These notes for CBSE Class 12 Biology, Chapter 1, "Reproduction in Organisms," explain the fundamental concepts of life and reproduction. It defines lifespan as the period from birth to natural death, providing examples like elephants and fruit flies. The chapter details two primary modes of reproduction: asexual, involving a single parent, and sexual, requiring two parents. Various asexual reproduction methods are discussed, including binary fission (Amoeba, Paramoecium), multiple fission (Plasmodium), budding (Hydra, Yeast), conidia, and zoospores. In plants, vegetative reproduction using units like runners and tubers is highlighted. The notes also cover the three phases of lifespan: juvenile, reproductive, and senescent. Key events in sexual reproduction, such as pre-fertilization (gametogenesis, gamete transfer), fertilization (external and internal), and post-fertilization events (zygote formation, embryogenesis), are elaborated. It also touches upon parthenogenesis and the formation of clones. These notes are designed to aid students in revising the core concepts for their exams.
Last optimized 10 Aug 2026
Jump to solutions, books, papers, or another subject — without leaving NCERT Help.
• Life Span • Period from birth till natural death. Every organism lives only for certain period of time Eg Elephant 60 -90 years, Fruit fly 4-5 weeks. Every organism live only for a certain period of time.
Producing young-ones of their kind, generation after generation.
• Asexual reproduction :single parent capable of producing offsprings .Somatogenic reproduction • Sexual reproduction : two parents are invovled in producing offspring
• Binary fission : parent body divides into two halves, genetically identical to parent.
Amoeba: It is simple or irregular.
• Paramoecium : Transverse binary fission. Organisms considered
immortal
• Multiple fission : parent body divides into many daughter organisms Plasmodium.
• Budding : daughter organisms grow from small buds arising in parent body. Exogenous budding: out side the body eg. Hydra, Yeast.
• Budding in Yeast Endogenous budding : inside the body eg. Gemmule in sponge. Oidia of rhizopus
• Conidia : non-motile, exogenous spores in chains eg. Fungi.
• Zoospores : microscopic motile structures eg. Algae.
• In plants : term vegetative reproduction
frequently used instead of asexual reproduction, units of vegetative propagation called
vegetative propagules.Eg runner, rhizome, sucker, tuber offset, bulb give rise to new plant
• All organisms show remarkable similarity. Vast difference in their reproductive structure. Similar
pattern or phases in their life cycles
• Juvenile phase : The phase of growth before reproductive maturity. • Reproductive phase : Reproductive maturity. • Senescent phase : Phase between reproductive maturity and
i. Pre-fertilisation events
a. Gametogenesis
• The process of formation of male and female gametes by meiosis (cell-division).
? Homogamete (Isogamete) : - gametes similar eg. Algae
? Heterogamete(an-isogamete) : - morphologically dissimilar gamete ,male gamete (antherozoid or
sperm) ,female gamete (egg or ovum) eg. Human.
? Sexuality in organisms : In plants Bisexual term is used for Homothallic and Monoecious
plants
Both male and female reproductive structures in same plant eg. Higher plants, cucurbits and coconut.
? Unisexual term used for Heterothallic and Dioecious plants Male and female reproductive-structure on different plants.
Flowering plants – male flower–staminate flower and female flower–pistillate flower eg. papaya
and date-palm.
• Animals – Bisexual term is used for Hermaphrodite animals -eg. Earth-worm, Tape-worm,
Leech, Sponge.
• Unisexual animals have male & female sexes in separate individuals -e.g. insects, frogs,
humanbeings
• Cell division during gamete formation :
• Haploid-parent (n) produces haploid gametes (n) by mitotic division, eg. Monera, fungi, algae and
bryophytesDiploid parent (2n) produces haploid gametes(n) by meiosis division (possess only one set of
chromosomes)and such specialized parent cell is called meiocyte or gamete mother cell..
