NCERT Solutions Class 12 Biology - All 14 Chapters

All 14 Chapters

Click on any chapter to view detailed solutions

Reproduction in Organisms

Asexual and sexual reproduction

› 2

Sexual Reproduction in Flowering Plants

Pollination, Fertilization, Seed formation

› 3

Human Reproduction

Male & female reproductive system, Menstrual cycle

› 4

Reproductive Health

Contraception, STDs, Amniocentesis

› 5

Principles of Inheritance and Variation

Mendelism, Linkage, Mutations

› 6

Molecular Basis of Inheritance

DNA, Replication, Transcription, Translation

› 7

Evolution

Theories, Evidence, Hardy-Weinberg principle

› 8

Human Health and Disease

Immunity, Cancer, AIDS, Drugs

› 9

Strategies for Enhancement in Food Production

Animal husbandry, Plant breeding

› 10

Microbes in Human Welfare

Household products, Sewage treatment, Biogas

› 11

Biotechnology: Principles and Processes

Genetic engineering, PCR, Gel electrophoresis

› 12

Biotechnology and Its Applications

GM crops, Gene therapy, Bt cotton

› 13

Organisms and Populations

Ecological adaptations, Population interactions

› 14

Ecosystem

Energy flow, Nutrient cycling, Ecological succession

›

Chapter 1: Reproduction in Organisms

Q1. Define reproduction. What are its two main types?

Solution

Reproduction is the biological process by which new individual organisms (offspring) are produced from their parents.

Asexual reproduction: Involves single parent, no gamete fusion (e.g., binary fission, budding)
Sexual reproduction: Involves two parents, fusion of male and female gametes

Q2. What is the difference between asexual and sexual reproduction?

Solution

Asexual: One parent, no gametes, offspring are clones, faster
Sexual: Two parents, gamete fusion, genetic variation, slower

Q3. Define the following: (i) Spermiation (ii) Ovulation (iii) Implantation

Solution

(i) Spermiation: Release of sperm from Sertoli cells into the lumen of seminiferous tubules
(ii) Ovulation: Release of mature ovum from the Graafian follicle
(iii) Implantation: Embedding of blastocyst into the uterine wall

Q4. What is vegetative propagation? Give two examples.

Solution

Vegetative propagation is a form of asexual reproduction in plants where new plants arise from vegetative parts (roots, stems, leaves).

Examples: (1) Potato - from tubers (2) Bryophyllum - from leaf buds

Q5. What are hermaphrodites? Give examples.

Solution

Organisms that possess both male and female reproductive organs.

Examples: Earthworm, Leech, Hydra, Tapeworm, Sponge

Chapter 2: Sexual Reproduction in Flowering Plants

Q1. What is double fertilization? Why is it significant?

Solution

Double fertilization involves two events: (1) One male gamete fuses with egg to form zygote (syngamy), (2) Other male gamete fuses with two polar nuclei to form Primary Endosperm Nucleus (PEN).

Significance: Ensures food supply for developing embryo through endosperm formation.

Q2. Distinguish between perfect and imperfect flowers.

Solution

Perfect: Both stamens and carpels present (e.g., Hibiscus, Mustard)
Imperfect: Either stamens or carpels absent (e.g., Corn, Papaya)

Q3. What are the three types of pollination based on source of pollen?

Solution

1. Self-pollination: Pollen from same flower or plant
2. Cross-pollination: Pollen from different plant of same species
3. Geitonogamy: Pollen from different flower on same plant

Q4. What is the structure of an ovule? Name its parts.

Solution

Parts of ovule: (1) Funicle (stalk) (2) Hilum (attachment point) (3) Integuments (protective layers) (4) Nucellus (nutritive tissue) (5) Embryo sac (female gametophyte) containing egg cell, synergids, polar nuclei, antipodals

Q5. What are the effects of emasculation and bagging in plant breeding?

Solution

Emasculation: Removal of anthers from bisexual flowers before they mature, to prevent self-pollination.

Bagging: Covering emasculated flowers with bags to prevent unwanted pollen from reaching the stigma.

