CBSE Class 12 Biology Revision Notes

CBSE Class 12 Biology Revision Notes summarise the main processes, terms, diagrams and examples covered across the current 13-chapter syllabus. They help CBSE students revise reproduction, genetics, evolution, human welfare, biotechnology and ecology in a structured form.

Class 12 Biology studies how living organisms reproduce, inherit characters, evolve and interact with their surroundings. It also explains human health, useful microbes and modern biotechnology.

These CBSE Class 12 Biology Revision Notes follow the current 2026–27 chapter sequence. Use them to revise definitions, biological processes, examples, diagrams and important differences before examinations.

Key Takeaways

• 13 chapters: The current Class 12 Biology textbook contains 13 chapters across five units.
• Five units: The syllabus covers Reproduction, Genetics and Evolution, Biology in Human Welfare, Biotechnology, and Ecology.
• 20 marks: Genetics and Evolution carries the highest unit weightage in the theory paper.
• Diagram-based learning: Flower structure, reproductive systems, DNA processes, biotechnology tools and ecological cycles require careful visual revision.

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Chapter-Wise CBSE Class 12 Biology Notes

The current Class 12 Biology NCERT textbook contains 13 chapters. The chapters are organised into five connected units.

Unit VI: Reproduction

This unit explains sexual reproduction in flowering plants and humans. It also covers reproductive health and responsible reproductive choices.

Chapter 1: Sexual Reproduction in Flowering Plants

A flower is the reproductive structure of an angiosperm. Stamens form the male reproductive part, while the pistil forms the female reproductive part.

The anther contains pollen sacs where microspores develop into pollen grains. The ovule contains the embryo sac, which represents the female gametophyte.

Important processes include:

• Microsporogenesis
• Megasporogenesis
• Pollination
• Pollen-pistil interaction
• Double fertilisation
• Endosperm development
• Embryo development
• Seed and fruit formation

Pollination is the transfer of pollen grains from anther to stigma. It may occur through wind, water or animals.

Double fertilisation is a characteristic feature of flowering plants. One male gamete fuses with the egg to form the zygote. The other fuses with two polar nuclei to form the primary endosperm nucleus.

Chapter 2: Human Reproduction

Human reproduction involves the formation of gametes, fertilisation, embryo development and birth.

The male reproductive system includes testes, accessory ducts, glands and external genitalia. The female reproductive system includes ovaries, oviducts, uterus, cervix and vagina.

The formation of sperm is called spermatogenesis. The formation of female gametes is called oogenesis.

The menstrual cycle includes:

• Menstrual phase
• Follicular phase
• Ovulatory phase
• Luteal phase

Fertilisation usually occurs at the junction of the ampulla and isthmus of the oviduct. The resulting zygote undergoes cleavage and forms a blastocyst.

Implantation occurs when the blastocyst attaches to the uterine wall. The placenta supports the exchange of nutrients, gases and wastes between the mother and developing embryo.

Chapter 3: Reproductive Health

Reproductive health means total well-being in all aspects of reproduction. It includes physical, emotional, behavioural and social well-being.

Important concerns include:

• Population growth
• Contraception
• Medical termination of pregnancy
• Sexually transmitted infections
• Infertility
• Assisted reproductive technologies

Natural, barrier, intrauterine, oral, injectable and surgical methods may be used for contraception.

Sexually transmitted infections include gonorrhoea, syphilis, genital herpes, chlamydiasis, hepatitis-B and HIV infection. Early diagnosis and proper treatment can prevent complications in several cases.

Assisted reproductive technologies may help some infertile couples. These methods include IVF, ZIFT, GIFT and intracytoplasmic sperm injection.

Unit VII: Genetics and Evolution

This unit explains how characters pass from parents to offspring. It also connects heredity with molecular biology and evolution.

Chapter 4: Principles of Inheritance and Variation

Inheritance is the transmission of characters from parents to offspring. Variation refers to differences among individuals of the same species.

Gregor Mendel studied inheritance using pea plants. His work established the basic principles of heredity.

Important terms include:

• Gene
• Allele
• Dominant trait
• Recessive trait
• Homozygous
• Heterozygous
• Genotype
• Phenotype

A monohybrid cross studies one pair of contrasting characters. A dihybrid cross studies two pairs.

Mendel’s law of segregation states that two alleles of a gene separate during gamete formation.

The law of independent assortment states that alleles of different genes assort independently when the genes are unlinked.

The chapter also covers incomplete dominance, codominance, multiple alleles, chromosomal theory of inheritance, linkage and recombination.

Human genetic disorders include haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria, thalassaemia and chromosomal disorders.

