Class 12 Biology – Short Definitions of All Terminologies (NCERT Based)

Chapter 1: Reproduction in Organisms

• Reproduction – A biological process by which living organisms produce offspring similar to themselves.

• Asexual reproduction – Formation of new individuals without the fusion of gametes; a single parent produces offspring identical to itself.

• Clone – A group of individuals genetically identical to the parent.

• Binary fission – Division of a parent body into two halves; common in bacteria and amoeba.

• Budding – Outgrowth (bud) from a parent that detaches to form a new individual; e.g., yeast, Hydra.

• Zoospores – Microscopic, motile spores with flagella; produced by algae and fungi.

• Conidia – Non‑motile, asexual spores produced in chains; e.g., Penicillium.

• Gemmules – Internal buds formed in sponges for asexual reproduction.

• Vegetative propagation – New plants develop from vegetative parts (root, stem, leaf) without seeds.

• Sexual reproduction – Fusion of male and female gametes to form a diploid zygote.

• Juvenile phase – Period of growth before attaining sexual maturity.

• Vegetative phase – Phase in plants before they start flowering.

• Senescence – Ageing phase leading to death.

• Annuals – Plants that complete their life cycle in one season.

• Biennials – Plants that complete their life cycle in two seasons.

• Perennials – Plants that live for several years and flower repeatedly.

• Oviparous – Animals that lay eggs; e.g., birds, reptiles.

• Viviparous – Animals that give birth to live young; e.g., humans, dogs.

Chapter 2: Sexual Reproduction in Flowering Plants

• Flower – Reproductive structure of angiosperms.

• Stamen – Male reproductive organ; consists of anther and filament.

• Anther – Bilobed structure containing four microsporangia; produces pollen grains.

• Microsporogenesis – Formation of microspores (pollen grains) from microspore mother cells by meiosis.

• Pollen grain – Male gametophyte; contains two male gametes and a vegetative cell.

• Pistil (carpel) – Female reproductive organ; consists of stigma, style, and ovary.

• Megasporogenesis – Formation of megaspores from megaspore mother cell by meiosis.

• Embryo sac – Female gametophyte; typically a 7‑celled, 8‑nucleate structure.

• Pollination – Transfer of pollen grains from anther to stigma.

• Autogamy – Transfer of pollen to the stigma of the same flower.

• Geitonogamy – Transfer of pollen to the stigma of another flower on the same plant.

• Xenogamy – Transfer of pollen to the stigma of a different plant of the same species.

• Agents of pollination – Wind (anemophily), water (hydrophily), animals (zoophily).

• Double fertilisation – Fusion of one male gamete with the egg (forming zygote) and the other with polar nuclei (forming endosperm).

• Endosperm – Triploid (3n) nutritive tissue for the developing embryo.

• Seed – A mature ovule containing an embryo and stored food.

• Fruit – Mature ovary; may be true (from ovary) or false (from other parts).

• Apomixis – Formation of seeds without fertilisation.

• Parthenocarpy – Development of fruit without fertilisation; results in seedless fruits.

Chapter 3: Human Reproduction (covered in your previous notes – key definitions included)

• Gametogenesis – Process of formation of gametes (sperms and ova).

• Spermatogenesis – Formation of spermatozoa in seminiferous tubules.

• Oogenesis – Formation of an ovum in the ovaries.

• Sertoli cells – Nurse cells that nourish developing germ cells in the testis.

• Leydig cells – Interstitial cells that secrete androgens.

• Menstrual cycle – Cyclic changes in the female reproductive system (~28 days).

• Menarche – First menstruation.

• Menopause – Cessation of menstrual cycles (~50 years).

• Fertilisation – Fusion of male and female gametes to form a zygote.

• Implantation – Embedding of blastocyst into the endometrium.

• Placenta – A structural and functional unit between the foetus and the mother.

• Parturition – Process of childbirth.

• Lactation – Milk production by mammary glands.

• Colostrum – Antibody‑rich milk secreted during first few days after birth.

Chapter 4: Reproductive Health

• Reproductive health – Total well‑being in all aspects of reproduction (physical, emotional, social).

• Family planning – Planned Parenthood uses contraceptive methods.

• Contraception – Methods to prevent pregnancy.

• Natural methods – Periodic abstinence, withdrawal, lactational amenorrhea.

