CBSE Class 12 Biology
Chapters: 6
1. Reproduction
Sexual Reproduction in Flowering Plants · Human Reproduction · Reproductive Health
- Sexual Reproduction in Flowering Plants – A flower is the sex organ of a plant. The stamen makes pollen grains (male gametophytes). The ovule holds the embryo sac (female gametophyte, 7 cells, 8 nuclei). Pollination brings pollen to the stigma; the pistil checks it; a pollen tube carries two male gametes to the embryo sac. One fuses with the egg (zygote, 2n) and the other with two polar nuclei (primary endosperm nucleus, 3n). This is double fertilisation. The ovule becomes the seed and the ovary becomes the fruit.
- Human Reproduction – Humans reproduce sexually and are viviparous (the baby grows inside the mother). Testes make sperm in seminiferous tubules; ovaries make ova in follicles. Gametogenesis uses meiosis: 1 spermatogonium gives 4 sperm, but 1 oogonium gives just 1 ovum plus polar bodies. The menstrual cycle (about 28 days) is run by FSH, LH, oestrogen and progesterone; an LH surge near day 14 causes ovulation. Fertilisation in the ampulla makes a zygote, which divides into a morula and a blastocyst that implants in the uterus. The placenta feeds the fetus; birth comes after about 9 months, followed by lactation.
- Reproductive Health – Reproductive health means being well in body, mind, behaviour and social life in all matters of reproduction. It needs correct information, safe and hygienic habits, prevention of sexually transmitted infections (STIs), and access to safe birth control, legal abortion (MTP) and care for infertility. Birth-control methods act at different points: natural methods avoid fertile days, barriers block sperm, IUDs stop sperm and implantation, pills stop ovulation and surgery cuts the tubes. Assisted reproductive technologies such as IVF-ET, ZIFT, IUT, GIFT, ICSI and IUI help couples who cannot conceive.
2. Genetics and Evolution
Principles of Inheritance and Variation · Molecular Basis of Inheritance · Evolution
- Principles of Inheritance and Variation – Genes come in pairs (alleles). Each parent passes one allele of each pair to the child. Mendel showed dominance (3 : 1), segregation and independent assortment (9 : 3 : 3 : 1). Real life has twists: incomplete dominance (1 : 2 : 1), co-dominance (AB blood), multiple alleles, pleiotropy (one gene, many effects) and polygenic traits (many genes, one trait). Genes sit on chromosomes, so genes on the same chromosome are linked and are separated only by crossing over. Sex chromosomes decide sex (XX-XY in humans, ZW in birds, haplo-diploid in honeybees) and carry sex-linked genes (haemophilia, colour blindness). Mistakes in genes cause Mendelian disorders (thalassemia, sickle-cell) and mistakes in chromosome number cause Down, Turner and Klinefelter syndromes.
- Molecular Basis of Inheritance – DNA is the genetic material in most living things (some viruses use RNA). It is a double helix: two antiparallel strands of nucleotides, A pairs with T by 2 hydrogen bonds and G with C by 3. Long DNA is packed on histones into nucleosomes and then chromatin. DNA copies itself semi-conservatively (Meselson–Stahl). The central dogma says DNA → RNA → protein. Transcription makes RNA from one strand; in eukaryotes the hnRNA is capped, tailed and spliced. The genetic code is a triplet, has 64 codons (61 for amino acids, 3 stops), starts with AUG, is nearly universal and degenerate. Ribosomes translate mRNA into protein with tRNA adaptors. Genes are switched on and off; the lac operon is the classic example. The Human and Rice Genome Projects read whole genomes, and DNA fingerprinting uses repeat DNA (VNTRs) to identify people.
- Evolution: Origin of Life, Mechanisms and Human Evolution – Life began on the early Earth from simple chemicals: Oparin and Haldane proposed it and Miller made amino acids in a flask. Evidence of evolution comes from fossils, homologous and analogous organs, embryos, molecules and changes we can watch (industrial melanism, drug resistance). Darwin explained it by natural selection acting on variation in populations; the modern synthetic theory adds genes: mutation, recombination, gene flow, genetic drift and natural selection change allele frequencies. If none of these act, frequencies stay constant: Hardy–Weinberg, p² + 2pq + q² = 1. Selection can be stabilising, directional or disruptive. One ancestor spreading into many habitats gives adaptive radiation (Darwin’s finches, Australian marsupials). Humans evolved from Dryopithecus-like apes through Australopithecus, Homo habilis, Homo erectus and Neanderthals to Homo sapiens.
