Botany वनस्पतिशास्त्र

Quick summary

Genetics is the study of heredity and variation. Gregor Johann Mendel discovered the basic laws of inheritance by working on the garden pea. This topic covers Mendel's experiments, monohybrid and dihybrid crosses, his laws, test cross and back cross, and deviations from Mendelian ratios such as incomplete dominance, codominance, epistasis and multiple alleles.

Key points

• Gregor Johann Mendel (1822–1884), the "father of genetics", worked on the garden pea Pisum sativum (1856–1863) and published his results in 1865/1866.
• His work was rediscovered in 1900 by Hugo de Vries, Carl Correns and Erich von Tschermak.
• The term genetics was coined by William Bateson (1905); the word gene by W. Johannsen (1909).
• Mendel studied seven pairs of contrasting characters, e.g. tall/dwarf stem, round/wrinkled seed, yellow/green seed, violet/white flower.
• Reasons for success: pea is self-pollinating, has clear contrasting characters, short life cycle; he used pure lines, studied one character at a time and used statistics.
• Monohybrid cross (tall × dwarf): F₁ all tall; F₂ phenotypic ratio 3 : 1, genotypic ratio 1 : 2 : 1.
• Dihybrid cross (round yellow × wrinkled green): F₂ phenotypic ratio 9 : 3 : 3 : 1.
• Law of dominance: in a heterozygote, one factor (dominant) expresses itself and the other (recessive) does not.
• Law of segregation (purity of gametes): the two factors of a pair separate during gamete formation; each gamete gets only one.
• Law of independent assortment: factors of different characters are distributed to gametes independently of each other.

Important terms

• Allele – alternative forms of a gene (e.g. T and t).
• Homozygous / heterozygous – having identical (TT, tt) / different (Tt) alleles.
• Genotype / phenotype – genetic make-up / visible character.
• Test cross – cross of F₁ with the homozygous recessive parent; monohybrid test cross ratio 1 : 1, dihybrid 1 : 1 : 1 : 1.
• Back cross – cross of F₁ with either parent.
• Incomplete dominance – F₁ shows an intermediate character; e.g. Mirabilis jalapa (4 o'clock plant) red × white → pink; F₂ ratio 1 : 2 : 1 (both phenotypic and genotypic).
• Codominance – both alleles fully expressed in the heterozygote; e.g. AB blood group, roan coat in cattle.
• Epistasis – one gene masks the effect of another non-allelic gene.
• Multiple alleles – more than two alleles of one gene in a population; e.g. Iᴬ, Iᴮ, i of ABO blood groups.

Formulas

• Number of gamete types = 2ⁿ (n = number of heterozygous gene pairs).
• Number of F₂ genotypes = 3ⁿ; F₂ phenotypes (complete dominance) = 2ⁿ; F₂ combinations in Punnett square = 4ⁿ.
• Example: for a dihybrid (n = 2): gametes 4, genotypes 9, phenotypes 4, combinations 16.

5 important questions

Q1. State the law of segregation.

The two alleles of a gene, which stay together in an individual, separate during gamete formation. Each gamete receives only one allele, so gametes are always pure.

Q2. What is a test cross? Give its significance.

A test cross is the cross between an individual of dominant phenotype and its homozygous recessive. A 1 : 1 result shows the individual is heterozygous; all dominant offspring show it is homozygous.

Q3. Explain a dihybrid cross with a Punnett square.

• Parents: RRYY (round yellow) × rryy (wrinkled green).
• F₁: RrYy – all round yellow.
• F₁ gametes: RY, Ry, rY, ry.
• F₂ (16 boxes): 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
• Proves law of independent assortment.

Q4. Explain incomplete dominance with an example.

• Neither allele is fully dominant.
• Mirabilis jalapa: RR (red) × rr (white) → Rr (pink).
• F₂: 1 red : 2 pink : 1 white.
• Phenotypic and genotypic ratios are the same.
• Shows that factors do not blend; they separate again in F₂.

Q5. Explain complementary gene interaction (9 : 7).

• Two dominant genes needed together to produce a character.
• Bateson and Punnett: two white-flowered sweet pea varieties (CCpp × ccPP) gave purple F₁ (CcPp).
• F₂: 9 purple (C_P_) : 7 white (C_pp, ccP_, ccpp).
• Reason: one gene makes a precursor, the other converts it into pigment.

Exam tip

Always draw Punnett squares neatly and write both phenotypic and genotypic ratios. Memorise the gene-interaction ratio table.

Quick summary

Not all inheritance follows simple Mendelian ratios. Some traits such as height, seed colour intensity and yield are controlled by many genes with small additive effects (polygenic or quantitative inheritance). Other traits are inherited through genes outside the nucleus, in chloroplasts and mitochondria (extranuclear or cytoplasmic inheritance). This topic explains both with classic examples.