• Barrier methods – Condoms, diaphragms, cervical caps (prevent sperm entry).

• IUDs – Intrauterine devices; e.g., Copper‑T, Lippes loop.

• Oral contraceptives – Pills containing hormones (progesterone + estrogen) to inhibit ovulation.

• Surgical methods – Vasectomy (male) and tubectomy (female) for permanent sterilisation.

• MTP (Medical Termination of Pregnancy) – Induced abortion.

• STIs – Sexually Transmitted Infections (e.g., gonorrhoea, syphilis, AIDS).

• IVF – In vitro fertilisation (fertilisation outside the body).

• ZIFT – Zygote Intrafallopian Transfer.

• ICSI – Intracytoplasmic Sperm Injection (sperm injected into the ovum).

Chapter 5: Principles of Inheritance and Variation

• Genetics – Study of heredity and variation.

• Heredity – Transmission of traits from parents to offspring.

• Variation – Differences among individuals of the same species.

• Gene – Unit of inheritance; segment of DNA coding for a protein/RNA.

• Allele – Alternative form of a gene.

• Dominant – Allele that expresses itself in a heterozygous condition.

• Recessive – An allele that expresses only in a homozygous condition.

• Genotype – Genetic constitution of an organism.

• Phenotype – Observable physical/physiological characteristics.

• Homozygous – Same alleles for a trait (e.g., TT, tt).

• Heterozygous – Different alleles for a trait (e.g., Tt).

• Monohybrid cross – A cross involving one pair of contrasting characters.

• Dihybrid cross – A cross involving two pairs of contrasting characters.

• Law of Segregation – Alleles separate during gamete formation, so each gamete gets one allele.

• Law of Independent Assortment – Alleles of different genes assort independently during gamete formation.

• Incomplete dominance – Heterozygote shows intermediate phenotype (e.g., snapdragon flower colour).

• Codominance – Both alleles express equally in a heterozygote (e.g., AB blood group).

• Multiple alleles – More than two alleles of a gene (e.g., ABO blood groups).

• Polygenic inheritance – Trait controlled by several genes (e.g., skin colour).

• Pleiotropy – One gene influences multiple traits (e.g., sickle‑cell anaemia).

• Sex‑linked inheritance – Genes located on sex chromosomes (e.g., haemophilia, colour blindness).

• Mendelian disorders – Single‑gene disorders; e.g., haemophilia, sickle‑cell anaemia.

• Chromosomal disorders – Due to chromosomal abnormalities (e.g., Down’s syndrome, Turner’s syndrome).

Chapter 6: Molecular Basis of Inheritance

• DNA – Deoxyribonucleic acid; genetic material in most organisms.

• RNA – Ribonucleic acid; functions in protein synthesis.

• Nucleotide – Building block of nucleic acids; contains pentose sugar, phosphate, and nitrogenous base.

• Chargaff’s rule – A = T, G = C in DNA.

• Double helix – DNA structure proposed by Watson & Crick; two antiparallel strands.

• DNA replication – Synthesis of new DNA strands using the parent strand as a template.

• Semiconservative replication – Each daughter DNA contains one parent and one new strand.

• Transcription – Synthesis of RNA from DNA template.

• Translation – Protein synthesis from mRNA at ribosomes.

• Genetic code – Relationship between nucleotide sequence and amino acid sequence.

• Codon – A triplet of nucleotides coding for one amino acid.

• Lac operon – Inducible unit of gene expression in E. coli.

• Central dogma – Flow of information: DNA → RNA → Protein.

• Reverse transcription – Synthesis of DNA from RNA (e.g., retroviruses).

• Human Genome Project – An international project to map all human genes.

• DNA fingerprinting – A technique to identify individuals based on DNA polymorphism.

Chapter 7: Evolution

• Evolution – Descent with modification; change in allele frequencies over generations.

• Origin of life – Abiogenesis (life from non‑living matter) vs. biogenesis (life from pre‑existing life).

• Miller‑Urey experiment – Simulated early Earth conditions to produce amino acids.

• Natural selection – The process by which organisms with favourable traits survive and reproduce.

• Adaptation – A trait that enhances survival and reproduction.

• Speciation – Formation of new species.

• Hardy‑Weinberg principle – Allele frequencies remain constant in the absence of evolutionary influences.

• Gene flow – Movement of alleles between populations.

• Genetic drift – Random change in allele frequency in small populations.

