Biology

Chapter 1 of the Class 12 Biology NCERT textbook, "Sexual Reproduction in Flowering Plants", covers sexual reproduction in angiosperms, explaining how flowers serve as the site of reproduction through processes including pollination, double fertilisation, and seed and fruit formation.

• 1A typical anther is bilobed, dithecous, and tetrasporangiate; pollen grains develop inside four microsporangia through meiosis (microsporogenesis).
• 2Pollen grains have a tough outer exine made of sporopollenin and an inner intine of cellulose and pectin; they contain a vegetative cell and a generative cell.
• 3The mature embryo sac (female gametophyte) is 7-celled and 8-nucleate: egg apparatus (egg + 2 synergids) at the micropylar end, 3 antipodals at the chalazal end, and 2 polar nuclei in the large central cell.
• 4Double fertilisation — unique to angiosperms — involves syngamy (male gamete + egg = diploid zygote) and triple fusion (male gamete + 2 polar nuclei = triploid primary endosperm nucleus).
• 5After fertilisation, the ovary develops into a fruit and ovules develop into seeds; endosperm development always precedes embryo development.

Chapter 2 of the Class 12 Biology NCERT textbook, "Human Reproduction", covers the male and female reproductive systems, gametogenesis (spermatogenesis and oogenesis), the menstrual cycle, fertilisation, implantation, pregnancy, embryonic development, parturition, and lactation.

• 1The male reproductive system includes paired testes (each 4–5 cm long) housed in the scrotum, which maintains a temperature 2–2.5°C lower than body temperature—essential for spermatogenesis.
• 2Each testis contains about 250 testicular lobules, with seminiferous tubules lined by spermatogonia (male germ cells) and Sertoli cells; Leydig cells in interstitial spaces secrete androgens.
• 3Spermatogenesis begins at puberty stimulated by GnRH, LH, and FSH; a single sperm consists of head (with acrosome), neck, middle piece (packed with mitochondria), and tail.
• 4The human male ejaculates 200–300 million sperms, of which at least 60% must have normal shape and size and 40% must show vigorous motility for normal fertility.
• 5The female reproductive system features paired ovaries (2–4 cm long) that produce the ovum and steroid hormones; the oviduct (10–12 cm) extends from the ovary to the uterus via the infundibulum and ampulla.

Chapter 3 of the Class 12 Biology NCERT textbook, "Reproductive Health", explains reproductive health as total well-being in all aspects of reproduction — physical, emotional, behavioural and social per WHO — and addresses strategies for population stabilisation, contraception, MTP, STIs and infertility.

• 1WHO defines reproductive health as total well-being — physical, emotional, behavioural, and social — in all aspects of reproduction.
• 2India initiated family planning programmes in 1951, among the first nations to do so at a national level.
• 3Contraceptive methods are broadly categorised as natural, barrier, IUDs, oral contraceptives, injectables, implants, and surgical (vasectomy/tubectomy).
• 4Saheli, a non-steroidal once-a-week oral contraceptive for females, was developed at CDRI, Lucknow.
• 5MTP was legalised in India in 1971; it is considered safe only in the first trimester (up to 12 weeks).

Chapter 4 of the Class 12 Biology NCERT textbook, "Principles of Inheritance and Variation", explains how Mendel's laws of dominance, segregation and independent assortment govern the passing of traits from parents to offspring, along with sex determination, mutation and genetic disorders.

• 1Mendel studied 7 contrasting traits in pea plants over 7 years (1856–1863) and proposed three laws: Law of Dominance, Law of Segregation, and Law of Independent Assortment.
• 2In a monohybrid cross, the F1 generation shows only the dominant trait; F2 produces a 3:1 phenotypic ratio and 1:2:1 genotypic ratio (TT:Tt:tt).
• 3In a dihybrid cross, F2 offspring appear in a 9:3:3:1 phenotypic ratio, demonstrating that allele pairs segregate independently of each other.
• 4ABO blood groups illustrate co-dominance (IA and IB both expressed) and multiple alleles (three alleles: IA, IB, i) controlling the same character.
• 5Sex determination in humans is XY type: females are XX, males are XY; the sex of the child is determined by whether the sperm carries an X or Y chromosome.

Chapter 5 of the Class 12 Biology NCERT textbook, "Molecular Basis of Inheritance", explains the structure of DNA, its replication, transcription, the genetic code, translation, gene regulation, the Human Genome Project and DNA fingerprinting.

