Biology XII

All Chapters

Chapter 1

Reproduction in Organisms

~4 marks

Chapter 2

Sexual Reproduction in Flowering Plants

~6 marks

Chapter 3

Human Reproduction

~5 marks

Chapter 4

Reproductive Health

~3 marks

Chapter 5

Principles of Inheritance

~6 marks

Chapter 6

Molecular Basis of Inheritance

~8 marks

Chapter 7

Evolution

~4 marks

Chapter 8

Human Health & Disease

~5 marks

Chapter 9

Strategies for Enhancement in Food

~4 marks

Chapter 10

Microbes in Human Welfare

~4 marks

Chapter 11

Biotechnology: Principles & Processes

~6 marks

Chapter 12

Biotechnology & Its Applications

~5 marks

Chapter 13

Organisms & Populations

~4 marks

Chapter 14

Ecosystem

~5 marks

Chapter 15

Biodiversity & Conservation

~4 marks

Chapter 16

Environmental Issues

~3 marks

🌿 Chapter 1: Reproduction in Organisms

Asexual · Sexual · Life Span · Vegetative Propagation

📖 Core Theory

1.1 Types of Reproduction

Reproduction ensures continuity of species. Two fundamental modes exist.

Asexual Reproduction

Single parent, no gametes. Offspring genetically identical (clones). Binary fission (Amoeba), budding (Hydra, yeast), spore formation (Rhizopus), fragmentation (Spirogyra), vegetative propagation (plants).

Sexual Reproduction

Two parents, gamete fusion. Genetic variation. All sexually reproducing organisms have gametogenesis → gamete transfer → fertilisation → development.

Life Span

Duration from birth to natural death. Rose: few months; Banyan: hundreds of years; Parrot: 150 yr; Crow: 15 yr; Butterfly: few weeks; Tortoise: 100+ yr. NOT related to size.

Vegetative Propagation

Runner (grass), rhizome (ginger), tuber (potato), bulb (onion), leaf buds (Bryophyllum). Produces plants with characters of parent; rapid, no seeds needed.

1.2 Events in Sexual Reproduction

🔑 Phases of Sexual Reproduction

Pre-fertilisation: gametogenesis (gamete formation) + gamete transfer

Fertilisation: syngamy (fusion of gametes) → zygote

Post-fertilisation: embryogenesis (zygote → embryo → offspring)

Parthenogenesis: development without fertilisation (bees, some lizards, Apis — drones from unfertilised eggs)

Oestrus cycle (non-primate mammals) vs menstrual cycle (primates — including humans)

✏️ Worked Examples

Binary Fission vs Budding (Board 2023, 2M)

Q: Distinguish binary fission (Amoeba) from budding (Hydra).

Binary fission: parent divides into two equal daughter cells; both daughters are new individuals

Budding: a small outgrowth (bud) forms on parent; bud detaches and grows into new individual

Binary fission: both daughter cells same size; complete division of parent

Budding: parent remains; bud is initially smaller than parent

Binary fission: Amoeba, Paramecium, bacteria. Budding: Hydra, yeast.

✅ Both asexual; fission = equal division; budding = unequal outgrowth

Vegetative Propagation (Board 2022, 2M)

Q: Name the vegetative propagule in potato and ginger. What is its significance?

Potato: stem tuber. "Eyes" of potato are nodes with axillary buds.

Ginger: rhizome (underground modified stem with nodes and internodes)

Significance: rapid multiplication; new plants genetically identical to parent

No need for seeds — useful when seeds not produced or germination rate is low

Commercial advantage: desirable characters maintained exactly (mango grafting, sugarcane sets)

✅ Potato = stem tuber; Ginger = rhizome; both ensure rapid, clonal multiplication

Parthenogenesis (Board 2024, 2M)

Q: What is parthenogenesis? Give two examples.

Parthenogenesis = development of egg into individual WITHOUT fertilisation

No syngamy; egg divides mitotically (or meiotically with later chromosome doubling)

Examples: (1) Honey bee drones (males) develop from unfertilised eggs of queen (haploid)

(2) Rotifers, water fleas (Daphnia) during favourable conditions

Advantage: rapid reproduction; disadvantage: no genetic variation

✅ Parthenogenesis = egg development without fertilisation; examples: honey bee drones, rotifers

⚖️ Comparisons

Asexual vs Sexual Reproduction

Vegetative vs Sexual Reproduction in Plants

Single parent only

Two parents (usually)

No gametes, no meiosis

Meiosis to form gametes; syngamy

Offspring = genetic clone

Genetic variation in offspring

Rapid; no mate needed

Slower; mate/pollinator needed

Examples: rhizome, spore, fission

Seeds, flowers, fruits

🎯 Quick Activity

Think and Answer

Q: Parthenogenesis produces males (drones) in honeybees. Why are the drones haploid?

▶ Show Answer

A: Queen bee produces eggs by meiosis. Unfertilised eggs (haploid, n) develop by parthenogenesis into drones (males). Fertilised eggs (diploid, 2n) develop into workers and queens. This is an example of haplodiploidy.

🌿 Chapter 2: Sexual Reproduction in Flowering Plants

Microsporogenesis · Megasporogenesis · Pollination · Double Fertilisation

📖 Core Theory

2.1 Flower Structure and Development

Flower is the site of sexual reproduction in angiosperms. Stamen (male: anther + filament) and carpel/pistil (female: stigma + style + ovary) are essential organs.

Microsporogenesis

Formation of microspores (pollen) in anther. Microsporocyte (2n) → meiosis → 4 microspores (n) = pollen tetrad. Tapetum nourishes developing pollen. Pollen grain has 2 layers: exine (sporopollenin) and intine (cellulose-pectin).

Megasporogenesis

Formation of megaspore in ovule. MMC (2n) → meiosis → 4 megaspores; only 1 functional (Polygonum type = most common). Functional megaspore → 3 mitotic divisions → 8-nucleate embryo sac.

Embryo Sac (7-celled, 8-nucleate)

Egg cell (1) + synergids (2) at micropylar end; antipodal cells (3) at chalazal end; central cell with 2 polar nuclei (1 cell). Egg + 2 synergids = egg apparatus.

Double Fertilisation

Unique to angiosperms. One male gamete + egg cell → zygote (2n). Second male gamete + 2 polar nuclei → primary endosperm nucleus (3n, PEN). PEN → endosperm (food for embryo).

🔑 Pollination and Outbreeding Devices

Self-pollination (autogamy): pollen from same flower — e.g. wheat, rice, pea

Cross-pollination (allogamy): pollen from different plant — promotes variation

Agents: wind (anemophily), water (hydrophily), insects (entomophily), birds (ornithophily), bats (chiropterophily)

Outbreeding devices: dicliny, dichogamy, self-incompatibility, heterostyly

Apomixis: seed formation without fertilisation — dandelion, grasses; polyembryony: multiple embryos in one seed

✏️ Worked Examples

Embryo Sac Nuclei (Board 2023, 3M)

Q: Draw and label the structure of a mature embryo sac. How many cells and nuclei are present?

Mature embryo sac = 7 cells, 8 nuclei (Polygonum type)

Micropylar end: egg cell (1) + 2 synergids = egg apparatus (3 cells)

Chalazal end: 3 antipodal cells

Centre: large central cell with 2 polar nuclei (1 cell, 2 nuclei)

Total: 7 cells, 8 nuclei (central cell counted as 1 cell but has 2 nuclei)

✅ Embryo sac = 7 cells (3+1+3) and 8 nuclei

Double Fertilisation (Board 2022, 5M)

Q: Explain double fertilisation and its significance.

Pollen tube enters ovule through micropyle, releases 2 male gametes

First fertilisation (syngamy): male gamete + egg cell → zygote (2n) → embryo

Second fertilisation (triple fusion): second male gamete + 2 polar nuclei → PEN (3n) → endosperm

Endosperm = nutritive tissue for developing embryo

Significance: endosperm ensures efficient nutrition; prevents wastage (only fertilised ovules develop into seeds)

✅ Syngamy (2n zygote) + triple fusion (3n PEN) = double fertilisation; PEN → nutritive endosperm

Sporopollenin (Board 2024, 2M)

Q: Why is pollen well preserved as fossils? Name the substance responsible.

Pollen grain exine (outer wall) made of sporopollenin

Sporopollenin is one of the most resistant biological molecules known

Resistant to high temperatures, strong acids and alkalis, biological decomposition

This is why pollen grains are preserved for millions of years as fossils (palynology)

Sporopollenin content and composition is genetically determined — pollen identification possible

✅ Sporopollenin (on exine) — extremely resistant to decomposition → pollen preserved as fossils

⚖️ Comparisons

Autogamy

Cross-pollination (Geitonogamy / Xenogamy)

Pollen from same flower/plant

Pollen from different plant

No genetic variation

Promotes genetic variation

No pollinator needed — more reliable

Needs pollinator; risky but evolutionarily advantageous

Examples: wheat, rice, pea, tomato

Examples: maize (wind), rose (insect), orchids

🎯 Quick Activity

Think and Answer

Q: Cleistogamous flowers guarantee autogamy. What advantage does this give the plant?

▶ Show Answer

A: Cleistogamous flowers never open — self-pollination is assured even without pollinators. This ensures seed production in unfavourable conditions (no pollinators) and conserves energy (no need to attract pollinators). Trade-off: no genetic variation.

🌿 Chapter 3: Human Reproduction

Gametogenesis · Menstrual Cycle · Fertilisation · Implantation · Parturition

📖 Core Theory

3.1 Male Reproductive System

Testes (in scrotum at 2–2.5°C below body temp for normal spermatogenesis) produce sperms and testosterone. Accessory glands: seminal vesicles, prostate, Cowper's gland — produce seminal plasma.

Spermatogenesis

Spermatogonia (2n) → Primary spermatocyte → Meiosis I → Secondary spermatocytes (2) → Meiosis II → Spermatids (4) → Spermiogenesis → Spermatozoa (4 haploid). Takes ~74 days.

Sperm Structure

Head (acrosome + condensed nucleus), middle piece (mitochondria — energy for motility), tail (flagellum). Acrosome contains hydrolytic enzymes for zona pellucida penetration.

Oogenesis

Oogonia (2n) → Primary oocyte → Meiosis I (arrested at birth in prophase I) → Secondary oocyte + 1st polar body → Meiosis II (arrested at metaphase II, completed only after fertilisation) → Ovum + 2nd polar body.

Menstrual Cycle

~28 days. Menstrual phase (1–5, lining shed), Follicular/Proliferative (6–13, FSH → Graafian follicle, estrogen surge), Ovulation (day 14, LH surge), Luteal (15–28, LH → corpus luteum → progesterone).