• Founder effect – Genetic drift when a new colony is started by a few individuals.

• Bottleneck effect – Sharp reduction in population size, reducing genetic diversity.

• Fossils – Preserved remains of past organisms.

• Homologous organs – Same basic structure, different function (e.g., forelimbs of vertebrates).

• Analogous organs – Different structure, similar function (e.g., wings of a butterfly and a bird).

• Convergent evolution – Unrelated species evolve similar traits due to a similar environment.

• Divergent evolution – Related species become dissimilar due to different environments.

• Darwin’s finches – Example of adaptive radiation.

• Human evolution – Origin and diversification of hominids; ancestors: Australopithecus, Homo habilis, Homo erectus, Homo sapiens.

Chapter 8: Human Health and Disease

• Health – State of complete physical, mental and social well‑being.

• Disease – Disturbance in normal body structure or function.

• Infectious diseases – Caused by pathogens; transmitted from one person to another.

• Pathogen – Disease‑causing organism (virus, bacteria, fungus, protozoan, helminth).

• Immunity – Ability to resist infection.

• Innate immunity – Non‑specific, inborn defence (physical barriers, phagocytes).

• Acquired immunity – Specific, developed after exposure to an antigen.

• Active immunity – Antibodies produced by the body after infection/vaccination.

• Passive immunity – Ready‑made antibodies transferred (e.g., colostrum, antiserum).

• Antigen – A foreign substance that triggers an immune response.

• Antibody – A protein produced by plasma cells that neutralises antigens.

• Vaccination – Introduction of weakened/inactivated pathogen to induce immunity.

• Allergy – Hypersensitivity to harmless substances (allergens).

• Autoimmunity – The immune system attacks self‑cells.

• AIDS – Acquired Immune Deficiency Syndrome; caused by HIV.

• Cancer – Uncontrolled cell division; malignant tumour.

• Drug abuse – Harmful use of addictive substances (opioids, cannabinoids, cocaine, etc.).

Chapter 9: Strategies for Enhancement in Food Production

• Animal husbandry – Scientific management of livestock.

• Dairy farming – Management of animals for milk production.

• Poultry – Rearing of fowl for eggs and meat.

• Bee‑keeping – Apiculture; production of honey and wax.

• Fisheries – Cultivation of fish (aquaculture, pisciculture).

• Plant breeding – Genetic improvement of crops.

• Inbreeding – Mating of closely related individuals.

• Outbreeding – Mating of unrelated individuals.

• Hybridisation – Crossing genetically different plants.

• Heterosis (hybrid vigour) – Superiority of the hybrid over the parents.

• Biofortification – Breeding crops with higher nutrient content.

• Single Cell Protein (SCP) – Protein from microorganisms (e.g., Spirulina).

• Tissue culture – Growing cells/organs in sterile medium.

• Somatic hybridisation – Fusion of protoplasts of two different plants.

• Mutagenesis – Inducing mutations to create new varieties.

Chapter 10: Microbes in Human Welfare

• Bioactive molecules – Compounds produced by microbes with therapeutic value (e.g., antibiotics, statins).

• Antibiotics – Substances produced by microbes that kill or inhibit other microbes (e.g., penicillin).

• Probiotics – Live beneficial microbes (e.g., Lactobacillus in curd).

• Biocontrol agents – Biological agents used to control pests (e.g., Bacillus thuringiensis).

• Biofertilisers – Microbes that enrich soil nutrients (e.g., Rhizobium, Azospirillum).

• Biogas – Methane‑rich fuel produced by anaerobic digestion of organic waste.

• Floc – A mass of bacteria and fungi in sewage treatment that helps settle organic matter.

• Activated sludge – Flocs after bacterial digestion in secondary treatment.

Chapter 11: Biotechnology: Principles and Processes

• Biotechnology – Use of living organisms to develop useful products.

• Recombinant DNA (rDNA) technology – Joining DNA from different sources.

• Restriction enzymes – Molecular scissors; cut DNA at specific sequences.

• Vector – DNA carrier (plasmid, virus) used to transfer foreign DNA into the host.

• Plasmid – Circular extra‑chromosomal DNA in bacteria.

• DNA ligase – An enzyme that joins DNA fragments.

• Polymerase Chain Reaction (PCR) – A technique to amplify specific DNA sequences.

• Gel electrophoresis – A technique to separate DNA fragments by size.