• 1DNA is a double helix with anti-parallel strands held by hydrogen bonds: A-T (2 H-bonds) and G-C (3 H-bonds), with a pitch of 3.4 nm and ~10 base pairs per turn.
• 2DNA replicates semiconservatively — each daughter molecule retains one parental strand — proved by Meselson and Stahl (1958) using heavy nitrogen (15N) in E. coli.
• 3The genetic code is a triplet codon system: 61 codons specify amino acids and 3 are stop codons (UAA, UAG, UGA); AUG serves as the start codon and codes for methionine.
• 4In eukaryotes, primary RNA transcripts (hnRNA) undergo splicing (introns removed, exons joined), capping, and tailing before becoming functional mRNA for translation.
• 5The lac operon in E. coli is the classic model of gene regulation: in the absence of lactose the repressor blocks transcription; lactose (inducer) inactivates the repressor, allowing gene expression.

Chapter 6 of the Class 12 Biology NCERT textbook, "Evolution", explains the origin of life through chemical evolution, Darwin's theory of natural selection, evidences for evolution, adaptive radiation, the Hardy-Weinberg principle and the evolutionary history of life forms including the origin of man.

• 1The universe is approximately 13.8 billion years old; Earth formed about 4.5 billion years ago, and life first appeared nearly 4 billion years ago.
• 2Miller's 1953 experiment demonstrated that amino acids can form abiotically by passing electric discharge through CH₄, H₂, NH₃, and water vapour at 800°C, supporting the chemical evolution hypothesis.
• 3Darwin's theory of natural selection holds that heritable variations conferring better fitness enable individuals to leave more progeny, driving gradual evolution; Alfred Wallace independently reached similar conclusions.
• 4Homologous structures (e.g., forelimbs of whales, bats, cheetahs, and humans) indicate divergent evolution and common ancestry, while analogous structures (e.g., wings of birds and butterflies) indicate convergent evolution.
• 5Hardy-Weinberg principle states that allele frequencies in a population remain constant across generations (genetic equilibrium); disturbances caused by gene flow, genetic drift, mutation, recombination, or natural selection result in evolution.

Chapter 7 of the Class 12 Biology NCERT textbook, "Human Health and Disease", explains immunity types, common infectious diseases such as typhoid, malaria, pneumonia and AIDS, cancer, and drug and alcohol abuse in adolescents.

• 1Health is defined as complete physical, mental and social well-being; affected by genetic disorders, infections, and lifestyle
• 2Immunity is of two types: innate (non-specific, present at birth with physical, physiological, cellular and cytokine barriers) and acquired (pathogen-specific, memory-based, involving B and T lymphocytes)
• 3Malaria is caused by Plasmodium falciparum (most dangerous species) transmitted by the bite of an infected female Anopheles mosquito; the toxic substance haemozoin causes recurring chills and fever
• 4AIDS is caused by HIV (a retrovirus) that destroys helper T-lymphocytes; it spreads through sexual contact, contaminated blood, shared needles or from infected mother to child — not by touch
• 5Cancer arises when cells lose contact inhibition and divide uncontrollably; malignant tumours spread to distant sites via blood (metastasis) and are caused by carcinogens including radiation, chemicals and oncogenic viruses

Chapter 8 of the Class 12 Biology NCERT textbook, "Microbes in Human Welfare", covers the beneficial roles of microbes — in household food products, industrial production of antibiotics and chemicals, sewage treatment, biogas generation, biocontrol of pests, and as biofertilisers.

• 1Lactic acid bacteria (LAB) convert milk to curd by producing acids that coagulate milk proteins and increase vitamin B12; small amount of curd acts as inoculum for the next batch.
• 2Penicillin, the first antibiotic, was discovered by Alexander Fleming from the mould Penicillium notatum; Fleming, Chain, and Florey received the Nobel Prize in 1945 for this discovery.
• 3Sewage treatment involves primary (physical removal of solids) and secondary (biological reduction of BOD using aerobic microbes forming flocs) stages; anaerobic digestion of sludge produces biogas.
• 4Methanogens such as Methanobacterium break down cellulosic material to produce methane, CO2, and H2; cattle dung (gobar) is the key input for rural biogas plants developed by IARI and KVIC.
• 5Bacillus thuringiensis (Bt) spores are used as biopesticides to kill insect larvae without harming other insects; Trichoderma fungi control plant pathogens as biocontrol agents.

Chapter 9 of the Class 12 Biology NCERT textbook, "Biotechnology: Principles and Processes", covers recombinant DNA technology — how restriction enzymes, cloning vectors, and bioreactors are used to produce useful proteins and genetically modified organisms.