🔑 Fertilisation and Development

Fertilisation: ampullary region of fallopian tube. Acrosomal reaction → sperm penetrates zona pellucida

Cortical reaction: prevents polyspermy (blocks zona pellucida to other sperms after entry of one)

Cleavage: mitotic divisions of zygote → 2, 4, 8, 16 cells (morula) → blastocyst

Implantation: blastocyst embeds in endometrium (day 6–10 after fertilisation) — trophoblast helps

Parturition: triggered by oxytocin (positive feedback loop); uterine contractions → delivery (~38 weeks)

✏️ Worked Examples

Menstrual Cycle Hormones (Board 2023, 5M)

Q: Describe the hormonal events during the menstrual cycle.

Day 1–5: Menstrual phase — low estrogen + progesterone → endometrium sheds

Day 6–13: FSH from anterior pituitary → Graafian follicle develops → secretes estrogen

Day 13: Estrogen surge → LH surge (LH peak triggers ovulation on day 14)

Day 15–28: Luteal phase — LH → corpus luteum → secretes progesterone (+ estrogen)

Progesterone maintains endometrium (for implantation); if no fertilisation, CL degenerates → hormone drop → menstruation

✅ FSH→follicle→estrogen→LH surge→ovulation→corpus luteum→progesterone→menstruation if no pregnancy

Polyspermy Prevention (Board 2022, 3M)

Q: How is polyspermy prevented after fertilisation?

Polyspermy = entry of more than one sperm — would create polyploid, non-viable zygote

Cortical reaction (fast block): Ca²⁺ release from egg cortex when first sperm enters

Cortical granules fuse with egg membrane → release enzymes into perivitelline space

Enzymes harden zona pellucida (zona reaction) → impermeable to other sperms

First block: depolarisation of egg membrane immediately on sperm entry (fast block)

✅ Cortical reaction + zona hardening = slow block; membrane depolarisation = fast block against polyspermy

Oogenesis vs Spermatogenesis (Board 2024, 3M)

Q: Why does oogenesis give only 1 ovum but spermatogenesis gives 4 sperms from one primary gametocyte?

Both start with one primary gametocyte (2n) and undergo meiosis I then meiosis II

Spermatogenesis: meiosis I → 2 secondary spermatocytes; meiosis II → 4 spermatids → 4 sperms (equal division)

Oogenesis: meiosis I → secondary oocyte + 1st polar body (unequal, cytoplasm mostly to oocyte)

Meiosis II → ovum + 2nd polar body (again unequal); polar bodies are tiny and non-functional

Purpose: one large yolk-rich egg cell instead of 4 small, non-viable eggs

✅ Spermatogenesis: equal division → 4 sperms; Oogenesis: unequal → 1 large ovum + 3 polar bodies

⚖️ Comparisons

Spermatogenesis

Oogenesis

Begins at puberty; continuous throughout life

Begins in foetal life; arrested until puberty

Occurs in seminiferous tubules of testes

Occurs in ovarian follicles

One primary spermatocyte → 4 sperms

One primary oocyte → 1 ovum + 3 polar bodies

Meiosis completed before release

Meiosis II completed only after fertilisation

🎯 Quick Activity

Think and Answer

Q: Graafian follicle is the mature follicle. What happens to it after ovulation?

▶ Show Answer

A: After ovulation, the empty Graafian follicle transforms into the corpus luteum (yellow body) under the influence of LH. Corpus luteum secretes progesterone (and estrogen) which maintains the endometrium. If fertilisation occurs, CL is maintained by hCG from placenta. If not, CL degenerates → menstruation.

🌿 Chapter 4: Reproductive Health

Problems · STIs · Contraception · ART · MTP

📖 Core Theory

4.1 Reproductive Health and STIs

Reproductive health (WHO) = total well-being in all aspects of reproduction — physical, emotional, social. India: first country to launch National Family Planning Programme (1951).

STIs (Sexually Transmitted Infections)

HIV/AIDS (retrovirus), gonorrhoea (Neisseria), syphilis (Treponema), genital herpes (HSV), hepatitis-B (HBV), chlamydiasis, genital warts (HPV). Transmitted via sex, blood, mother → foetus. HIV destroys T-helper cells.

Contraceptive Methods

Natural (periodic abstinence, coitus interruptus, lactational amenorrhoea <6 months postpartum). Barrier (condoms, diaphragm, cervical caps). IUDs (CuT, LNG-20). Oral pills (combined estrogen+progestin). Surgical (vasectomy, tubectomy).

Medical Termination of Pregnancy (MTP)

Legal in India up to 20 weeks (MTP Act 1971, amended 2021 — up to 24 weeks in special cases). Used for unwanted pregnancies or foetal anomalies. NOT a contraceptive method.

Assisted Reproductive Technologies (ART)

IVF-ET (test tube baby — Edwards, Steptoe, 1978; first: Louise Brown). GIFT (gamete intrafallopian transfer). ZIFT (zygote intrafallopian transfer). AI (artificial insemination). ICSI (intracytoplasmic sperm injection).

🔑 IUDs and Hormonal Contraception

Non-medicated IUDs: Lippes loop — increase phagocytosis of sperm; interfere with uterine motility

Cu-releasing IUDs (CuT, Cu7, Multiload 375): Cu ions suppress sperm motility; fertilisation prevented

Hormone-releasing IUDs (LNG-20, Progestasert): thicken cervical mucus; prevent implantation

Emergency contraceptives: high-dose progestin within 72 hours → prevent/delay ovulation or implantation

Amniocentesis: foetal chromosomal analysis (fluid from amniotic sac) — for genetic disorders, NOT sex determination (illegal)

✏️ Worked Examples

IUD Mechanism (Board 2023, 3M)

Q: How do copper-releasing IUDs act as contraceptives?

IUDs are placed inside uterus by medical professional

Cu-releasing IUDs (CuT-380A, Multiload 375) release Cu²⁺ ions continuously

Cu²⁺ ions are spermicidal — inhibit sperm motility and fertilising capacity

Also affect uterine environment (pH) making it hostile to sperm

Prevent fertilisation; if fertilisation occurs, also prevent implantation

Most effective for 5–10 years; reversible on removal

✅ Cu²⁺ ions are spermicidal + alter uterine environment → prevent fertilisation

ART Comparison (Board 2022, 3M)

Q: Distinguish between IVF-ET and ZIFT.

IVF-ET (In Vitro Fertilisation-Embryo Transfer):

Eggs collected from wife → fertilised with husband's sperm in lab (in vitro)

Resulting embryo (8-cell stage) transferred to uterus (ET) or fallopian tube

ZIFT (Zygote Intrafallopian Transfer):

Fertilisation in lab (same as IVF) → resulting ZYGOTE transferred to FALLOPIAN TUBE

Key difference: IVF-ET embryo goes to uterus; ZIFT zygote goes to fallopian tube (more natural position)

✅ IVF-ET: embryo to uterus; ZIFT: zygote to fallopian tube — both involve external fertilisation

STI Prevention (Board 2024, 2M)

Q: Why is HIV not classified as a typical STI by some?

STIs are primarily sexually transmitted; HIV can also be transmitted by:

(1) Contaminated blood transfusions

(2) Sharing needles (IV drug users)

(3) Mother to child (trans-placental or breast milk)

HIV is therefore classified as a blood-borne AND sexually transmitted infection

HIV specifically destroys CD4⁺ T-helper lymphocytes → immune system collapse → AIDS

✅ HIV = STI + blood-borne + vertical transmission; destroys T-helper cells → AIDS

⚖️ Comparisons

Vasectomy

Tubectomy

Male sterilisation method

Female sterilisation method

Vas deferens cut and tied

Fallopian tubes cut/ligated/blocked (Pomeroy method)

Minor surgery; local anaesthetic

Requires general anaesthetic; more invasive

Sperm cannot reach ejaculate; hormones unaffected

Egg cannot reach uterus; hormones unaffected

🎯 Quick Activity

Think and Answer

Q: Lactational amenorrhoea is a natural contraceptive method. Under what conditions is it effective?

▶ Show Answer

A: Lactational amenorrhoea (no periods during breastfeeding) is effective as contraceptive ONLY when: (1) mother is fully breastfeeding (on-demand, day and night), (2) baby is less than 6 months old, and (3) mother has not yet had a menstrual period. Prolactin (from suckling) suppresses GnRH → no FSH/LH → no ovulation. Maximum 98% effective under these strict conditions.

🌿 Chapter 5: Principles of Inheritance and Variation

Mendelism · Chromosomal Theory · Linkage · Sex Determination · Mutations

📖 Core Theory

5.1 Mendel's Laws

Gregor Mendel (1865) worked on garden pea (Pisum sativum) — 7 pairs of contrasting traits, each controlled by a single gene with 2 alleles (dominant/recessive).

Law of Dominance

In a heterozygous individual, only dominant allele is expressed in F1 (phenotype). Recessive allele remains latent but present in genotype.

Law of Segregation

During gamete formation, alleles of a gene separate so each gamete has one allele. Explains 3:1 ratio in F2 of monohybrid cross.

Law of Independent Assortment

Genes for different traits assort independently (if on non-homologous chromosomes). Explains 9:3:3:1 ratio in F2 of dihybrid cross. Does NOT apply to linked genes.

Chromosomal Theory of Inheritance

Sutton and Boveri (1902): genes located on chromosomes. Experimental proof: Morgan's fruit fly (Drosophila) linkage groups correspond to chromosome number.

🔑 Extensions of Mendelism

Incomplete dominance: F1 intermediate (RR×rr → Rr pink); F2 = 1:2:1 (red:pink:white)

Co-dominance: both alleles expressed (AB blood type — IAIA × IBIB → IAIB)

Polygenic inheritance: multiple genes control one trait — skin colour, height (continuous variation)

Pleiotropy: one gene affects multiple traits — sickle cell anaemia (HbS gene)

Sex-linked inheritance: haemophilia (X-linked recessive); females: carrier (XHXh) or affected (XhXh); all sons of carrier mother have 50% chance

✏️ Worked Examples

Monohybrid Cross (Board 2023, 5M)

Q: Tall (TT) × Dwarf (tt) pea. Show F1, F2 genotypes and phenotypes. State the law illustrated.

P generation: TT (tall) × tt (dwarf)

F1: all Tt (tall) — Law of Dominance (T dominant over t)

F1 × F1: Tt × Tt

F2: TT : Tt : tt = 1 : 2 : 1 (genotype); Tall : Dwarf = 3 : 1 (phenotype)

Law illustrated: Law of Segregation — T and t alleles separate during gamete formation

This 3:1 ratio is the MONOHYBRID ratio

✅ F1 = all Tt (tall); F2 = 3 Tall : 1 Dwarf; Law of Segregation illustrated

Sex Determination (Board 2022, 3M)

Q: Explain XY sex determination in humans. Why is father responsible for sex of child?

