the body breaks into distinct
pieces (fragments) and each fragment grows into an adult Asexual reproduction is the common method in simple
capable of producing offspring. E.g. Hydra. organisms like algae and fungi. During adverse conditions,
d. Vegetative propagation: It is the production of offspring they can shift to sexual method.
from vegetative propagules in plants. Higher plants reproduce asexually (vegetative) & sexually.
Vegetative propagules are units of vegetative propagation. But most of the animals show only sexual reproduction.
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, SEXUAL REPRODUCTION
- It is the reproduction that involves formation of male and Gametes (haploid cells) are 2 types:
female gametes, either by the same individual or by a. Homogametes (isogametes): Similar gametes. They cannot
different individuals of the opposite sex. categorize into male & female gametes. E.g. Some algae
- It results in offspring that are not identical to the parents or like Cladophora.
amongst themselves. b. Heterogametes: The male and female gametes are
- It is an elaborate, complex and slow process as compared distinct types. Male gamete is called antherozoid
to asexual reproduction. (sperm) and female gamete is called egg (ovum). E.g.
- The period of growth to reach in maturity for sexual Fucus (an alga), Human beings etc.
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. Sexuality (bisexual or unisexual) in organisms:
• Bamboo species flower only once in their lifetime (after a. Bisexual: Male & female reproductive structures present
50-100 years), produce large number of fruits and die. in the same individual.
• Strobilanthus kunthiana flowers once in 12 years. Bisexual plants: E.g. Hibiscus, Pisum.
- In animals, juvenile phase is followed by morphological & In flowering plants, male flower is staminate (bears
physiological changes prior to reproductive behaviour. stamens) and female flower is pistillate (bears pistils).
- Birds living in nature lay eggs only seasonally. However, If male & female flowers are present on same plant, it is
birds in captivity (e.g. poultry) can be made to lay eggs called monoecious. E.g. Cucurbits, coconuts, Chara.
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 Bisexual animals (hermaphrodites): E.g. Earthworms,
favourable seasons. leech, sponge, tapeworm, etc.
b. Continuous breeders: They are reproductively active b. Unisexual: Male and female reproductive structures are
throughout their reproductive phase. present on different individuals.
Senescence (old age): If male & female flowers are present on different plants,
- It is the last phase of lifespan and end of reproductive phase. it is called dioecious. E.g. papaya, date palm, Marchantia.
- 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
Unisexual animals: E.g. Cockroach, higher animals etc.
3 stages: Pre-fertilisation, Fertilisation & Post-
fertilisation events. Fungi may be homothallic (bisexual) or heterothallic
(unisexual).
1. Pre-fertilisation Events
Cell division during gamete formation:
These are the events prior to the fusion of gametes.
- Many monerans, fungi, algae & bryophytes have haploid
They include gametogenesis and gamete transfer. parental body. They produce haploid gametes by mitosis.
a. Gametogenesis - Pteridophytes, gymnosperms, angiosperms & animals have
It is the formation of male and female gametes. diploid parental body. They produce haploid gametes by
meiosis of meiocytes (gamete mother cell).
2
, Name of Chromosome number E.g. most aquatic organisms (many algae, bony fishes
organism In meiocytes (2n) In gametes (n) etc.) and amphibians.
Human being 46 23 Such organisms show synchrony between the sexes and
Housefly 12 6 release large number of gametes into the surrounding
Rat 42 21
Dog 78 39 medium to ensure syngamy.
Cat 38 19 Disadvantage: The offspring are extremely vulnerable to
Fruit fly 8 4 predators threatening their survival up to adulthood.
Ophioglossum 1260 630
Apple 34 17
b. Internal fertilisation: Syngamy occurs inside the body
Rice 24 12 of the organism. E.g. terrestrial organisms, belonging to
Maize 20 10 fungi, animals (reptiles, birds, mammals) & plants
Potato 48 24 (bryophytes, pteridophytes, gymnosperms & angiosperms).