Name of organism |
Meiocyte (2n) |
gamete (n) |
| Human | 46 | 23 |
| Housefly | 12 | 6 |
| Ophioglossum (fern) | 1260 | 630 |
| Potato | 48 | 24 |
b) Gamete transfer :- to facilitate fusion. ? Male gametes mostly motile and female non-motile, exception few fungi and in algae both gametes are motile in some cases ? Water medium for gamete transfer- in lower plants. Large number of male gametes produced to compensate loss ? Higher plants, pollen-grains are transferred by pollination. ? Fertilization :- Fusion of male and female gametes diploid zygote. ? Parthenogenesis :- development into new organism without fertilisation eg. Rotifers, honey-bees, some lizard, bird(turkey). ii. Fertilization
• External fertilisation :- outside the body of organism in external- medium (water) eg. majority of
algae, fishes, amphibians.
• Advantage :- show great synchrony between the sexes –
1. Release of large number of gametes into surrounding medium
2. Large number of offsprings produced.
• Disadvantage :- offspring vulnerable to predators, natural disasters.
• Internal fertilisation :- fusion occurs inside female body eg. majority of plants and animals. Egg
non-motile and formed inside female body. Male gamete motile, produced in large numbers to
reach egg and fuse with it. In seed plants, non- motile male gamete carried to female gamete by
pollen-tube.
a. Zygote. One celled , diploid, vital link between two generations.
• External fertilization :- zygote formed in external medium water eg. Frog,
• Internal fertilization :– zygote formed inside the body eg. Human beings. Development of zygote
depends on type of life cycle and environment. Some develop thick wall ( prevent damage and desiccation) & undergo period of rest eg. Algae, fungi.
• Haplontic life cycle :- zygote (2n) divides by meiosis to form haploid (n) spores.
• Diplontic life-cycle :- zygote (2n) divides mitotically, develops into embryo (2n).
• Oviparous animals lay eggs out :- side the female body.Eggs can be fertilized/ unfertilized. Fertilized
eggs covered which hard calcareous shell, laid in safe place in the environment. Unfertilised eggs
laid in water. Example- fishes, frogs, reptiles, birds
• Viviparous animals bear and rear the embryo inside female body, give birth to young-ones.
Advantage- proper embryonic care, protection, survival chances of young-ones greater. Examplecows,
whales, human beings .
• Embryogenesis :- development of embryo from zygote by cell division (mitosis) and cell
differentiation.
• Cell :- division increases the number of cells in the developing embryo
Cell differentiation - groups of cells undergo certain modifications for the formation of different
kinds of tissues and organs.
• In flowering plants :- zygote formed inside ovule
| Sepal | Fall off |
| Petal | Fall off |
| Stamen | Fall off |
| Zygote | Embryo |
| Primary endosperm nucleus | Endosperm (3 N) |
| Synergid | Disintegrate |
| Antipodals | Disintegrate |
| Ovary | Fruit |
| Ovule | Seed |
| Ovary wall | Pericarp (epicarp + mesocarp + endocarp) |
| Integument | Seed coat (testa + tegmen) |
• Parthenogenesis :Female gamete develops into new organism. • Seedless fruits formed by parthenogenesis • Clone : A group of individuals of the same species that are morphologically and genetically similar to each other & their parents • Turion : Fleshy overwintering buds in aquatic plants help in perrenation Eg potomegaton, utricularia • Bulbil : Fleshy buds that produce new plant Eg Agave and Oxalis
Lifespan is defined as the period from birth till natural death of an organism.
The two main types of reproduction are asexual reproduction, involving a single parent, and sexual reproduction, involving two parents.
Binary fission is a mode of asexual reproduction where the parent body divides into two genetically identical halves, as seen in Amoeba and Paramoecium.
The three phases of lifespan are the juvenile phase (growth before maturity), the reproductive phase (reproductive maturity), and the senescent phase (aging).
Gametogenesis is the process of formation of male and female gametes, usually by meiosis.
External fertilization occurs outside the organism's body in an external medium like water, while internal fertilization occurs inside the female body.
Post-fertilization events include the formation of the zygote and embryogenesis, which is the development of the embryo from the zygote.
Parthenogenesis is the development of a new organism from a female gamete without fertilization.
Copyright @ ncerthelp.com A free educational website for CBSE, ICSE and UP board.