3. Biology and Human Welfare
Human Health and Disease · Microbes in Human Welfare
- Human Health and Disease – Health means the body, mind and social life all work well. Diseases can be caused by germs (pathogens) like bacteria, viruses, protozoa, worms and fungi. Our body fights them with two kinds of immunity: innate (we are born with it) and acquired (it learns and remembers). Vaccines use this memory. HIV destroys helper T cells and causes AIDS. Cancer is cells dividing without control. Drugs and alcohol harm the body and mind, especially in teenagers.
- Microbes in Human Welfare – Microbes are tiny living things: bacteria, fungi, some protozoa, algae and viruses. Many of them help us. They turn milk into curd, make dough rise, and produce drinks, acids, enzymes and medicines in big factories. They clean sewage, make biogas fuel, kill crop pests safely and add nutrients to soil. Antibiotics like penicillin come from microbes, but we must use them carefully so germs do not become resistant.
4. Biotechnology and its Applications
Biotechnology: Principles and Processes · Biotechnology and its Applications
- Biotechnology and its Applications – The tools of genetic engineering are used in medicine, farming and research. Bacteria make human insulin; yeast makes safe vaccine proteins. Stem cells can become many cell types. Gene therapy puts a correct gene into a patient's cells. GM crops like Bt cotton carry a gene for a protein that kills pests. Transgenic animals carry foreign genes to help study disease or make medicines. Because these are powerful, India controls them through GEAC, and laws guard against biopiracy and unfair patents.
- Biotechnology: Principles and Processes (Recombinant DNA Technology) – Genetic engineering means changing the genes of a living thing on purpose. We cut the gene we want with restriction enzymes (molecular scissors), paste it into a carrier DNA called a vector using DNA ligase (molecular glue), put it into a host cell, pick out the cells that took it, and grow them in big tanks (bioreactors) to collect the product. PCR makes millions of copies of a gene, and gel electrophoresis sorts DNA pieces by size.
5. Ecology and Environment
Organisms and Populations · Ecosystem · Biodiversity and its Conservation
- Organisms and Populations: Growth and Interactions – A population is a group of the same species living in one area at one time. It has features that a single organism does not have: density, birth rate, death rate, sex ratio and age structure. N grows by births and immigration and shrinks by deaths and emigration. With unlimited food it grows fast (J-curve, dN/dt = rN); with limited food it slows and stops at the carrying capacity K (S-curve, dN/dt = rN(1 − N/K)). Different species affect each other: mutualism (+/+), competition (−/−), predation and parasitism (+/−), commensalism (+/0) and amensalism (−/0).
- Ecosystem: Structure, Productivity, Energy Flow and Pyramids – An ecosystem is a working unit of nature where living things (biotic) and non-living things (abiotic) interact. Plants make food at a rate called productivity: gross (GPP) minus respiration gives net (NPP = GPP − R). Dead matter is broken down in five steps: fragmentation, leaching, catabolism, humification and mineralisation. Energy enters as sunlight, flows one way through trophic levels and only about 10% passes on each time. Ecological pyramids of number, biomass and energy show this; number and biomass pyramids can be inverted, but the energy pyramid is always upright.
- Biodiversity and its Conservation – Biodiversity is the variety of life at three levels: genetic, species and ecological. It is highest near the equator and grows with area (log S = log C + Z log A). Every species matters, like rivets on a plane. We are losing species fast because of the 'evil quartet': habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinction. The IUCN Red List (Red Data Book) ranks species from Least Concern to Extinct. We protect life in situ (hotspots, national parks, sanctuaries, biosphere reserves, sacred groves, Ramsar wetlands) and ex situ (zoos, botanical gardens, seed and gene banks).
6. Formative-only topic
Environmental Issues
- Geographical Perspective on Selected Issues and Problems (Class 12) – Pollution means harmful things added to air, water, land or sound. Air pollution comes from burning fuel, factories and vehicles; water pollution from sewage, industry and farm chemicals; land pollution from garbage and chemicals; noise pollution from traffic and loudspeakers. Cities make huge amounts of solid waste that should be sorted, recycled and safely disposed. People move from villages to cities because of push factors (poverty, no work) and pull factors (jobs, services); many end up in slums with poor housing and services. Land degradation from erosion, waterlogging, salinity, mining and overgrazing can be healed, as shown by watershed work in Jhabua.