Butterfly 380 190
Onion 16 8 In this, non-motile egg is formed inside the female body
to where motile male gamete reaches and fuses.
b. Gamete Transfer In seed plants, the non-motile male gametes are carried to
- Male gametes need a medium to move towards female female gamete by pollen tubes.
gametes for fertilisation. There is large number of sperms produced but the number
- In most organisms, male gamete is motile and the female of eggs is very low.
gamete is stationary.
In some fungi and 3. Post-fertilisation Events
algae, both types of These are the events after the formation of zygote.
gametes are motile. Zygote
- In simple plants - Development of the zygote depends on the type of life
(algae, bryophytes & cycle of the organism and the nature of environment.
pteridophytes), - In fungi and algae, zygote develops a thick wall that is
gamete transfer takes resistant to desiccation and damage. It undergoes a period
place through water medium. To compensate the loss of of rest before germination.
male gametes during transport, large number of male - In organisms with haplontic life cycle, zygote divides by
gametes is produced. meiosis into haploid spores that grow into haploid
- In seed plants, pollen grains (in anthers) carry male individuals.
gametes and ovule carries the egg. Pollen grains are - Sexually reproducing organisms begin life as a zygote.
transferred to the stigma. - Zygote is the vital link between organisms of one
- In bisexual self-fertilizing plants (e.g. peas), anthers & generation and the next.
stigma are closely located for easy transfer of pollen grains.
Embryogenesis
- In cross pollinating plants (including dioecious plants),
pollination helps in transfer of pollen grains. Pollen grains - It is the development of embryo from the zygote.
germinate on the stigma and the pollen tubes carrying the - During embryogenesis, zygote undergoes cell division
male gametes reach the ovule and discharge male gametes (mitosis) and cell differentiation.
near the egg. - Cell divisions increase the number of cells in the embryo.
- In dioecious animals, the fertilisation helps for successful Cell differentiation causes the modifications of groups of
transfer and coming together of gametes. cells into various tissues and organs to form an organism.
Based on place of zygote development, animals are 2 types:
2. Fertilisation (syngamy)
a. Oviparous: Here, animals lay fertilized/unfertilized eggs.
- It is the fusion of gametes to form a diploid zygote.
E.g. Reptiles & birds lay fertilized eggs covered by hard
- In rotifers, honeybees, some lizards, birds (turkey) etc.,
calcareous shell. After incubation, young ones hatch out.
female gamete develops to new organisms without
b. Viviparous: Here, zygote develops into a young one
fertilisation. This is called parthenogenesis.
inside the female body. Later, the young ones are
Types of fertilization: delivered out of the body. E.g. most of mammals.
a. External fertilisation: Syngamy occurs in the external It shows proper care and protection. So the chances of
medium (water), i.e. zygote is formed outside the body. survival of young ones are greater.
Embryogenesis in flowering plants (see next chapter)
3
, MODEL QUESTIONS
1. Write technical terms for the following:
(a) Morphologically different types of gametes.
(b) Process of formation of male and female gametes.
(c) Formation of new organisms without fertilization.
(d) Cell which undergo meiosis.
(e) Male and female sex organs in the same animal.
(f) Development of fruits without fertilization.
2. Observe the relationship of the first two and fill in the blanks.
(a) Sponge: gemmules Hydra: .........................
(b) Homothallic Plants: Monoecious & bisexual Heterothallic Plants: .........................
3. Match the following:
A B
A. Buds (‘eyes’) 1. Bryophyllum
B. Rhizomes 2. Water hyacinth
C. Adventitious buds 3. Banana & ginger
D. Bulbil 4. Potato tuber
E. Offset 5. Agave
4. Distinguish between continuous breeders and seasonal breeders with examples.
5. Differentiate between homogametes and heterogametes with examples.
6. Some plants exhibit unusual flowering. Give any 2 examples.
7. Differentiate between
(a) Asexual reproduction & Sexual reproduction
(b) Juvenile phase & Senescent phase
(c) Monoecious and Dioecious
(d) Oviparous & Viviparous
8. Classify the following organisms, according to the nature of fertilization.
Shark, Tortoise, Tape worm, Frog, Chlamydomonas, Ficus Tree
9. Viviparity is more advantageous than oviparity. Do you agree? Justify.
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