Females: 44 + XX; Males: 44 + XY (23 pairs total)

Females produce only one type of egg: 22 + X

Males produce two types of sperm: 22 + X (gynosperm) and 22 + Y (androsperm) in equal numbers

X-sperm + egg → 44+XX = female; Y-sperm + egg → 44+XY = male

Since females contribute only X, the sex of offspring is entirely determined by which sperm fertilises the egg

Hence father (sperm producer) is responsible for determining sex of child

✅ Father's X-sperm → daughter; Y-sperm → son; mother always contributes X → father determines sex

Sickle Cell Anaemia (Board 2024, 3M)

Q: Explain the molecular basis of sickle cell anaemia.

Autosomal recessive disorder; mutation in HBB gene (chromosome 11)

Single nucleotide substitution: GAG → GTG at 6th codon of β-globin

Results in: Glutamic acid → Valine at 6th position of β-globin chain

HbS (abnormal) polymerises under low O₂ → forms fibrous precipitates → sickling of RBCs

Sickle cells clog capillaries, are fragile → anaemia

HbA/HbS heterozygotes (carriers): sickle cell trait — usually healthy but some sickling under very low O₂

✅ GAG→GTG mutation; Glu→Val; HbS polymerises → sickling; autosomal recessive

⚖️ Comparisons

Linkage

Crossing Over

Genes on same chromosome tend to be inherited together

Exchange of segments between non-sister chromatids of homologous chromosomes

Reduces independent assortment

Creates new combinations (recombinants)

Morgan showed with white eye and yellow body genes in Drosophila

Occurs during prophase I (pachytene) of meiosis I

Tight linkage = near genes; loose = far genes

Frequency of crossing over ∝ distance between genes (basis of chromosome mapping)

🎯 Quick Activity

Think and Answer

Q: A colour-blind father and a normal homozygous mother have a son. Will the son be colour blind?

▶ Show Answer

A: No. Colour blindness is X-linked recessive. Father is XcbY; mother is XCXC. Daughters get X from father (Xcb) and X from mother (XC) → all daughters XCXcb (carriers, normal vision). Sons get Y from father and X from mother (XC) → all sons XCY (normal vision). The son is NOT colour blind.

🌿 Chapter 6: Molecular Basis of Inheritance

DNA Structure · Replication · Transcription · Translation · Regulation

📖 Core Theory

6.1 DNA — Structure and Packaging

DNA (deoxyribonucleic acid) is the genetic material in most organisms. Proof: Griffith's transformation experiment → Avery, MacLeod, McCarty (1944) showed DNA is transforming principle; Hershey-Chase experiment (1952) with radioactive S³⁵ (protein) and P³² (DNA) confirmed DNA is genetic material.

DNA Double Helix (Watson-Crick, 1953)

Two antiparallel polynucleotide strands. Bases pair: A=T (2 H-bonds), G≡C (3 H-bonds). Pitch = 3.4 nm; 10 bp/turn; diameter 2 nm. Sugar-phosphate backbone on outside; bases inside.

DNA Packaging

Nucleosome: 200 bp DNA wound around histone octamer (H2A, H2B, H3, H4 × 2; H1 linker). Nucleosomes → 30 nm fibre → loops → chromosomes. Human genome: 3.1 billion bp; ~30,000 genes.

Replication

Semi-conservative (Meselson-Stahl, 1958, E. coli in ¹⁵N→¹⁴N medium). At origin: helicase unwinds, primase makes RNA primer, DNA Pol III elongates (5'→3'), DNA Pol I removes primer, ligase joins fragments. Leading strand continuous; lagging strand: Okazaki fragments.

Central Dogma

DNA → (Transcription) → RNA → (Translation) → Protein. Reverse transcription (retroviruses: RNA → DNA) is an exception (Temin).

🔑 Transcription and Translation

Transcription: template strand (3'→5') → mRNA (5'→3'); RNA Pol II in eukaryotes; promoter has TATA box

mRNA processing (eukaryotes): 5' capping, 3' poly-A tail, splicing (introns removed, exons joined)

Genetic code: 64 codons; 61 code amino acids; 3 stop codons (UAA, UAG, UGA); start = AUG (Met)

Code is degenerate (1 AA ≥ 2 codons), non-overlapping, non-ambiguous, comma-less, universal

Jacob-Monod lac operon: structural genes (lacZ, lacY, lacA) + operator + promoter; regulated by repressor (lacI gene)

✏️ Worked Examples

Semi-conservative Replication (Board 2023, 5M)

Q: Describe Meselson-Stahl experiment to prove semi-conservative replication.

E. coli grown in ¹⁵NH₄Cl (heavy nitrogen) for many generations → all DNA is heavy (¹⁵N-¹⁵N)

Transferred to ¹⁴NH₄Cl (normal nitrogen) medium

After 1 generation: CsCl density gradient centrifugation → only intermediate density band (¹⁵N-¹⁴N)

After 2 generations: equal light (¹⁴N-¹⁴N) and intermediate (¹⁵N-¹⁴N) bands

Intermediate = one old ¹⁵N strand + one new ¹⁴N strand = semi-conservative replication confirmed

✅ ¹⁵N→¹⁴N switch + CsCl gradient → intermediate band after 1 gen = semi-conservative confirmed

Lac Operon (Board 2022, 5M)

Q: Explain the regulation of lac operon with inducer present and absent.

WITHOUT inducer (lactose): lacI gene makes repressor → binds operator → blocks RNA Pol → no transcription

With lactose (inducer): allolactose (lactose metabolite) binds repressor → conformational change → repressor leaves operator

RNA Polymerase binds promoter → transcribes lacZ, lacY, lacA genes

lacZ: β-galactosidase (cleaves lactose); lacY: permease; lacA: transacetylase

When lactose exhausted: no inducer → repressor binds operator again → genes switched off (negative regulation)

✅ Repressor blocks without lactose; lactose → inducer removes repressor → genes transcribed

Genetic Code Properties (Board 2024, 3M)

Q: What is meant by "degenerate" and "wobble" in the genetic code?

Degenerate code: most amino acids coded by more than one codon (synonymous codons)

Example: Leu coded by UUA, UUG, CUU, CUC, CUA, CUG (6 codons)

Third position of codon is less specific ("wobble position")

Wobble hypothesis (Crick): anticodon base at 3' end of tRNA can pair with more than one base at 3rd codon position

Allows fewer tRNAs to recognize all 61 sense codons

✅ Degenerate = multiple codons per amino acid; wobble = 3rd codon position less specific

⚖️ Comparisons

Template Strand

Coding Strand

Used as template by RNA Pol (read 3'→5')

Has same sequence as mRNA (except T→U)

Also called antisense / non-coding strand

Also called sense strand

RNA Pol reads this to synthesise mRNA 5'→3'

Not used as template

Base complementary to mRNA

Same sequence as mRNA (with T instead of U)

🎯 Quick Activity

Think and Answer

Q: In eukaryotes, hnRNA is produced in the nucleus but mRNA reaches the ribosome. What processing converts hnRNA to mRNA?

▶ Show Answer

A: Three steps: (1) 5' capping — 7-methylguanosine cap added to 5' end, protects from degradation and aids ribosome attachment. (2) 3' polyadenylation — poly-A tail (100-200 A residues) added, increases stability. (3) Splicing — introns removed and exons joined by spliceosomes. The processed mRNA is then exported to cytoplasm.

🌿 Chapter 7: Evolution

Origin of Life · Darwinism · Evidence · Human Evolution

📖 Core Theory

7.1 Origin of Life

Earth formed ~4.5 billion years ago. First life ~3.8 billion years ago (prokaryotes). Oparin-Haldane hypothesis: chemical evolution — simple molecules → organic molecules → proto-cells.

Urey-Miller Experiment (1953)

Simulated primitive Earth atmosphere (CH₄, H₂, NH₃, H₂O, no O₂) with lightning (sparks). Produced amino acids, sugars, urea etc. Proved abiotic synthesis of organic molecules possible.

Darwinian Theory

Natural selection: organisms with favourable variations survive and reproduce more (survival of the fittest). Variations arise randomly (not directed). Evolution = gradual accumulation of variations over generations.

Evidence for Evolution

Homologous organs (same origin, different function — forelimbs of whale, bat, human, horse — common ancestry), Analogous organs (different origin, similar function — wings of birds and insects — convergent evolution), Fossils, Biochemical similarity (cytochrome c sequence).

Hardy-Weinberg Principle

In a large, randomly mating population with no mutation/selection/migration — allele frequencies remain constant. p+q=1; p²+2pq+q²=1. Deviations = evolution occurring.

🔑 Mechanisms of Evolution

Genetic drift: random change in allele frequency in small population (founder effect, bottleneck effect)

Gene flow: transfer of alleles between populations via migration — reduces differentiation

Mutation: random change in DNA → new alleles; ultimate source of variation

Industrial melanism (pepper moth): dark Biston betularia survived better on soot-darkened trees (natural selection)

Drug resistance: mutation + antibiotic pressure → resistant strains selected (not Lamarckian)

✏️ Worked Examples

Hardy-Weinberg Calculation (Board 2023, 3M)

Q: In a population, frequency of recessive allele (q) for albinism is 0.3. Find frequency of carriers.

Hardy-Weinberg: p + q = 1 → p = 1 - 0.3 = 0.7

Frequency of homozygous dominant (AA) = p² = 0.49

Frequency of affected individuals (aa) = q² = 0.09

Frequency of carriers (Aa) = 2pq = 2 × 0.7 × 0.3 = 0.42

42% of population are carriers of albinism gene

✅ Carriers (Aa) = 2pq = 2×0.7×0.3 = 0.42 = 42%

Homologous vs Analogous (Board 2022, 2M)

Q: Distinguish homologous and analogous organs with one example each.

Homologous organs: same basic structure/embryological origin; different functions (divergent evolution)

Example: forelimbs of whale (flipper), bat (wing), horse (leg), human (arm) — same bone pattern (humerus, radius, ulna, carpals)

Analogous organs: different origin; similar function (convergent evolution)

Example: wing of bird (modified forelimb, bones) vs wing of insect (outgrowth of exoskeleton, no bones)

Both are aerodynamic structures but structurally completely different

✅ Homologous: same origin, different function (forelimbs); Analogous: different origin, same function (wings)

Natural Selection Evidence (Board 2024, 3M)

Q: Explain industrial melanism as evidence for natural selection.