• Transformation – Uptake of foreign DNA by the host cell.

• Electroporation – Use of an electric field to make cells competent for DNA uptake.

• Bioreactor – Vessel for large‑scale production of biological products.

Chapter 12: Biotechnology and Its Applications

• Transgenic – Organisms carrying foreign genes.

• BT cotton – Cotton genetically modified with the Bt toxin gene from Bacillus thuringiensis to resist pests.

• Gene therapy – Introduction of a functional gene to correct a genetic disorder.

• Molecular diagnosis – Detection of pathogens/genes using PCR, ELISA, etc.

• ELISA – Enzyme‑Linked Immunosorbent Assay; detects antibodies/antigens.

• Biopharming – Production of therapeutic proteins in genetically modified animals/plants.

• GMO – Genetically Modified Organism.

• Patenting – Legal protection for inventions; includes patents for genes and organisms.

Chapter 13: Organisms and Populations

• Ecology – Study of interactions among organisms and their environment.

• Abiotic factors – Non‑living physical factors (temperature, water, light, soil).

• Biotic factors – Living components (other organisms).

• Niche – Functional role of a species in its ecosystem.

• Population – A group of individuals of the same species in an area.

• Population density – Number of individuals per unit area/volume.

• Birth rate – Number of births per individual per unit time.

• Death rate – Number of deaths per individual per unit time.

• Exponential growth – Population grows at a constant rate (J‑shaped curve).

• Logistic growth – Growth slows as carrying capacity is reached (S‑shaped curve).

• Carrying capacity (K) – Maximum population size an environment can sustain.

• Adaptations – Traits that increase fitness in a particular environment.

Chapter 14: Ecosystem

• Ecosystem – A functional unit of living and non‑living components interacting.

• Abiotic component – Non‑living physical and chemical factors.

• Biotic component – Producers (autotrophs), consumers (heterotrophs), decomposers.

• Food chain – Linear transfer of energy from one trophic level to the next.

• Food web – Interconnected food chains.

• Trophic level – Position of an organism in a food chain.

• Ecological pyramid – Graphical representation of biomass, number or energy at each trophic level.

• Energy flow – Unidirectional movement of energy from the sun to producers to consumers.

• Productivity – Rate of biomass production; gross primary productivity (GPP) vs net primary productivity (NPP).

• Decomposition – Breakdown of dead organic matter by decomposers.

• Humification – Formation of dark, organic humus from detritus.

• Mineralisation – Release of inorganic nutrients from humus.

• Ecological succession – Gradual change in species composition over time.

• Pioneer community – First organisms to colonise a barren area.

• Climax community – Stable, final community in succession.

Chapter 15: Biodiversity and Conservation

• Biodiversity – Variety of life forms at genetic, species and ecosystem levels.

• Species richness – Number of species in an area.

• Genetic diversity – Variation within a species.

• Ecosystem diversity – Variety of habitats and ecological processes.

• Endemism – Species confined to a particular region.

• Hotspots – Regions with high species richness and endemism under threat.

• Threatened species – Species at risk of extinction.

• Red Data Book – Record of endangered, vulnerable and rare species.

• In situ conservation – On‑site conservation (protected areas, national parks, sanctuaries).

• Ex situ conservation – Off‑site conservation (zoos, botanical gardens, seed banks).

• Sacred groves – Forest patches protected by communities for religious reasons.

• Ecological services – Benefits ecosystems provide to humans (e.g., pollination, oxygen).

Chapter 16: Environmental Issues

• Pollution – Harmful addition of substances or energy to the environment.

• Air pollution – Presence of harmful substances in air (particulates, SO₂, NOₓ, etc.).

• Water pollution – Contamination of water bodies (e.g., by sewage, industrial waste).

• Eutrophication – Nutrient enrichment of water bodies causes algal blooms and oxygen depletion.

• Biomagnification – Increase in concentration of persistent toxins along the food chain.

• Greenhouse effect – Trapping of heat by gases (CO₂, methane), causing global warming.

• Ozone depletion – Thinning of the ozone layer due to CFCs.

• CFCs – Chlorofluorocarbons; the main cause of the ozone hole.

• Solid waste management – Collection, disposal, and recycling of solid wastes.

• Deforestation – Removal of forests leads to soil erosion and climate change.

• Reforestation – Restoring forests by planting trees.