Class 12 Biology — Complete Notes and CBSE PYQs

Unit 1: Sexual Reproduction in Flowering Plants

Flower Structure and Function

Male Reproductive Part (Stamen): Anther (produces pollen) + Filament

Female Reproductive Part (Pistil/Carpel): Stigma + Style + Ovary (contains ovules)

Pollen Grain: Mature pollen = 3-celled (1 tube cell + 2 male gametes)

Ovule Structure: Nucellus + Integuments + Embryo sac Embryo Sac (7-celled, 8-nucleate):

• 2 synergids + 1 egg cell (at micropyle end) — egg apparatus
• 3 antipodal cells (at chalaza end)
• 1 large central cell with 2 polar nuclei

Double Fertilization (Unique to Angiosperms)

1. Pollen tube enters ovule through micropyle
2. Two male gametes released
3. Syngamy: 1 male gamete + egg cell → Zygote (2n) → Embryo
4. Triple fusion: 1 male gamete + 2 polar nuclei → Primary Endosperm Nucleus (3n) → Endosperm

Endosperm: Nutritive tissue for developing embryo.

Fruits and Seeds

Fruit: Ripened ovary (or ovary + other parts = false fruit)

• True fruit: From ovary only (mango, tomato)
• False fruit: From receptacle + ovary (apple, strawberry)
• Parthenocarpy: Fruit without fertilization (banana, seedless grapes)

---

Unit 2: Human Reproduction

Male Reproductive System

• Testes (in scrotal sac, below body temp for spermatogenesis): produce sperm + testosterone
• Seminiferous tubules: site of spermatogenesis
• Sertoli cells: nourish developing sperm
• Leydig cells: secrete testosterone
• Path: Testes → Rete testis → Vasa efferentia → Epididymis → Vas deferens → Ejaculatory duct → Urethra

Spermatogenesis: Spermatogonia (2n) → Primary spermatocyte (2n) → Secondary spermatocyte (n) → Spermatids → Sperm (n)

Sperm Structure: Head (acrosome + nucleus) + Neck + Middle piece (mitochondria) + Tail (flagellum)

Female Reproductive System

• Ovaries: Produce eggs + estrogen + progesterone
• Oviduct/Fallopian tube: Path of egg from ovary to uterus; fertilization occurs here
• Uterus: Site of embryo development

Menstrual Cycle (28 days):

• Days 1-5: Menstruation (shedding of endometrium)
• Days 6-13: Follicular phase (FSH → follicle grows, estrogen rises)
• Day 14: Ovulation (LH surge)
• Days 15-28: Luteal phase (corpus luteum secretes progesterone)

---

Unit 3: Genetics and Molecular Biology

Mendelian Genetics (Revisit)

Mendel's Laws:

1. Law of Segregation: Alleles separate during gamete formation
2. Law of Independent Assortment: Different genes assort independently

Monohybrid Cross: F2 ratio: 3:1 (phenotype), 1:2:1 (genotype) Dihybrid Cross: F2 ratio: 9:3:3:1

Incomplete Dominance: F1 intermediate; F2 = 1:2:1 (e.g., snapdragon flower color) Codominance: Both alleles expressed; F2 = 1:2:1 (e.g., ABO blood groups)

Blood Groups (ABO):

| Blood Group | Genotype | Antigen on RBC | Antibody in plasma | |---|---|---|---| | A | I^A I^A or I^A i | A | Anti-B | | B | I^B I^B or I^B i | B | Anti-A | | AB | I^A I^B | A and B | None (universal recipient) | | O | ii | None | Anti-A and Anti-B (universal donor) |

Rh Factor: Rh⁺ if D antigen present. Rh⁻ mother + Rh⁺ baby → erythroblastosis fetalis.

Chromosomal Theory

Morgan's Experiment (Drosophila):

• Linked genes violate Mendel's second law
• Crossing over during prophase-1 of meiosis → recombinant chromosomes
• Recombination frequency = (recombinant offspring / total offspring) × 100

Sex-Linked Traits: Genes on X chromosome

• Males (XY) show X-linked traits even with one copy
• Females (XX) need two copies for expression
• Examples: Color blindness, haemophilia A (both X-linked recessive)

Molecular Basis of Inheritance

DNA Structure (Watson and Crick, 1953):

• Double helix, antiparallel strands
• Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)
• 3.4 Å rise per base pair, 10 bp per turn = 34 Å pitch

DNA Replication (Semi-conservative): Meselson and Stahl experiment confirmed semi-conservative replication.