Chapter 5: Molecular Basis of Inheritance

DNA stores genetic information in most organisms. RNA acts as genetic material in some viruses.

A DNA molecule consists of two antiparallel polynucleotide chains. Each nucleotide contains a sugar, phosphate group and nitrogenous base.

The main biological processes are:

• DNA replication
• Transcription
• Genetic coding
• Translation
• Regulation of gene expression

DNA replication is semiconservative. Each new DNA molecule contains one parental strand and one newly formed strand.

Transcription transfers genetic information from DNA to RNA. Translation uses messenger RNA to form a polypeptide chain.

The genetic code is triplet, nearly universal and degenerate. Each codon specifies an amino acid or a termination signal.

The chapter also covers the lac operon, human genome project and DNA fingerprinting.

Chapter 6: Evolution

Evolution refers to changes in living organisms over generations. These changes may lead to adaptation and the formation of new species.

Evidence for evolution comes from:

• Fossils
• Comparative anatomy
• Embryology
• Molecular similarities
• Biogeography

Darwin explained evolution through natural selection. Individuals with favourable variations have a better chance of survival and reproduction.

Hardy-Weinberg equilibrium describes stable allele frequencies in a population:

p² + 2pq + q² = 1

Factors that disturb this equilibrium include:

• Gene migration
• Genetic drift
• Mutation
• Genetic recombination
• Natural selection

The chapter also discusses adaptive radiation and human evolution.

Unit VIII: Biology in Human Welfare

This unit explains diseases, immunity and the useful roles of microbes. It connects Biology with health, food production and industry.

Chapter 7: Human Health and Disease

Health is a state of physical, mental and social well-being. Disease disturbs the normal functioning of the body.

Disease-causing organisms include bacteria, viruses, protozoans, fungi and helminths.

Important diseases covered in this chapter include:

• Typhoid
• Pneumonia
• Common cold
• Malaria
• Amoebiasis
• Ascariasis
• Filariasis
• AIDS
• Cancer

Immunity protects the body from foreign organisms and substances.

Innate immunity is present from birth. Acquired immunity develops after exposure to a specific antigen.

Acquired immunity may be:

• Active immunity
• Passive immunity
• Humoral immunity
• Cell-mediated immunity

Vaccination develops immunological memory. It prepares the immune system to respond rapidly during future exposure.

The chapter also covers allergies, autoimmune responses, cancer, HIV infection and drug and alcohol abuse.

Chapter 8: Microbes in Human Welfare

Microbes are used in food preparation, industry, sewage treatment, energy production and agriculture.

Household products involving microbes include:

• Curd
• Bread
• Fermented batter
• Cheese
• Traditional beverages

Industrial products include antibiotics, organic acids, enzymes and alcoholic beverages.

Saccharomyces cerevisiae is used in fermentation. Lactobacillus converts milk into curd.

Sewage treatment involves primary and secondary treatment. Microbial flocs help reduce organic matter during secondary treatment.

Methanogens produce biogas from organic matter. They are commonly found in anaerobic sludge and the rumen of cattle.

Biological control uses living organisms to manage pests. Biofertilisers improve soil fertility through organisms such as Rhizobium, cyanobacteria and mycorrhiza.

Unit IX: Biotechnology

Biotechnology uses organisms, cells or biological molecules to develop useful products and processes. This unit focuses on genetic engineering and its applications.

Chapter 9: Biotechnology: Principles and Processes

Modern biotechnology mainly depends on genetic engineering and controlled bioprocessing.

The important tools of recombinant DNA technology include:

• Restriction enzymes
• DNA ligase
• Polymerases
• Cloning vectors
• Competent host cells

Restriction enzymes cut DNA at specific recognition sequences. DNA ligase joins DNA fragments.

A cloning vector carries foreign DNA into a host cell. Important vector features include the origin of replication, selectable marker and cloning sites.

The main steps in recombinant DNA technology are:

1. Isolation of genetic material
2. Cutting DNA at specific sites
3. Amplification of the desired gene
4. Ligation into a vector
5. Transfer into a host
6. Selection of transformed cells
7. Expression of the desired product
8. Downstream processing

Polymerase chain reaction amplifies a selected DNA segment. Bioreactors provide controlled conditions for large-scale production.

Chapter 10: Biotechnology and its Applications

Biotechnology has applications in agriculture, medicine and diagnostics.

Genetically modified crops may show resistance to pests, diseases or environmental stress.

Bt cotton contains genes obtained from Bacillus thuringiensis. These genes produce proteins that affect specific insect pests.

RNA interference can silence a selected gene. It has been used to develop resistance against certain parasites.