Before industrial revolution: tree bark pale → pale (melanic) moths camouflaged; dark moths easily seen by birds → eaten

After industrialisation: soot darkened tree bark → dark (carbonaria) moths camouflaged; pale moths eaten

Result: dark moths increased in proportion in industrial areas; pale moths increased in rural areas

When Clean Air Acts removed pollution: tree bark returned pale → pale moths again increased

This is direct experimental evidence of natural selection in action — population changes observed over ~50 years

✅ Pale moths → fit pre-industry; dark moths → fit post-industry; soot = selection pressure; populations shifted accordingly

⚖️ Comparisons

Darwinism

Neo-Darwinism (Modern Synthetic Theory)

Natural selection as mechanism

Natural selection + genetic basis (Hardy-Weinberg)

Could not explain variation source

Mutation + recombination = source of variation

No knowledge of genetics or DNA

Incorporates Mendel's genetics + population genetics

Proposed pangenesis (wrong)

Genetic drift, gene flow, natural selection all included

🎯 Quick Activity

Think and Answer

Q: Bacteria never exposed to antibiotics can develop resistance very quickly once antibiotics are introduced. Is this Lamarckian or Darwinian? Explain.

▶ Show Answer

A: Darwinian. Random mutations occur in bacterial populations continuously, even without antibiotic exposure. A few bacteria happen to carry a mutation that confers resistance. When antibiotics are introduced, susceptible bacteria die; only resistant ones survive and reproduce. The antibiotic is a SELECTION PRESSURE, not a cause of mutation (that would be Lamarckian). The variation pre-existed; the environment selected for it.

🌿 Chapter 8: Human Health and Disease

Pathogens · Immunity · AIDS · Cancer · Drugs

📖 Core Theory

8.1 Innate and Adaptive Immunity

Two types of immunity: Innate (non-specific) — present from birth, first line of defense (skin, mucus, phagocytes, natural killer cells, inflammation, fever) and Adaptive (acquired/specific) — develops after exposure to antigen; has memory.

Humoral Immunity (B-cells)

B lymphocytes produce antibodies (immunoglobulins). 5 classes: IgG, IgA, IgM, IgD, IgE. Structure: 2 heavy + 2 light chains (Y-shape). Fab region binds antigen. Fc region determines function.

Cell-mediated Immunity (T-cells)

T helper (CD4⁺): coordinate immune response; targets of HIV. T cytotoxic (CD8⁺): kill virus-infected/tumour cells. T regulatory: suppress immune response (prevent autoimmunity). Memory T-cells: rapid response on re-exposure.

Active vs Passive Immunity

Active: body makes own antibodies (natural infection or vaccination); slow but long-lasting (memory). Passive: readymade antibodies given (antitoxin, colostrum, Ig injection); immediate but no memory.

Allergies

Exaggerated response to non-harmful antigens (allergens). IgE antibodies + mast cells release histamine → sneezing, watery eyes, rash, asthma. Anaphylaxis = severe allergic reaction. Antihistamines block H₁ receptors.

🔑 HIV/AIDS, Cancer and Drugs

HIV (Human Immunodeficiency Virus): retrovirus (RNA genome + reverse transcriptase). Infects CD4⁺ T-cells → immune collapse → AIDS (Acquired Immunodeficiency Syndrome).

ELISA: Enzyme-Linked Immunosorbent Assay — used to confirm HIV infection (detects antibodies)

Cancer: uncontrolled cell division due to proto-oncogene → oncogene activation or tumour suppressor gene loss

Carcinogens: chemical (tobacco), physical (UV, X-rays, gamma), biological (viruses — HPV, EBV)

Opioids: morphine, heroin bind opioid receptors → euphoria + addiction; depresses CNS; withdrawal painful

✏️ Worked Examples

Antibody Structure (Board 2023, 3M)

Q: Draw a labelled diagram of antibody structure and explain the antigen-binding site.

Antibody (immunoglobulin) has 4 polypeptide chains: 2 identical heavy (H) + 2 identical light (L)

Connected by disulphide (S-S) bonds in Y-shape

Variable region (V): N-terminal part of both H and L chains; unique to each antibody specificity

Antigen-binding site: formed by V regions of one H and one L chain (Fab fragment)

2 antigen-binding sites per antibody molecule (bivalent) → can cross-link antigens (agglutination)

✅ Y-shaped; 2H+2L chains; variable regions form 2 antigen-binding sites (Fab); Fc = constant region

HIV Replication (Board 2022, 5M)

Q: Describe how HIV replicates inside a host cell.

HIV (retrovirus) carries RNA genome + reverse transcriptase enzyme

Enters CD4⁺ T-cell via gp120 protein binding CD4 receptor

Reverse transcriptase: viral RNA → DNA (reverse transcription)

Viral DNA integrates into host chromosome as provirus

Host cell transcribes provirus → viral RNA + mRNA → viral proteins + new RNA genomes

New virions assemble and bud off → infect more CD4⁺ cells → progressive immune decline → AIDS

✅ RNA→DNA (rev. transcriptase)→provirus integration→virus assembly→CD4⁺ destruction→AIDS

Active vs Passive Immunity (Board 2024, 2M)

Q: Give two differences between active and passive immunity with one example each.

Active immunity: host produces its own antibodies after antigen exposure; has immunological memory

Example: natural infection (chickenpox → lifetime immunity); vaccination (polio vaccine → long-lasting protection)

Passive immunity: readymade antibodies administered; no memory; immediate effect

Example: tetanus antitoxin given after injury; colostrum (mother's first milk) → IgA to newborn

Duration: active = long-lasting (years/lifetime); passive = short-lived (weeks to months)

✅ Active: own antibodies, memory, slow, long-lasting; Passive: readymade, no memory, immediate, short

⚖️ Comparisons

Benign Tumour

Malignant Tumour (Cancer)

Localised; stays in original tissue

Invasive; spreads to other tissues

Does not metastasise

Metastasises via blood/lymph to distant sites

Slow growing; well differentiated

Rapidly dividing; poorly differentiated

Usually curable by surgery alone

Requires surgery + radiation + chemotherapy

🎯 Quick Activity

Think and Answer

Q: Interferons are produced in response to viral infection. How do they protect non-infected cells?

▶ Show Answer

A: Virus-infected cells secrete interferons (α, β, γ) — small proteins that bind to receptors on neighbouring non-infected cells and trigger production of antiviral proteins in those cells, making them resistant to viral infection. Interferons don't kill the virus directly — they put surrounding cells in an "antiviral state". They also activate NK cells and macrophages. Key property: interferons are species-specific but virus non-specific — one interferon protects against many viruses.

🌿 Chapter 9: Strategies for Enhancement in Food Production

Plant Breeding · Animal Husbandry · SCP · Tissue Culture · Hybridisation

📖 Core Theory

9.1 Plant Breeding and Green Revolution

Scientific plant breeding = produce improved varieties with higher yield, disease resistance, better nutrition. Steps: germplasm collection → evaluation → selection → hybridisation → evaluation → release as variety.

Green Revolution

Semi-dwarf wheat varieties (Norman Borlaug, Mexico) introduced to India in 1960s by M.S. Swaminathan. Semi-dwarf rice (IR-8, Philippines). Result: India's food production tripled by 1980s. Key gene: Norin10 (dwarf gene in wheat).

Biofortification

Breeding for improved nutritional content: Atlas 66 wheat (high protein+lysine), Maize hybrids (lysine+tryptophan), Golden Rice (β-carotene via psy + crt1 genes), IARI spinach, bitter gourd, carrot (iron, vitamins).

Tissue Culture / Micropropagation

Totipotency: each plant cell can regenerate whole plant. Explant (shoot tip/axillary bud/root tip) → callus (unorganised mass on agar) → shoot/root initiation → plantlet → field transfer. Meristem culture: virus-free plants. Somaclonal variation possible.

Somatic Hybridisation

Protoplasts (cell walls removed by cellulase + pectinase) of two species fused by PEG or electrofusion → somatic hybrid. Example: Pomato (potato + tomato). Overcomes sexual incompatibility barriers.

9.2 Animal Husbandry and SCP

Animal Husbandry

Scientific management of farm animals. Inbreeding (same breed, 4-6 gen): fixes desirable traits, increases homozygosity; exposes recessive defects. Outbreeding (different breeds/species): hybrid vigour (heterosis) — greater milk yield, disease resistance.

Single Cell Protein (SCP)

Microbial biomass as protein source. Spirulina (60-70% protein; on sewage, waste water), Methylophilus methylotrophus (on methanol; ICI UK). Reduces land/water needed vs conventional protein. Addresses protein hunger.

🔑 Plant Breeding for Resistance

Disease resistance: Himgiri wheat (rust-resistant); TN1, Pankaj rice (bacterial blight, brown planthopper resistant)

MBER (Multiple Biotic and Abiotic Stress Resistance) varieties developed by IARI

Mutation breeding: induce mutations by X-rays, UV, mutagens → screen for desirable traits

Polyploidy induction (colchicine blocks spindle → allopolyploids); triticale (wheat × rye — first man-made crop)

✏️ Worked Examples

Resistance Breeding (Board 2023, 2M)

Q: What is disease resistance in plants? Name one disease-resistant variety of wheat and rice.

Disease resistance = ability of plant to withstand pathogen attack without major damage

Achieved by identifying resistance genes in wild varieties → hybridise with cultivated variety

Wheat: Himgiri — resistant to Puccinia rust (leaf, stripe rust) and hill bunt

Rice: Jaya and Ratna varieties — resistant to brown planthopper; Pankaj/TN1 — bacterial blight

Procedure: hybridise resistant (wild) × susceptible (high yield) → select resistant F2 → evaluate → release

✅ Wheat: Himgiri (rust resistant); Rice: Jaya (brown planthopper resistant)

Micropropagation (Board 2022, 3M)

Q: What is micropropagation? State two advantages over conventional vegetative propagation.

Micropropagation = rapid clonal multiplication of plants using tissue culture (in vitro)

Process: explant → surface sterilise → culture on MS/Murashige-Skoog medium → callus → shoot → root → plantlet

Advantage 1: Thousands of plants from single explant in weeks (vs months for cuttings)

Advantage 2: Virus-free plants if meristem tip used (meristems have no vascular tissue, virus cannot reach)

Advantage 3: Year-round production independent of season; space-efficient

✅ In vitro rapid clonal multiplication; advantages: faster, virus-free, season-independent

Hybrid Vigour (Board 2024, 2M)

Q: What is heterosis? Why does inbreeding depression occur?