• 1Restriction endonucleases cut DNA at specific palindromic recognition sequences, producing sticky ends that facilitate joining of DNA fragments using DNA ligase
• 2Stanley Cohen and Herbert Boyer constructed the first recombinant DNA in 1972 by linking an antibiotic resistance gene with a Salmonella typhimurium plasmid
• 3Cloning vectors must have an origin of replication (ori), a selectable marker, and suitable cloning sites to allow foreign DNA to replicate in host cells
• 4PCR (Polymerase Chain Reaction) uses thermostable DNA polymerase from Thermus aquaticus to amplify a target DNA segment up to approximately one billion copies
• 5DNA can be introduced into host cells by chemical treatment with calcium ions (heat shock method), micro-injection, biolistics (gene gun), or disarmed pathogen vectors

Chapter 10 of the Class 12 Biology NCERT textbook, "Biotechnology and Its Applications", covers how biotechnology is used in agriculture (GM crops, Bt toxin, RNAi pest resistance), medicine (recombinant insulin, gene therapy, molecular diagnosis), and transgenic animals, along with ethical issues surrounding genetic modification and biopiracy.

• 1Bt toxin from Bacillus thuringiensis is expressed in GM crops (Bt cotton, Bt corn) via cry genes (e.g., cryIAc, cryIIAb) to confer insect resistance without chemical pesticides.
• 2RNAi (RNA interference) is used to protect tobacco plants from the nematode Meloidegyne incognita by introducing nematode-specific dsRNA via Agrobacterium vectors.
• 3Recombinant human insulin was first produced in 1983 by Eli Lilly by separately expressing insulin chain A and chain B in E. coli plasmids and then combining them with disulfide bonds.
• 4The first clinical gene therapy was administered in 1990 to a 4-year-old girl with ADA (adenosine deaminase) deficiency, using retroviral vectors to introduce functional ADA cDNA into her lymphocytes.
• 5PCR and ELISA are key molecular diagnostic tools; PCR detects very low concentrations of pathogens (e.g., HIV) and mutations in cancer-suspected genes before symptoms appear.

Chapter 11 of the Class 12 Biology NCERT textbook, "Organisms and Populations", covers ecology at the population level — including population attributes, growth models (exponential and logistic), life history variation, and interspecific interactions such as predation, competition, parasitism, commensalism, and mutualism.

• 1Population attributes include birth rate, death rate, sex ratio, and age distribution — attributes that individual organisms do not possess.
• 2Population growth is exponential (dN/dt = rN) when resources are unlimited, producing a J-shaped curve; it becomes logistic (Verhulst-Pearl model) when resources are limiting, producing an S-shaped sigmoid curve bounded by carrying capacity (K).
• 3The intrinsic rate of natural increase (r) measures a population's inherent growth potential; for Norway rat r = 0.015, flour beetle r = 0.12, and India's human population in 1981 had r = 0.0205.
• 4Interspecific interactions are classified by outcome: mutualism (+/+), competition (−/−), predation (+/−), parasitism (+/−), commensalism (+/0), and amensalism (−/0).
• 5Gause's Competitive Exclusion Principle states that two closely related species competing for the same limiting resource cannot coexist indefinitely; the inferior competitor is eventually eliminated.

Chapter 12 of the Class 12 Biology NCERT textbook, "Ecosystem", covers the ecosystem as a functional unit of nature where organisms interact with each other and their physical environment, studied through four key processes: productivity, decomposition, energy flow, and nutrient cycling.

• 1An ecosystem functions through four processes: productivity, decomposition, energy flow, and nutrient cycling
• 2Gross primary productivity (GPP) minus respiration losses equals net primary productivity (NPP), which is the biomass available to heterotrophs
• 3Decomposition converts complex organic detritus into inorganic substances through fragmentation, leaching, catabolism, humification, and mineralisation
• 4Energy flow in ecosystems is unidirectional; only 10 per cent of energy is transferred from one trophic level to the next
• 5Ecological pyramids (number, biomass, energy) represent feeding relationships; the pyramid of energy is always upright and can never be inverted

Chapter 13 of the Class 12 Biology NCERT textbook, "Biodiversity and Conservation", explains the three levels of biodiversity (genetic, species, ecological), patterns of species distribution, causes of biodiversity loss, and strategies for conservation including in situ and ex situ methods.

• 1Biodiversity exists at three levels: genetic diversity (e.g., India has 50,000+ strains of rice), species diversity (e.g., Western Ghats vs Eastern Ghats amphibians), and ecological diversity (e.g., India's deserts, rain forests, coral reefs).
• 2Global species diversity is estimated at about 7 million by Robert May; only about 1.5 million have been formally described as of IUCN 2004.
• 3Species richness follows a latitudinal gradient, decreasing from the tropics toward the poles; the Amazonian rain forest harbours the greatest biodiversity on Earth.
• 4Current species extinction rates are 100 to 1,000 times faster than pre-human rates; the 'Evil Quartet' of causes comprises habitat loss, over-exploitation, alien species invasions, and co-extinctions.
• 5In situ conservation protects species in their natural habitat through biodiversity hotspots (34 worldwide, three covering India), biosphere reserves, national parks, and sacred groves.