Key enzymes:

• Helicase: Unwinds double helix
• SSB proteins: Keep strands apart
• Primase: Makes RNA primer
• DNA Pol III: Main replication enzyme (5'→3')
• DNA Pol I: Removes RNA primer, fills gap
• DNA Ligase: Seals nicks

Transcription: DNA → mRNA (using RNA polymerase) Template strand (3'→5') → mRNA (5'→3') In eukaryotes: hnRNA → RNA processing (splicing of introns, capping, tailing) → mRNA

Translation: mRNA → Protein (using ribosomes, tRNA, amino acids) Start codon: AUG (Methionine) Stop codons: UAA, UAG, UGA

Genetic Code Properties:

• Triplet (3 bases = 1 codon)
• Degenerate (multiple codons for one amino acid)
• Non-overlapping
• Universal (same in almost all organisms)

---

Unit 4: Evolution

Darwin's Theory: Natural Selection

• Variation exists in populations
• More offspring born than can survive (struggle for existence)
• Individuals with favorable variations survive (survival of the fittest)
• Favorable traits passed to offspring → evolution

Evidence for Evolution:

• Homologous organs: same structure, different function (forelimbs of mammals) → common ancestry
• Analogous organs: different structure, same function (wings of birds and insects) → convergent evolution
• Vestigial organs: functionless remnants (appendix, nictitating membrane)
• Fossils: preserved remains

Hardy-Weinberg Principle: p² + 2pq + q² = 1 (p = freq of dominant allele, q = freq of recessive) p + q = 1 Equilibrium disturbed by: mutation, gene flow, genetic drift, natural selection, non-random mating.

---

Unit 5: Ecology

Ecosystem: Biotic (living) + Abiotic (non-living) components interacting as a unit.

Food Chain: Grass → Grasshopper → Frog → Snake → Eagle Trophic Levels: Producers (T1) → Primary consumers (T2) → Secondary consumers (T3)

Ecological Pyramids:

| Pyramid | Unit | Always Upright? | |---|---|---| | Number | Number of organisms | Can be inverted (tree ecosystem) | | Biomass | Dry weight | Can be inverted (aquatic) | | Energy | Kcal/m²/yr | Always upright (10% law) |

10% Law (Lindemann): Only 10% of energy transferred from one trophic level to next.

Biogeochemical Cycles:

• Carbon Cycle: Photosynthesis (CO₂ absorbed), Respiration/Decomposition (CO₂ released)
• Nitrogen Cycle: N₂ fixation → Ammonification → Nitrification → Denitrification

Population Growth:

• Exponential growth: dN/dt = rN (unlimited resources; J-shaped curve)
• Logistic growth: dN/dt = rN((K-N)/K) (limited resources; S-shaped curve; K = carrying capacity)

---

Solved CBSE PYQs

Q1 (2023): Explain double fertilization and its significance. Two fertilizations occur: (1) Syngamy — male gamete + egg = zygote (future embryo); (2) Triple fusion — male gamete + 2 polar nuclei = PEN (3n, future endosperm). Significance: Unique to angiosperms; endosperm provides nutrition to developing embryo.

Q2 (2022): A woman with blood group A marries a man with blood group B. What blood groups are possible in children? If A is I^A i and B is I^B i: offspring can be I^A I^B (AB), I^A i (A), I^B i (B), ii (O). All 4 blood groups possible!

Q3 (2024): Explain the significance of pyramids of energy. Pyramid of energy is always upright — never inverted. Shows actual energy available at each trophic level. Based on 10% law, only 10% energy passes to next level. Explains why food chains rarely exceed 4-5 levels (insufficient energy at higher levels).

---

Quick Revision Checklist

• Embryo sac: 7-celled, 8-nucleate; location of cells
• Double fertilization: syngamy + triple fusion
• Menstrual cycle: 28 days, 4 phases
• Mendelian ratios: 3:1 monohybrid, 9:3:3:1 dihybrid
• Blood groups: ABO antigens and antibodies
• DNA: A=T (2H bonds), G≡C (3H bonds), antiparallel, semi-conservative
• 3 stop codons: UAA, UAG, UGA; Start: AUG
• Homologous vs analogous organs
• Hardy-Weinberg: p² + 2pq + q² = 1
• 10% energy law, pyramid of energy always upright