Heterosis (hybrid vigour): F1 hybrids of unrelated varieties show greater performance than either parent

Seen in: yield, disease resistance, growth rate, fertility

Inbreeding depression: continuous inbreeding → increased homozygosity → recessive deleterious alleles expressed

Example: inbred maize lines show reduced vigour; F1 hybrid of two inbred lines = vigorous commercial crop

Commercial use: Sharbati Sonora wheat, maize hybrids, broiler chicken breeds

✅ Heterosis = F1 superior to parents; inbreeding depression = recessive alleles expressed on homozygosity

⚖️ Comparisons

Inbreeding

Outbreeding

Mating between closely related animals (same breed)

Mating between unrelated animals (different breeds/species)

Increases homozygosity; exposes recessive defects

Increases heterozygosity; hybrid vigour (heterosis)

Fixes desirable traits; useful for establishing pure lines

Greater milk yield, disease resistance, growth rate

🎯 Quick Activity

Think and Answer

Q: What is the advantage of producing crops with somatic hybridisation over conventional cross-pollination?

▶ Show Answer

A: Conventional hybridisation requires sexual compatibility (usually same genus/species). Somatic hybridisation fuses protoplasts of two different species — even genera — creating hybrids impossible sexually. Example: pomato (potato + tomato). However, somatic hybrids are often sterile and difficult to regenerate into commercially useful plants.

🌿 Chapter 10: Microbes in Human Welfare

Fermentation · Antibiotics · Sewage Treatment · Biogas · Biological Control · Biofertilisers

📖 Core Theory

10.1 Microbes in Food and Industry

Microbes serve crucial roles in food, medicine, agriculture and environment. Many industrial processes depend on microbial metabolism.

Fermented Foods

Bread: Saccharomyces cerevisiae (CO₂ leavens). Curd/yoghurt: Lactobacillus (lactic acid). Dosa/Idli: Leuconostoc mesenteroides (ferments rice+urad). Cheese: Penicillium roquefortii (Roquefort blue veins); Propionibacterium shermanii (Swiss cheese holes + flavour).

Industrial Products

Ethanol (Saccharomyces, anaerobic). Citric acid (Aspergillus niger). Acetic acid/vinegar (Acetobacter aceti). Lactic acid (Lactobacillus). Statins (Monascus purpureus — cholesterol-lowering). Cyclosporin A (Trichoderma — immunosuppressant).

Antibiotics

Penicillin: Alexander Fleming (1929), from Penicillium notatum. Production: Howard Florey and Ernst Chain (Nobel 1945 with Fleming). Streptomycin (Streptomyces griseus — Waksman). Erythromycin, tetracycline, chloramphenicol — all from Streptomyces (soil bacteria).

Sewage Treatment

Primary treatment: physical — screening, sedimentation (removes 30-35% BOD). Secondary treatment: biological — aeration tanks (flocs of bacteria+fungi → decompose organics); effluent → settling tank → sludge (to anaerobic digestors → biogas). Tertiary: chemical/physical (removes N, P, pathogens).

10.2 Biogas, Biofertilisers and Biocontrol

Biogas

Methanogens (Methanobacterium) anaerobically digest cattle dung/organic waste → CH₄ (60-70%) + CO₂ + H₂S. KVIC (floating drum) and Janata (fixed dome) designs. Slurry = excellent fertiliser. Used for cooking+lighting in rural India.

Biofertilisers

Rhizobium (N-fixation in legume nodules — symbiotic); Azospirillum, Azotobacter (free-living N-fixation); Azolla (pteridophyte with Anabaena cyanobacteria — N-fixation, used in paddy); mycorrhiza (Glomus — phosphate solubilisation+uptake).

Biological Control

Bacillus thuringiensis (Bt): produces Cry proteins → kill insect larvae. Nucleopolyhedrovirus (NPV): baculovirus specific to insects, harmless to others — narrow spectrum, eco-friendly. Trichoderma: biocontrol agent against plant pathogens. Predators: Ladybird beetle (Coccinella) eats aphids.

🔑 BOD and Sewage

BOD = Biochemical Oxygen Demand = O₂ consumed by bacteria to decompose organics in 5 days at 20°C

High BOD = high pollution; treated sewage should have BOD <20 mg/L before discharge

Flocs in secondary treatment = masses of bacteria + fungal filaments forming mesh that traps organic matter

Anaerobic sludge digestors: bacteria digest sludge → biogas (CH₄+CO₂); methanogens are strict anaerobes

✏️ Worked Examples

Penicillin Discovery (Board 2023, 2M)

Q: How was penicillin discovered? Why is it significant?

Alexander Fleming (1929): Penicillium notatum mould contaminated Staphylococcus culture; bacteria around mould were killed

Fleming isolated and named the active substance "penicillin" but could not purify enough for clinical use

Ernst Chain and Howard Florey (1940-45): purified penicillin; proven clinical efficacy in WWII → Nobel Prize 1945

Significance: first antibiotic — revolutionised treatment of bacterial infections (pneumonia, septicaemia, STIs)

Penicillin inhibits bacterial transpeptidase → blocks cell wall synthesis → selective toxicity (humans have no cell wall)

✅ Fleming 1929, Penicillium notatum; Florey+Chain — production; inhibits bacterial cell wall synthesis

Sewage Treatment Steps (Board 2022, 5M)

Q: Describe the steps of sewage treatment with role of microorganisms.

Primary treatment (physical): sewage → bar screening (removes large objects) → grit chamber → settling tank (sedimentation removes 60% solids, 30-35% BOD) → primary sludge removed

Secondary treatment (biological): effluent pumped to aeration tanks with air and activated sludge (bacterial flocs)

Aerobic microorganisms (bacteria, fungi) in flocs decompose organic matter → BOD reduced significantly

Floc-containing effluent → secondary settling tank → sludge separated; treated water discharged into rivers

Primary + secondary sludge → anaerobic digestors → methanogens produce biogas (CH₄ + CO₂) → energy

✅ Primary (physical) → Secondary (biological, microbes decompose organics) → Sludge → Biogas

Biofertiliser Types (Board 2024, 3M)

Q: Distinguish free-living and symbiotic N-fixing biofertilisers. Give one example each.

Free-living N-fixers: live independently in soil; fix atmospheric N₂ without a plant host

Examples: Azotobacter (aerobic, soil), Azospirillum (rhizosphere of non-legumes — rice, wheat), Anabaena (cyanobacterium in Azolla fronds)

Symbiotic N-fixers: live in plant root tissue; fix N₂ mutuistically with host plant

Examples: Rhizobium (legume root nodules — pea, bean, soybean), Frankia (alder trees), Anabaena in Azolla

Rhizobium: infects root hair → forms nodule → nitrogenase (+leghaemoglobin) fixes N₂; plant provides carbohydrates

✅ Free-living: Azotobacter, Azospirillum; Symbiotic: Rhizobium (legume nodules) — both fix N₂ but differently

⚖️ Comparisons

Biocontrol Agents

Chemical Pesticides

Species-specific; minimal non-target impact

Broad spectrum; kill beneficial insects too

Biodegradable; no soil/water residue

Persist in environment; bioaccumulate (DDT)

Slower acting; expensive to produce

Fast-acting; cheap; widely available

Examples: Bt toxin (Bacillus thuringiensis), NPV, Trichoderma

Examples: DDT, BHC, organophosphates

🎯 Quick Activity

Think and Answer

Q: How does Rhizobium fix nitrogen? What makes the legume-Rhizobium association unique?

▶ Show Answer

A: Rhizobium infects root hairs of legumes → stimulates nodule formation. Inside nodules, nitrogenase (requires anaerobic conditions — provided by leghaemoglobin which scavenges O₂) converts N₂ → NH₃ → amino acids. Plant provides carbohydrates; Rhizobium provides fixed nitrogen — mutualism. Leghaemoglobin (pinkish — like haemoglobin) is unique to legume nodules; serves as O₂ buffer making an anaerobic micro-environment for nitrogenase.

🌿 Chapter 11: Biotechnology — Principles and Processes

rDNA · Restriction Enzymes · Vectors · PCR · Gel Electrophoresis · Transformation

📖 Core Theory

11.1 Tools of Genetic Engineering

Genetic engineering: deliberate manipulation of DNA to alter the phenotype of an organism using recombinant DNA technology. Three core requirements: (1) restriction enzymes to cut DNA, (2) vector to carry DNA, (3) host organism to express foreign gene.

Restriction Endonucleases

Molecular scissors — cut DNA at specific palindromic sequences. EcoRI: G↓AATTC (recognises 6-bp palindrome); creates 4-base sticky ends. BamHI: G↓GATCC; HindIII: A↓AGCTT. Same enzyme cuts vector + insert → complementary sticky ends → ligation by DNA ligase (molecular glue).

Cloning Vectors

Vehicle to carry insert into host cell. Plasmids (pBR322, pUC19): small circular self-replicating DNA; must have ori (origin of replication), selectable marker (ampR, tetR), MCS (multiple cloning site). Bacteriophage (λ, M13), cosmids, BAC/YAC for large inserts (chromosomes).

PCR (Polymerase Chain Reaction)

Amplify specific DNA in vitro. Kary Mullis (Nobel 1993). 3 steps per cycle: (1) Denaturation 94°C (strands separate), (2) Annealing ~55°C (primers bind), (3) Extension 72°C (Taq polymerase extends 5′→3′). After n cycles: 2ⁿ copies. Taq from Thermus aquaticus — heat-stable.

Gel Electrophoresis

Separate DNA/RNA/protein by size. Agarose gel; negative charge → DNA migrates toward anode. Smaller fragments travel farther. EtBr stain — orange bands under UV. Used to verify restriction digest fragment sizes. Southern blotting (DNA), Northern (RNA), Western (protein).

11.2 Gene Transfer and Selectable Markers

Transformation

Introduction of foreign DNA into host cell. Chemical method: CaCl₂ treatment makes bacteria competent (porous). Electroporation: electric pulse opens pores. Biolistics (gene gun): DNA-coated gold/tungsten particles fired into plant/animal cells. Microinjection: DNA injected into animal cell nucleus.

pBR322 Selectable Markers

ampR (ampicillin resistance) and tetR (tetracycline resistance). Recombinant selection: insert goes into tetR gene (insertional inactivation) → recombinants: ampR but tetR-sensitive. Non-recombinants: ampR AND tetR-resistant. Replica plating on both antibiotics identifies recombinants.

🔑 Key Definitions and Calculations

Palindromic sequence: reads same on both strands 5′→3′; e.g. 5′-GAATTC-3′ / 3′-CTTAAG-5′

Sticky ends: short single-stranded overhangs after restriction cut; allow annealing with complementary sticky ends

PCR product after 30 cycles: 2³⁰ ≈ 10⁹ copies of target sequence

Competent cells: cells able to take up foreign DNA (treated with CaCl₂ or by electroporation)

Ti plasmid (Agrobacterium tumefaciens): natural vector for plant transformation; T-DNA integrates into plant genome

✏️ Worked Examples

PCR Steps (Board 2023, 5M)

Q: Explain the steps of PCR with the role of Taq polymerase.