Chapter 3: Human Reproduction

Q1. What are the functions of the following: (i) Corpus luteum (ii) Fimbriae (iii) Acrosome

Solution

(i) Corpus luteum: Secretes progesterone to maintain endometrium for implantation
(ii) Fimbriae: Finger-like projections that catch the ovum from ovary
(iii) Acrosome: Contains enzymes to help sperm penetrate the ovum

Q2. Describe the menstrual cycle with its phases.

Solution

Phase 1 - Menstrual (1-5 days): Shedding of endometrium
Phase 2 - Follicular (6-13 days): FSH stimulates follicle growth, endometrium rebuilds
Phase 3 - Ovulatory (14th day): LH surge causes ovulation
Phase 4 - Luteal (15-28 days): Corpus luteum secretes progesterone

Q3. What is the role of HCG during pregnancy?

Solution

Human Chorionic Gonadotropin (HCG) is secreted by the placenta. It maintains the corpus luteum, which continues to secrete progesterone needed to maintain the endometrium during pregnancy.

Q4. What are the changes that occur in the uterus during the menstrual cycle?

Solution

Menstrual phase: Endometrium breaks down, bleeding occurs
Follicular phase: Endometrium regenerates, becomes thick and vascular
Luteal phase: Endometrium becomes highly vascular and glandular, ready for implantation

Q5. Name the hormones involved in parturition.

Solution

Oxytocin: Stimulates uterine contractions
Relaxin: Relaxes the cervix and ligaments
Prolactin: Stimulates milk production after birth

Chapter 4: Reproductive Health

Q1. What is the difference between natural and artificial methods of contraception?

Solution

Natural: Periodic abstinence, coitus interruptus, lactational amenorrhea
Artificial: Barrier methods (condoms), IUDs, oral pills, surgical methods (vasectomy, tubectomy)

Q2. What are sexually transmitted diseases? Name four common STDs.

Solution

STDs are diseases transmitted through sexual intercourse. Common ones: (1) AIDS (HIV) (2) Syphilis (3) Gonorrhea (4) Genital herpes

Q3. Why is amniocentesis banned in India?

Solution

Amniocentesis is banned because it was being misused for sex determination of the fetus, leading to female foeticide and skewed sex ratio.

Q4. What are the advantages of breast feeding?

Solution

1. Provides complete nutrition to infant
2. Contains antibodies (IgA) for immunity
3. Promotes mother-child bonding
4. Acts as natural contraceptive (lactational amenorrhea)

Chapter 5: Principles of Inheritance and Variation

Q1. State Mendel's Law of Segregation and Law of Independent Assortment.

Solution

Law of Segregation: Allele pair separates during gamete formation, so each gamete carries only one allele.
Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Q2. A cross between Tt x Tt gives 3:1 ratio. Explain with a Punnett square.

Solution

T t
T | TT Tt
t | Tt tt

Genotypic ratio: 1 TT : 2 Tt : 1 tt
Phenotypic ratio: 3 Tall : 1 Dwarf

Q3. What is linkage? How does it affect Mendel's law of independent assortment?

Solution

Linkage is the tendency of genes located on the same chromosome to be inherited together. It violates the law of independent assortment because linked genes do not assort independently - they tend to stay together during gamete formation.

Q4. What is a test cross? Why is it performed?

Solution

A test cross involves crossing an organism with a dominant phenotype but unknown genotype with a homozygous recessive individual.

Purpose: To determine the genotype of the organism with dominant trait.
If all offspring show dominant trait → homozygous dominant
If 50% show recessive trait → heterozygous

Q5. What are the different types of chromosomal mutations?

Solution

Structural: Deletion, Duplication, Inversion, Translocation
Numerical: Aneuploidy (loss/gain of chromosomes), Euploidy (complete set changes)

Chapter 6: Molecular Basis of Inheritance

Q1. What evidence supported the role of DNA as genetic material?

Solution

1. Griffith's experiment (1928) - Transforming principle
2. Avery, MacLeod, McCarty (1944) - Identified DNA as transforming material
3. Hershey-Chase experiment (1952) - Confirmed DNA is genetic material

Q2. What is the Central Dogma of Molecular Biology?