Medical applications include recombinant insulin, gene therapy and molecular diagnosis.

Recombinant human insulin is produced using genetically engineered microorganisms. Molecular diagnosis uses methods such as PCR and enzyme-linked immunosorbent assay.

The chapter also explains transgenic animals, biopatents and biopiracy.

Unit X: Ecology

Ecology studies relationships among organisms and their environment. This unit covers populations, ecosystems and biodiversity.

Chapter 11: Organisms and Populations

Organisms respond to environmental factors such as temperature, water, light and soil.

Responses to environmental changes include:

• Regulation
• Conformation
• Migration
• Suspension

Adaptations help organisms survive in their habitats. Desert plants and animals show features that reduce water loss.

Population attributes include:

• Population density
• Birth rate
• Death rate
• Age distribution
• Sex ratio

Population growth may follow an exponential or logistic pattern.

Exponential growth:

dN/dt = rN

Logistic growth:

dN/dt = rN(K − N)/K

Population interactions include mutualism, competition, predation and parasitism.

Chapter 12: Ecosystem

An ecosystem contains biotic and abiotic components. Organisms interact with each other and with the physical environment.

The main ecosystem processes are:

• Productivity
• Decomposition
• Energy flow
• Nutrient cycling

Primary productivity is the rate at which producers form organic matter.

Gross primary productivity is the total rate of photosynthesis. Net primary productivity is the biomass available after respiratory losses.

NPP = GPP − R

Decomposition involves fragmentation, leaching, catabolism, humification and mineralisation.

Energy moves through food chains and food webs. Only a small proportion of energy passes from one trophic level to the next.

Ecological pyramids represent numbers, biomass or energy at different trophic levels.

Chapter 13: Biodiversity and Conservation

Biodiversity refers to variation at genetic, species and ecosystem levels.

Species diversity is not distributed evenly across Earth. Tropical regions generally contain greater diversity than temperate regions.

The species-area relationship is expressed as:

log S = log C + Z log A

Major causes of biodiversity loss include:

• Habitat loss and fragmentation
• Over-exploitation
• Alien species invasions
• Co-extinctions

Conservation may be in situ or ex situ.

In situ conservation protects species in their natural habitats. Examples include biosphere reserves, national parks and wildlife sanctuaries.

Ex situ conservation protects organisms outside their natural habitats. Examples include zoological parks, botanical gardens, seed banks and cryopreservation.

Major Concepts Covered in Class 12 Biology Notes

Class 12 Biology important concepts are connected across units. Reproduction produces offspring, inheritance transfers information, evolution changes populations and ecology explains survival within environments.

Reproduction and Development

Plant and human reproduction involve gamete formation, fertilisation and development. The structures and sequence of events differ, but both depend on specialised reproductive cells.

Heredity and Molecular Biology

Inheritance explains the transmission of characters. Molecular biology explains how DNA stores and expresses genetic information.

Mendelian genetics studies observable traits. Molecular genetics studies genes, nucleic acids and protein synthesis.

Health and Useful Microbes

Microbes may cause disease or support human welfare. Their role depends on the organism, host and environment.

Some microbes produce antibiotics, fermented foods and biogas. Others cause infections such as typhoid, malaria and AIDS.

Biotechnology

Biotechnology applies biological processes to solve practical problems. It combines genetic material, enzymes, vectors, host cells and controlled production systems.

Ecology and Conservation

Ecology connects organisms with their physical and biological environment. Conservation protects the variety required for stable ecosystems.

Important Class 12 Biology Terms for Quick Revision

Diagrams and Processes to Revise

Biology diagrams help explain structures and processes that are difficult to describe through text alone.

Important diagrams include:

• Longitudinal section of a flower
• Structure of anther
• Mature embryo sac
• Male reproductive system
• Female reproductive system
• Menstrual cycle
• Monohybrid and dihybrid crosses
• DNA double helix
• DNA replication
• Transcription unit
• Lac operon
• Plasmid vector
• PCR cycle
• Population growth curves
• Ecological pyramids
• Carbon cycle

Label every part clearly. Use straight label lines and avoid crowding the diagram.

For process-based answers, present events in the correct sequence. This is especially important for fertilisation, DNA replication, transcription, translation, sewage treatment and recombinant DNA technology.

CBSE Class 12 Biology Unit-Wise Marks Distribution

The theory paper carries 70 marks. The practical assessment carries 30 marks.

Genetics and Evolution carries the highest theory weightage. Reproduction also forms a major part of the paper.

Weightage helps organise revision, but every chapter remains important. Questions may test definitions, processes, diagrams, comparisons, examples and applications.