PCR amplifies a specific DNA region using two synthetic oligonucleotide primers flanking the target

Step 1 — Denaturation (94°C, 30s): heat breaks H-bonds; double-stranded DNA separates into two single strands

Step 2 — Annealing (~55°C, 30s): primers (20-25 nucleotides) bind to complementary sequences on each strand

Step 3 — Extension (72°C, 1-2 min): Taq DNA polymerase adds nucleotides 5′→3′ from each primer

Taq polymerase from Thermus aquaticus (hot spring) — heat-stable; survives 94°C denaturation step; optimum at 72°C

✅ Denature (94°C) → Anneal (~55°C) → Extend with Taq (72°C); 2ⁿ copies after n cycles

Restriction Enzymes (Board 2022, 3M)

Q: What are restriction endonucleases? Why are they called "molecular scissors"?

Restriction endonucleases are bacterial enzymes that cut DNA at specific short sequences (restriction sites)

Named by bacterial source: EcoRI = E. coli Ry13, first restriction enzyme identified; BamHI = Bacillus amyloliquefaciens

They recognise palindromic sequences (4-8 bp) and cut at specific positions within or near them

Called "molecular scissors" because they cut DNA precisely — like scissors cut paper at a specific mark

In bacteria: restriction enzymes protect against bacteriophage infection (cut foreign DNA); own DNA is methylated (protected)

✅ Bacterial enzymes cutting DNA at palindromic recognition sequences; natural function = defense against phages

Recombinant Selection (Board 2024, 3M)

Q: How are recombinant bacteria identified using pBR322 and insertional inactivation?

pBR322 has two markers: ampR (ampicillin resistance) and tetR (tetracycline resistance)

Foreign DNA (insert) cloned into tetR gene (via BamHI or SalI site) → tetR gene disrupted

After transformation: plate all bacteria on ampicillin medium → only transformed cells survive (have plasmid)

Replica plate onto tetracycline medium: non-recombinant (intact tetR) → grow; recombinant (disrupted tetR) → do NOT grow

Colonies that grow on amp but NOT on tet = recombinants (have insert); pick and culture

✅ Insert disrupts tetR → recombinants are ampR but tetS (grow on amp, die on tet) → identified by replica plating

⚖️ Comparisons

Sticky Ends

Blunt Ends

Short single-stranded overhangs after cut

No overhang; both strands cut at same position

More efficient ligation; directional cloning easier

Any two blunt ends can ligate; less efficient

EcoRI, BamHI, HindIII produce sticky ends

HaeIII, SmaI produce blunt ends

🎯 Quick Activity

Think and Answer

Q: pBR322 was one of the first cloning vectors. What does "selectable marker" mean, and why is it essential?

▶ Show Answer

A: A selectable marker is a gene (usually antibiotic resistance — ampR and tetR in pBR322) that allows selection of transformed bacteria from non-transformed ones. After transformation, plate all bacteria on ampicillin medium → only those with plasmid survive (non-transformed die). To identify recombinants (plasmid WITH insert): insert goes into tetR gene (insertional inactivation) → recombinant colonies cannot grow on tetracycline medium but grow on ampicillin. This identifies recombinants without costly DNA analysis of every colony.

🌿 Chapter 12: Biotechnology and Its Applications

GM Crops · Bt Cotton · Golden Rice · Transgenic Animals · Insulin · Gene Therapy · Biopiracy

📖 Core Theory

12.1 GM Organisms in Agriculture

Genetically Modified (GM) organisms contain an alien gene introduced by rDNA technology. Applications in agriculture: pest resistance, herbicide tolerance, improved nutrition, stress tolerance, delayed ripening.

Bt Cotton (Bollgard)

Cry1Ac gene from Bacillus thuringiensis inserted into cotton. Cry proteins (crystal proteins) = inactive protoxin → activated in alkaline insect gut (pH 10) → forms pores in gut epithelial cells → insect dies. Specific to Lepidoptera (cotton bollworm Helicoverpa). Not harmful to humans (acid stomach, no receptor).

Golden Rice

β-carotene (provitamin A) produced in rice endosperm (normally white/β-carotene absent). Genes introduced: psy (phytoene synthase from Narcissus daffodil) + crtI (lycopene desaturase from Erwinia uredovora). Yellow colour from β-carotene. Addresses vitamin A deficiency (leading cause of childhood blindness in developing countries).

Recombinant Insulin (Humulin)

Human insulin gene (A-chain + B-chain separately) cloned into E. coli (Eli Lilly, 1982). Proinsulin produced → C-peptide removed → A + B chains joined → active insulin. Replaced pig/cattle insulin (allergy, BSE risk). First approved GM pharmaceutical product.

Transgenic Animals

Rosie (transgenic cow, 1997): human alpha-lactalbumin in milk — more nutritious for human infants. Transgenic sheep: human protein alpha-1-antitrypsin in milk (for emphysema). Gene pharming (molecular farming): use transgenic animals to produce therapeutic proteins.

12.2 Gene Therapy and Bioethics

Gene Therapy

Introduction of functional gene into patient's cells to correct genetic defect. ADA (adenosine deaminase) deficiency — first gene therapy case (1990, Ashanti DeSilva, USA). Retroviral vectors. Somatic gene therapy (body cells — not heritable) vs germline (reproductive cells — heritable; major ethical concerns).

Biopiracy

Use of bioresources (plants, microbes, traditional knowledge) by multinational companies without proper compensation to originating community/country. Examples: Turmeric (wound healing patent granted to University of Mississippi → challenged and revoked by India); Basmati (US company patented certain varieties). Addressed by CBD (Convention on Biological Diversity) and TRIPS.

🔑 Applications Summary

Bt toxin: different Cry proteins for different pests — Cry1 (Lepidoptera), Cry2 (Lepidoptera+Diptera), Cry3 (Coleoptera)

RNAi (RNA interference): dsRNA → silences specific mRNA → prevents nematode infection of tobacco roots

Flavr Savr tomato: antisense RNA blocks polygalacturonase → delayed ripening; first GM food crop (1994, USA)

ADA gene therapy: lymphocytes extracted → retroviral vector with ADA gene → transformed cells reinfused; requires repeat treatments

Ethical issues: GM crops → ecological impact (gene escape to wild relatives), allergenicity, resistance development

✏️ Worked Examples

Bt Toxin Mechanism (Board 2023, 3M)

Q: How does Bt toxin kill insects but not harm other organisms?

B. thuringiensis produces Cry proteins as inactive crystalline protoxin (inclusion bodies)

When insect larva ingests plant material: protoxin reaches alkaline gut (pH 10+)

Alkaline pH activates protoxin; activated toxin binds to specific receptor proteins on gut epithelial cells

Forms pores in gut membrane → massive ion imbalance → cell lysis → insect dies from gut damage and septicaemia

NOT harmful to humans/mammals: stomach is acidic (pH 2) → protoxin NOT activated; no specific receptor present

✅ Alkaline insect gut activates protoxin → pores in gut cells → death; human acidic stomach = protoxin stays inactive

Recombinant Insulin (Board 2022, 5M)

Q: How is recombinant human insulin produced? What are its advantages over animal insulin?

Human insulin gene cloned: A-chain and B-chain DNA prepared separately (synthetic or isolated)

Each gene inserted into plasmid with β-galactosidase (lacZ) promoter → transformed into E. coli

E. coli produces fusion protein (β-gal + insulin chain) → CN-Br treatment cleaves β-gal → insulin A-chain and B-chain obtained

A and B chains mixed under oxidising conditions → disulphide bonds form → active human insulin

Advantages: (1) No allergy risk (human sequence, not porcine/bovine), (2) No BSE/prion risk, (3) Unlimited supply, (4) Consistent purity

✅ A+B chains in E. coli separately → join in vitro → human insulin; advantages: no allergy, no animal disease risk

Biopiracy (Board 2024, 2M)

Q: What is biopiracy? Give one example from India.

Biopiracy = use of bioresources or traditional knowledge without permission or compensation to originating communities

Example 1: Turmeric (Curcuma longa) — US patent granted (1995) for wound-healing use; India challenged, showing this was traditional Indian knowledge; patent revoked (1997)

Example 2: Basmati rice — US company RiceTec patented certain Basmati-like varieties; India challenged genetic infringement

India's response: Traditional Knowledge Digital Library (TKDL) — documents traditional knowledge in patent-searchable format

International response: Convention on Biological Diversity (CBD) + Nagoya Protocol — require benefit sharing with originating countries

✅ Biopiracy = bioresource use without consent/compensation; Turmeric and Basmati are India's landmark cases

⚖️ Comparisons

Somatic Gene Therapy

Germline Gene Therapy

Target: body (somatic) cells — not passed to offspring

Target: fertilised egg/early embryo — change passed to all generations

Ethically accepted (corrects disease in individual)

Ethically controversial (permanent change in gene pool)

ADA deficiency gene therapy (1990) is an example

No approved human germline therapy; CRISPR babies (China 2018) — controversial

🎯 Quick Activity

Think and Answer

Q: What is ADA deficiency? How was it treated by gene therapy? What were the limitations?

▶ Show Answer

A: ADA (adenosine deaminase) deficiency: genetic disorder where lymphocytes cannot break down adenosine → toxic to B and T cells → severe combined immunodeficiency (SCID). Gene therapy (1990): ADA gene introduced via retroviral vector into patient's lymphocytes (extracted, transformed, reinfused). Success: immune function restored. Limitations: (1) Lymphocytes are non-dividing — gene lost over time, repeat treatments needed; (2) Bone marrow stem cells would be better (permanent cure); (3) Patient still needed PEG-ADA enzyme replacement as backup.

🌿 Chapter 13: Organisms and Populations

Habitat · Population Attributes · Growth Models · Interspecific Interactions

📖 Core Theory

13.1 Ecology — Levels and Adaptations

Ecology = scientific study of the relationships of organisms with each other and their environment. Levels: organism → population → community → ecosystem → biome → biosphere.

Adaptations

Morphological (cactus spines, camel hump), Physiological (kangaroo rat — metabolic water, no need to drink; desert animals concentrate urine), Behavioural (migration, hibernation, aestivation).

Population Attributes

Birth rate (natality), Death rate (mortality), Sex ratio, Age pyramid (expanding/stable/declining), Density. Population size (N) changes by: N = N₀ + (B−D) + (I−E) where B=births, D=deaths, I=immigration, E=emigration.

Logistic Growth

Most populations in nature show S-shaped (sigmoid) growth. dN/dt = rN[(K−N)/K]. K = carrying capacity. At N=K, growth rate=0. Lag phase → exponential → deceleration → plateau at K.