Solution

DNA → RNA → Protein
Replication Transcription Translation

Flow of genetic information: DNA → (transcription) → mRNA → (translation) → Protein

Q3. What are Okazaki fragments?

Solution

Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are joined by DNA ligase to form a continuous strand.

Q4. What is the role of tRNA in protein synthesis?

Solution

tRNA (transfer RNA) carries amino acids to the ribosome during translation. It has an anticodon that base pairs with the codon on mRNA, ensuring the correct amino acid is added to the growing polypeptide chain.

Chapter 7: Evolution

Q1. State the Hardy-Weinberg principle. What factors disturb it?

Solution

p² + 2pq + q² = 1 (genotype frequencies remain constant across generations)
Factors disturbing H-W equilibrium: (1) Gene flow (2) Genetic drift (3) Mutation (4) Natural selection (5) Non-random mating

Q2. What is the difference between natural selection and artificial selection?

Solution

Natural selection: Nature selects advantageous traits, increases fitness
Artificial selection: Humans select desired traits for breeding (e.g., dog breeds, crop varieties)

Q3. What evidence supports organic evolution?

Solution

1. Fossil record (comparative anatomy)
2. Comparative embryology
3. Biogeography
4. Molecular biology (DNA homology)
5. Vestigial organs

Chapter 8: Human Health and Disease

Q1. What is the difference between innate and acquired immunity?

Solution

Innate: Present from birth, non-specific (e.g., skin, stomach acid, WBCs)
Acquired: Developed after exposure, specific (e.g., antibodies, memory cells)

Q2. How does the body fight a bacterial infection?

Solution

1. Phagocytes engulf bacteria (phagocytosis)
2. B-cells produce antibodies (IgM, IgG)
3. Complement system activates
4. Opsonization marks bacteria for destruction
5. Memory cells provide long-term immunity

Q3. What are allergens? How do they cause allergic reactions?

Solution

Allergens are substances that cause exaggerated immune responses (allergies). Upon first exposure, they stimulate IgE production. On re-exposure, allergens bind to IgE on mast cells, triggering release of histamine, causing symptoms like sneezing, itching, and swelling.

Q4. What is cancer? Distinguish between benign and malignant tumors.

Solution

Benign: Localized, does not spread, non-cancerous
Malignant: Invades surrounding tissues, spreads through metastasis, cancerous

Chapter 9: Strategies for Enhancement in Food Production

Q1. What is the difference between inbreeding and outbreeding?

Solution

Inbreeding: Mating between closely related individuals, increases homozygosity
Outbreeding: Mating between unrelated individuals, increases heterozygosity and hybrid vigor

Q2. What is meant by single cell protein?

Solution

Single cell protein (SCP) refers to edible proteins extracted from pure or mixed cultures of microorganisms (algae, fungi, yeast, bacteria) grown on agricultural wastes, used as protein-rich food supplements.

Q3. What are the objectives of plant breeding?

Solution

1. Increased crop yield
2. Improved quality
3. Resistance to diseases and pests
4. Tolerance to environmental stress
5. Shorter maturity period

Q4. What is the Green Revolution? Name two achievements.

Solution

The Green Revolution (1960s-70s) increased food grain production through improved agricultural practices and high-yielding varieties.

Achievements: (1) Introduction of semi-dwarf wheat and rice varieties (2) India became self-sufficient in food grain production

Chapter 10: Microbes in Human Welfare

Q1. How are microbes used in sewage treatment?

Solution

Primary treatment: Physical removal of floating debris
Secondary treatment: Microbes decompose organic matter aerobically
Tertiary treatment: Chemical/physical purification for recycling

Q2. What is biogas? Name the bacteria involved in its production.

Solution

Biogas is a mixture of methane, CO₂, and H₂S produced by anaerobic decomposition of organic matter.

Methanogens: Methanobacterium - produce methane (CH₄)

Q3. What are the uses of Lactobacillus in daily life?

Solution

1. Conversion of milk to curd (lactic acid fermentation)
2. Production of cheese and yogurt
3. Probiotic supplements for gut health

Q4. What is mycorrhiza? How does it benefit the plant?

Solution

Mycorrhiza is a symbiotic association between fungus and plant roots.

Benefits: (1) Increased absorption of minerals (especially P) (2) Drought resistance (3) Protection from pathogens

Chapter 11: Biotechnology: Principles and Processes

Q1. What are the steps involved in recombinant DNA technology?

Solution

1. Isolation of desired gene
2. Cutting DNA with restriction enzymes
3. Joining DNA with ligase
4. Insertion into vector
5. Transfer into host cell
6. Selection and screening
7. Expression of desired gene

Q2. What is PCR? Name its three steps.

Solution

Polymerase Chain Reaction (PCR) amplifies specific DNA sequences.

Step 1 - Denaturation: 94-96°C (separate DNA strands)
Step 2 - Annealing: 55-65°C (primers bind)
Step 3 - Extension: 72°C (Taq polymerase extends primers)

Q3. What are restriction enzymes? How are they named?

Solution

Restriction enzymes are molecular scissors that cut DNA at specific recognition sequences.

Naming: Genus initial + species initials + strain + number
E.g., EcoRI = Escherichia coli RY13, first enzyme isolated

Chapter 12: Biotechnology and Its Applications

Q1. What is Bt cotton? How does it protect against bollworms?

Solution

Bt cotton contains the cry gene from Bacillus thuringiensis that produces the Cry protein (Bt toxin).

Mechanism: When bollworm eats Bt cotton, the alkaline gut activates the toxin, which binds to gut receptors, creating pores, leading to insect death.

Q2. What is gene therapy? Give an example.

Solution

Gene therapy is the introduction of a functional gene to correct a genetic disorder.

Example: ADA deficiency - Adenosine deaminase gene is introduced into patient's cells to treat SCID (Severe Combined Immunodeficiency).

Q3. What are transgenic animals? Name one example and its use.

Solution

Transgenic: Animals with foreign genes inserted into their genome
Example: Transgenic cow (Rosie) - produces human alpha-lactalbumin in milk, more nutritionally balanced

Q4. What are molecular diagnosis techniques?

Solution

1. PCR (amplify pathogen DNA)
2. ELISA (detect antigens/antibodies)
3. DNA probes (detect specific sequences)
4. Western blotting (detect proteins)

Chapter 13: Organisms and Populations

Q1. What are the different types of interactions between organisms?

Solution

1. Mutualism: Both benefit (+/+)
2. Commensalism: One benefits, other unaffected (+/0)
3. Parasitism: One benefits, other harmed (+/-)
4. Predation: One kills and eats other
5. Competition: Both harmed (-/-)

Q2. What is an ecological niche?

Solution

Ecological niche is the unique functional role and position of a species in its environment, including its habitat, food, interactions, and tolerances. Gause's principle states that two species cannot occupy the same niche indefinitely.

Q3. What are the attributes of a population?

Solution

1. Birth rate (natality)
2. Death rate (mortality)
3. Immigration rate
4. Emigration rate
5. Age distribution
6. Population density

Chapter 14: Ecosystem

Q1. What is an ecological pyramid? Name three types.

Solution

Pyramid of numbers: Number of organisms at each level
Pyramid of biomass: Total biomass at each level
Pyramid of energy: Energy flow at each level (always upright)

Q2. What is the 10% law of energy transfer?

Solution

Only about 10% of energy is transferred from one trophic level to the next. The rest is lost as heat through respiration.

Q3. What is ecological succession? Name its types.

Solution

Primary succession: On bare/uninhabited land (e.g., lava, sand)
Secondary succession: On previously inhabited but disturbed land (e.g., abandoned farmland)

Q4. What are biogeochemical cycles? Name two types.

Solution

Gaseous: Carbon cycle, Nitrogen cycle (atmosphere as reservoir)
Sedimentary: Phosphorus cycle, Sulphur cycle (rocks/soil as reservoir)

Q5. What is biodiversity? Why is it important?

Solution

Biodiversity is the variety of life forms at genetic, species, and ecosystem levels.

Importance: (1) Ecological services (2) Food and medicine (3) Industrial products (4) Aesthetic value (5) Ecosystem stability