Interspecific Interactions

Mutualism (+/+): lichens (fungus+alga), mycorrhiza; Commensalism (+/0): orchid on tree; Parasitism (+/−): roundworm in intestine; Predation (+/−): leopard-deer; Competition (−/−): Gause's competitive exclusion; Amensalism (0/−): Penicillium secretes penicillin → kills bacteria.

✏️ Worked Examples

Logistic Growth (Board 2023, 3M)

Q: Write the equation for logistic growth. Explain each term and draw the S-shaped curve.

dN/dt = rN[(K−N)/K]

N = current population size; t = time; r = intrinsic rate of natural increase (birth rate − death rate)

K = carrying capacity (maximum population that habitat can support)

(K−N)/K = unused portion of carrying capacity; → 0 as N approaches K

When N is small: (K−N)/K ≈ 1 → growth is nearly exponential (J-shaped)

As N → K: growth slows → S-shaped sigmoid curve; at N=K, dN/dt = 0 (population stable)

✅ dN/dt = rN[(K−N)/K]; S-curve: lag → log → deceleration → plateau at K

Gause's Principle (Board 2022, 2M)

Q: State competitive exclusion principle. Give an example.

Gause's competitive exclusion principle: two species competing for identical resources CANNOT coexist indefinitely

The competitively superior species will eventually exclude the other

Example: Paramecium aurelia and P. caudatum when cultured together in same medium → P. aurelia (more efficient) wins; P. caudatum eliminated

Coexistence possible only through niche differentiation (different resources, times or spaces used)

MacArthur's warblers: 5 species coexist in spruce tree by using different parts of canopy

✅ Two species with identical niches cannot coexist; P. aurelia excludes P. caudatum

🎯 Quick Activity

Think and Answer

Q: What is the advantage of lichens being "composite organisms" (algae + fungi)?

▶ Show Answer

A: The fungal partner provides: physical structure, anchorage, moisture retention, mineral absorption from substrate. The algal (or cyanobacterial) partner provides: photosynthetic energy (carbohydrates). This mutualism allows lichens to colonise bare rocks, extreme temperatures and deserts where neither organism could survive alone. Lichens are pioneer species — their organic acids break down rock, beginning soil formation (ecological succession).

🌿 Chapter 14: Ecosystem

Structure · Energy Flow · Nutrient Cycling · Productivity · Succession

📖 Core Theory

14.1 Ecosystem Structure

An ecosystem is a functional unit where biotic and abiotic components interact through nutrient cycling and energy flow. Components: producers, consumers, decomposers + abiotic factors.

Energy Flow (10% Law)

Only ~10% of energy is transferred from one trophic level to next (Lindemann 1942). Rest lost as heat (respiration, excretion). 3-4 trophic levels maximum in most ecosystems — beyond that, too little energy remains.

Productivity

GPP (Gross Primary Productivity) = total photosynthesis. NPP = GPP − respiration loss. Net Primary Production available to consumers. Tropical rain forests and estuaries = highest NPP; deserts and open ocean = lowest.

Nutrient Cycling

Gaseous cycles (C, N, O): reservoir = atmosphere/hydrosphere. Sedimentary cycles (P, S): reservoir = earth crust. Decomposers (bacteria, fungi) essential — release minerals from organic matter.

Ecological Succession

Directional change in community over time. Pioneer community → seral stages → climax community (stable). Primary succession: starts on bare rock/sand (no soil). Secondary succession: starts after disturbance (fire, flood) — faster. Hydrarch (wet → dry); Xerarch (dry → wet).

✏️ Worked Examples

Energy Pyramid (Board 2023, 3M)

Q: Why is an energy pyramid always upright? Can it ever be inverted?

Energy pyramids represent energy at each trophic level; always taper toward apex (upright)

Energy lost (~90%) at each trophic level as heat → higher level always has less energy

Energy CANNOT flow backwards — entropy (2nd law of thermodynamics)

Energy pyramids are ALWAYS upright (never inverted)

Biomass pyramids can be inverted (phytoplankton biomass < zooplankton at any instant in aquatic systems)

Number pyramids can be inverted (one oak tree supports thousands of insects)

✅ Energy pyramids always upright (energy lost as heat at each level; entropy); biomass pyramids can invert

Carbon Cycle (Board 2022, 3M)

Q: Describe the main processes in the carbon cycle.

Carbon source: CO₂ in atmosphere/hydrosphere

Fixation (removal): photosynthesis (autotrophs fix CO₂ → organic matter)

Return: respiration (all organisms → CO₂); decomposition (decomposers → CO₂, CH₄)

Combustion of fossil fuels → CO₂ (major human impact, greenhouse effect)

Oceans: dissolve CO₂ → carbonate/bicarbonate; marine organisms → shells (CaCO₃) → limestone

Geological return: volcanic activity releases CO₂ from carbonate rocks over millions of years

✅ Photosynthesis = C fixation; respiration/decomposition/combustion = C release; oceans = major C reservoir

🎯 Quick Activity

Think and Answer

Q: Decomposers are sometimes called "reducers." What would happen to an ecosystem if all decomposers suddenly disappeared?

▶ Show Answer

A: Decomposers break down dead organic matter → release inorganic minerals (N, P, K, Ca) back into soil and water. Without decomposers: (1) Dead organic matter accumulates indefinitely; (2) Nutrients locked in organic form → not available to producers; (3) Mineral nutrient pools depleted → plant growth stops → ecosystem collapses. Decomposers are as essential as producers — ecosystems cannot function without nutrient cycling.

🌿 Chapter 15: Biodiversity and Conservation

Biodiversity Types · Threats · Conservation · Hotspots · IUCN Categories

📖 Core Theory

15.1 Types and Patterns of Biodiversity

Biodiversity (E.O. Wilson popularised term): variety of life at genetic, species and ecosystem levels. Current estimates: 1.5 million described species; true number 5-50 million.

Genetic Diversity

Variation in DNA among individuals of same species. High genetic diversity = better adaptability. Sahyadri range rice = 50,000+ varieties; India = >1000 mango varieties. Allelic diversity.

Species Diversity

Variety of species in an area. Measured by species richness (number) and species evenness (relative abundance). Tropics (equatorial) more species-rich than temperate/polar (latitudinal gradient in biodiversity).

Ecosystem Diversity

Variety of ecosystems in region. India: deserts, rainforests, mangroves, coral reefs, wetlands, grasslands — all within one country.

Biodiversity Hotspots

Norman Myers (1988). Regions with exceptionally high species richness AND high endemism AND severe threats. 34 hotspots globally, cover <2.5% Earth land but hold >50% terrestrial species. India has 4: Western Ghats & Sri Lanka, Himalaya, Indo-Burma, Sundaland.

🔑 Threats (HIPPO) and Conservation

H = Habitat loss/fragmentation (biggest threat); I = Invasive alien species; P = Pollution; P = Population explosion (human); O = Overexploitation

In-situ conservation: protect in natural habitat — biosphere reserves, national parks, wildlife sanctuaries, sacred groves

Ex-situ conservation: protect outside habitat — zoos, botanical gardens, seed banks, cryopreservation, tissue culture

IUCN Red List: Extinct, Extinct in Wild, Critically Endangered, Endangered, Vulnerable, Near Threatened, Least Concern

India protected areas: 103 national parks, 544 wildlife sanctuaries, 18 biosphere reserves (10 in UNESCO MAB network)

✏️ Worked Examples

Hotspot Criteria (Board 2023, 3M)

Q: What criteria must a region meet to be declared a biodiversity hotspot?

Criterion 1: Must contain at least 1500 species of vascular plants as endemic species (endemism)

Criterion 2: Must have lost at least 70% of its original habitat (HIGH threat level, already mostly lost)

Both criteria must be met simultaneously

India's 4 hotspots: (1) Western Ghats + Sri Lanka (2) Himalaya (Eastern) (3) Indo-Burma (4) Sundaland (Nicobar Islands)

Hotspots cover only 2.4% of land but contain 60%+ of world's species — conservation priority areas

✅ 1500+ endemic vascular plant species AND 70%+ original habitat already destroyed = hotspot criteria

In-situ vs Ex-situ (Board 2022, 3M)

Q: Distinguish between in-situ and ex-situ conservation with two examples each.

In-situ (on-site): species conserved within natural habitat

Examples: (1) Jim Corbett National Park (tigers, elephants) (2) Kalakad-Mundanthurai Biosphere Reserve (Western Ghats species)

Ex-situ (off-site): species maintained outside natural habitat

Examples: (1) Kew Gardens, Royal Botanic Gardens (seed banks) (2) Crocodile bank, Chennai (crocodiles bred in captivity)

In-situ preferred — preserves ecological interactions; ex-situ = last resort for critically endangered species

✅ In-situ: national parks, sanctuaries (natural habitat); Ex-situ: zoos, seed banks (protected outside habitat)

🎯 Quick Activity

Think and Answer

Q: Alexander von Humboldt observed that within a region, species richness increases with area. Express this as an equation and interpret each term.

▶ Show Answer

A: Species-Area relationship (Arrhenius, 1921): S = C × Aᶻ, or log S = log C + z log A. S = species richness; A = area; C = Y-intercept (related to taxa density); z = slope (0.1-0.2 for small islands, 0.6-1.2 for continental areas). Larger area → more resources → more habitats → more species. This is why habitat fragmentation (converting large forests into isolated patches) causes species extinction even if TOTAL area is same — edges, isolation effects reduce effective area per patch.

🌿 Chapter 16: Environmental Issues

Pollution · Greenhouse Effect · Ozone · Deforestation · Solid Waste

📖 Core Theory

16.1 Air Pollution

Air pollution = addition of contaminants to atmosphere beyond threshold. Major pollutants: CO, SOₓ, NOₓ, particulates, hydrocarbons, lead, CFCs.

Greenhouse Effect and Global Warming

CO₂, CH₄, N₂O, CFCs, water vapour = greenhouse gases. Trap IR radiation from Earth's surface → warming. CO₂ increased 270 ppm (pre-industrial) → 420+ ppm now. Predicted effects: melting ice caps, sea level rise, extreme weather, coral bleaching, species shifts.

Ozone Layer Depletion

Stratospheric ozone (15-35 km altitude) absorbs UV-B and UV-C. CFCs (chlorofluorocarbons — Freons in refrigerants, aerosols) release Cl· radicals which destroy O₃ catalytically. Antarctic ozone hole — CFC accumulation in polar vortex. Montreal Protocol (1987) — phase-out of CFCs.

Water Pollution

BOD (Biochemical Oxygen Demand): O₂ consumed by bacteria to decompose organic waste in 5 days at 20°C. High BOD = more pollution. Eutrophication: nutrient enrichment → algal bloom → O₂ depletion → fish death. Biomagnification: DDT concentrates up food chain.

Solid Waste

MSW (municipal solid waste): domestic, commercial, construction waste. Categories: recyclable, biodegradable, hazardous, electronic waste (e-waste). Sanitary landfill, composting, recycling, incineration (energy recovery). E-waste: improper disposal → heavy metal leaching (Pb, Hg, Cd).

✏️ Worked Examples

Ozone Depletion (Board 2023, 3M)

Q: Explain how CFCs deplete the ozone layer.

CFCs (chlorofluorocarbons) used in refrigerators, AC, aerosols are chemically inert at surface

Rise to stratosphere where UV radiation cleaves C-Cl bond: CF₂Cl₂ → CF₂Cl· + Cl·

Cl· attacks ozone: Cl· + O₃ → ClO· + O₂

ClO· reacts with another O₃: ClO· + O₃ → Cl· + 2O₂

Cl· is regenerated → catalytic cycle; ONE Cl· can destroy 100,000 ozone molecules

Result: ozone hole (especially over Antarctica) → increased UV-B reaching Earth → skin cancer, cataracts, crop damage

✅ CFCs → Cl· radicals (UV) → catalytic O₃ destruction; each Cl· destroys 100,000 O₃ molecules

Biomagnification (Board 2022, 2M)

Q: Explain biomagnification of DDT in an aquatic food chain.

DDT (dichlorodiphenyltrichloroethane): persistent organic pollutant; fat-soluble, NOT excreted

Phytoplankton: 0.003 ppm DDT → zooplankton (0.04 ppm) → small fish (0.5 ppm) → larger fish (2 ppm) → fish-eating birds (25+ ppm)

Concentration increases at each trophic level (biomagnification, bioaccumulation)

Not diluted because DDT stored in fatty tissue; every time organism eats, DDT accumulates

Effects: osprey, bald eagle egg shell thinning (DDE inhibits Ca deposition) → reproductive failure → population decline

✅ DDT increases concentration up food chain: 0.003 ppm → 25+ ppm in fish-eating birds → reproductive failure

🎯 Quick Activity

Think and Answer

Q: Why is BOD a measure of water pollution? What does high BOD indicate?

▶ Show Answer

A: BOD (Biochemical Oxygen Demand) measures the amount of dissolved oxygen consumed by bacteria to decompose organic matter in a water sample over 5 days at 20°C. High BOD = high organic load (sewage, industrial effluent) = more bacteria consuming O₂. This depletes dissolved O₂ → aquatic life (fish, invertebrates) suffocate and die. Clean rivers: BOD <2 mg/L; polluted: >10 mg/L. BOD is the standard parameter for monitoring sewage treatment efficiency — treated sewage should have BOD <20 mg/L before discharge.

📜 PYQ Bank

Previous year board questions — Biology XII

2023 | 3 marks | Flowering Plants

Draw a labelled diagram of a mature embryo sac and mention the number of cells and nuclei present in it.

7 cells, 8 nuclei: 3 antipodals, 2 synergids, 1 egg cell, 1 central cell (with 2 polar nuclei). Draw diagram with labels. Egg apparatus at micropylar end, antipodals at chalazal end.

2024 | 5 marks | Molecular Biology

Explain the process of transcription in prokaryotes. How does it differ from eukaryotes?

Prokaryotes: single RNA polymerase, no nuclear membrane, translation can begin while transcription is ongoing, no post-transcriptional processing. Eukaryotes: 3 types of RNA pol, mRNA processing (capping, tailing, splicing), occurs in nucleus, translated in cytoplasm.

2023 | 5 marks | Biotechnology

Explain the process of preparation of recombinant DNA with the help of a suitable example. Name the enzyme used and explain its action.

Restriction enzyme (e.g., EcoRI) cuts DNA at palindromic site 5'GAATTC3', creating sticky ends. Foreign DNA cut with same enzyme, mixed with vector DNA, DNA ligase joins them → recombinant DNA. Sticky ends enable precise joining.

2023 | 3 marks | Ecosystem

What is biomagnification? Explain how DDT causes it with an example from an aquatic ecosystem.

Biomagnification: progressive increase in concentration of a substance through food chain. DDT: water (0.003 ppb) → zooplankton (0.04 ppb) → small fish (0.5 ppb) → large fish (2 ppb) → birds (25 ppb). Not biodegradable, stored in fat tissue, accumulates.

❌ Mistake Bank

Common errors in Biology XII exams

Confusing embryo sac cells

Embryo sac = 7 cells, 8 nuclei. Central cell has 2 polar nuclei. Synergids assist in fertilisation. Antipodals have no direct role — just be clear on each function.

Interchanging codons and anticodons

mRNA has codons (5'→3'). tRNA has anticodons (antiparallel, reads 3'→5' of mRNA). Start codon: AUG on mRNA; corresponding anticodon on tRNA: UAC.

Lamarckism vs Darwinism

Lamarck: use and disuse of organs leads to inherited changes (giraffe neck — wrong). Darwin: variations already exist; natural selection acts on them. Don't mix these up.

In-situ vs Ex-situ confusion

In-situ = conservation at natural habitat (national park, biosphere reserve). Ex-situ = conservation outside natural habitat (zoo, botanical garden, seed bank).

👁️ Examiner's Eye

What examiners check in Biology XII answers

Always label diagrams properly

In Biology, diagrams without labels or with missing labels lose 50% marks. Key diagrams: LS of flower, embryo sac, female reproductive system, pBR322 plasmid, lac operon.

Scientific names must be correct and italicised

Plasmodium falciparum (not P. falciparum alone in first mention), Bacillus thuringiensis, Rhizobium, Wuchereria bancrofti — underline when hand-writing. Missing genus letter costs marks.

Distinguish similar terms precisely

GPP vs NPP, Primary vs Secondary productivity, In-situ vs Ex-situ, Active vs Passive immunity, Natality vs Fecundity — examiners check if you can use the right term in context.

🔥 High-Probability Questions

Most likely to appear in your board exam

⭐ 98% probability

Embryo sac diagram (3–5 marks): Draw and label. Always include cell names, number of cells (7) and nuclei (8). Appears in almost every year's paper.

⭐ 95% probability

Molecular basis — central dogma and genetic code (5 marks): Transcription, translation, codons, anticodons. High-priority topic every year.

⭐ 90% probability

rDNA technology steps (5 marks): Restriction enzymes, ligation, transformation, selection. Often includes a diagram of cloning vector.

⭐ 85% probability

Ecosystem energy flow and 10% law (3–5 marks): Pyramid of energy, food chain, GPP/NPP calculations.

⭐ 80% probability

Hardy-Weinberg principle (3 marks): State principle, equation, list 5 factors disturbing equilibrium. Appears regularly in section B.

📝 Practice Papers

Full-length 70-mark board-pattern papers

BIOLOGY XII — PRACTICE PAPER 1

Time: 3 Hours | Max Marks: 70 | As per CBSE 2024-25 pattern

SECTION A — MCQ (16×1=16)

Q1. Which cell of embryo sac in flowering plants directly participates in fertilisation? (a) Synergid (b) Antipodal (c) Egg cell (d) Central cell [Ans: c]

Q2. Which enzyme is used to join two DNA molecules? (a) Restriction endonuclease (b) DNA polymerase (c) DNA ligase (d) RNA polymerase [Ans: c]

Q3. According to Hardy-Weinberg principle, allele frequencies in a population remain constant if: (a) Mutations occur (b) Large population with random mating (c) Migration occurs (d) Natural selection acts [Ans: b]

SECTION B — Short (5×2=10)

Q4. Differentiate between in-situ and ex-situ conservation with one example each. [In-situ: National Park (Jim Corbett); Ex-situ: Zoological garden (Delhi Zoo)]

Q5. What is biomagnification? Give one example. [Progressive increase in non-biodegradable toxic substance along food chain; DDT in aquatic ecosystem]

SECTION C — Short II (7×3=21)

Q6. Explain double fertilisation. What are the products formed? [Two male gametes: one + egg = zygote (2n); one + 2 polar nuclei = PEN (3n) = triploid endosperm]

Q7. Draw a labelled diagram of pBR322 plasmid. Mention two features of an ideal cloning vector. [ampR, tetR genes, ori, MCS; features: small size, selectable marker, origin of replication]

SECTION D — Long (3×5=15)

Q8. Explain Lac operon with a suitable diagram. What happens in the presence and absence of lactose? [No lactose → repressor binds operator → genes OFF. Lactose (inducer) → repressor inactive → genes ON → β-galactosidase, permease, transacetylase produced]

Q9. Explain ecological succession starting from bare rock to forest. Name pioneer species and climax community. [Lichens → mosses → herbs → shrubs → trees; pioneer = lichens; climax = forest; primary succession]

BIOLOGY XII — PRACTICE PAPER 2

Time: 3 Hours | Max Marks: 70 | Focus: Genetics + Biotech

SECTION C (7×3=21)

Q1. A woman heterozygous for colour blindness marries a normal man. What is the probability of their sons being colour blind? Show cross. [Mother X^c X → 50% sons (X^c Y) colour blind; daughters all normal carriers or normal]

Q2. Describe the structure of a mature pollen grain. [Exine (sporopollenin — most resistant substance known, contains germ pores), intine (pectocellulose), vegetative cell + generative cell (2 male gametes)]

SECTION D (3×5=15)

Q3. Explain the steps involved in production of recombinant human insulin. Why was it needed? [Natural insulin from pigs/cattle — allergic to some patients. rDNA: A and B chains separately in E. coli plasmids → expressed → chains extracted → joined by disulphide bonds → Humulin]

BIOLOGY XII — PRACTICE PAPER 3

Time: 3 Hours | Max Marks: 70 | Full Syllabus

SECTION D (3×5=15)

Q1. Explain the role of microbes in sewage treatment. What is BOD? [Primary: screening, sedimentation; Secondary: aerobic microbes digest organic matter, reduces BOD; BOD = O₂ needed to oxidise all organic matter; lower BOD = cleaner water]

Q2. Describe Hardy-Weinberg equilibrium. List 5 factors that disturb it. [p+q=1, p²+2pq+q²=1; disturbed by: mutation, gene flow, genetic drift, non-random mating, natural selection → these factors cause evolution]

Q3. Explain human reproductive system in females including oogenesis. [Ovaries → oogonium → primary oocyte → secondary oocyte (ovulation day 14) → ovum after fertilisation; fallopian tube site of fertilisation; implantation in uterus after 7 days]

🎯 Exam Simulator

35-question timed mock — Biology XII

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