Key points

• Qualitative traits show clear classes (tall/dwarf). Quantitative traits show continuous variation (height, weight, yield) and are measured, not counted.
• Polygenic inheritance: several genes (polygenes) each add a small, equal and cumulative (additive) effect; environment also affects the trait.
• Nilsson-Ehle (1909) – kernel colour in wheat: two gene pairs (R₁R₁R₂R₂ dark red × r₁r₁r₂r₂ white). F₁ is medium red; F₂ gives five colour classes in the ratio 1 : 4 : 6 : 4 : 1 (by number of dominant alleles 4, 3, 2, 1, 0).
• With three gene pairs (e.g. Nilsson-Ehle's other wheat crosses), F₂ gives 7 classes in the ratio 1 : 6 : 15 : 20 : 15 : 6 : 1.
• Rule: with n gene pairs, F₂ has 2n + 1 phenotypic classes, and the fraction like either parent is (1/4)ⁿ.
• Graph of a polygenic trait is a bell-shaped normal curve.
• Other examples: ear length in maize (East), human skin colour, plant height.
• Extranuclear inheritance: controlled by plasmagenes in chloroplast DNA (cpDNA) or mitochondrial DNA (mtDNA).
• Features: reciprocal crosses give different results; inheritance is mostly maternal (uniparental) because the egg gives almost all the cytoplasm; Mendelian ratios are not seen; traits are not linked to chromosomes.
• **Plastid inheritance in Mirabilis jalapa (four o'clock plant) – Correns (1909)**: branches can be green, white or variegated. Seeds from a flower on a white branch give only white seedlings, whatever the pollen; from a green branch only green; from a variegated branch green, white and variegated.
• Cytoplasmic male sterility (CMS): a mitochondrial gene makes pollen sterile; inherited through the female. Used in hybrid seed production of maize, jowar, bajra, rice, sunflower; fertility is restored by nuclear restorer (Rf) genes.
• Other examples: Chlamydomonas streptomycin resistance (Sager), poky mutant of Neurospora, "kappa" particles in Paramecium.

Important terms

• Polygenes – several genes with small additive effects on one trait.
• Continuous variation – variation with many intermediate grades.
• Plasmagene – gene located in the cytoplasm (organelle DNA).
• Maternal inheritance – inheritance through the female parent only.
• CMS – male sterility controlled by mitochondrial genes.

Formulas

• Phenotypic classes in F₂ = 2n + 1 (n = number of gene pairs).
• Proportion of F₂ like one parent = (1/4)ⁿ.
• Worked example: 2 gene pairs → classes = 2(2) + 1 = 5; proportion like the dark red parent = (1/4)² = 1/16.

5 important questions

Q1. What is quantitative inheritance?

Inheritance of traits that show continuous variation and are controlled by many genes, each adding a small cumulative effect, along with the environment. Example: kernel colour in wheat.

Q2. Why do reciprocal crosses differ in cytoplasmic inheritance?

Because the cytoplasm, with its plastid and mitochondrial genes, comes mostly from the egg. The female parent therefore decides the trait.

Q3. Explain kernel colour inheritance in wheat.

Nilsson-Ehle's cross; gene symbols; F₁; F₂ checkerboard or binomial expansion (a + b)⁴; ratio 1 : 4 : 6 : 4 : 1; normal curve; features of polygenic inheritance.

Q4. Describe plastid inheritance in Mirabilis jalapa.

Correns' experiment; three branch types; results in a table for each female branch; conclusion – maternal plastid inheritance; diagram.

Q5. Write a note on cytoplasmic male sterility and its use.

Origin in mitochondrial genome; A-line (male sterile), B-line (maintainer), R-line (restorer); hybrid seed production; example in maize (Texas cytoplasm).

Exam tip

Remember "2n + 1 classes" and the wheat ratio 1 : 4 : 6 : 4 : 1. For cytoplasmic inheritance, always mention that reciprocal crosses differ.

Quick summary

Chromosomes are thread-like structures in the nucleus that carry genes. Cytogenetics studies chromosomes and their link with heredity. This topic covers chromosome structure and types, special chromosomes, the chromosome theory of inheritance, linkage and crossing over, sex determination, and changes in chromosome structure and number.

Key points

• The term chromosome was given by W. Waldeyer (1888). Chromosome is गुणसूत्र in Marathi.
• A metaphase chromosome has two sister chromatids joined at the centromere (primary constriction). Some also have a secondary constriction (nucleolar organiser) and a satellite.
• Tips of chromosomes are telomeres; they protect the ends.
• Chromatin: euchromatin (lightly stained, active genes) and heterochromatin (darkly stained, mostly inactive).
• The DNA–histone unit is the nucleosome (about 200 base pairs of DNA around an octamer of histones).
• Types by centromere position: metacentric (middle; V-shape at anaphase), submetacentric (J/L-shape), acrocentric (near one end), telocentric (at the end).
• Special chromosomes: polytene chromosomes (salivary glands of Drosophila larvae; many DNA strands; show bands and puffs/Balbiani rings) and lampbrush chromosomes (oocytes of amphibians; lateral loops).
• Chromosome theory of inheritance: Sutton and Boveri (1902–1903) – genes lie on chromosomes, and chromosome behaviour in meiosis explains Mendel's laws.
• Linkage (studied in Drosophila by T. H. Morgan) – genes on the same chromosome tend to be inherited together.
• Crossing over – exchange of segments between non-sister chromatids of homologous chromosomes during pachytene of meiosis I; it forms recombinants. The visible cross points are chiasmata.

Important terms

• Karyotype – chromosome set of a species arranged by size and shape; its diagram is an idiogram.
• Deletion, duplication, inversion, translocation – structural changes in chromosomes.
• Euploidy – change in whole sets (monoploid, triploid, tetraploid).
• Aneuploidy – gain or loss of one or a few chromosomes (2n + 1 trisomy, 2n − 1 monosomy).
• Autopolyploid – multiple sets from the same species; allopolyploid – sets from different species (e.g. wheat, Raphanobrassica).
• Down syndrome – trisomy of chromosome 21 (47 chromosomes).
• Turner syndrome – 45, XO; Klinefelter syndrome – 47, XXY.

Formulas

• Recombination frequency (%) = (number of recombinant offspring ÷ total offspring) × 100.
• 1% recombination = 1 map unit (centimorgan, cM).
• Example: in a test cross of 1000 offspring, 80 are recombinants. RF = 80/1000 × 100 = 8%. The genes are 8 map units apart.
• The maximum recombination frequency between two genes is 50%.

5 important questions

Q1. What are polytene chromosomes?

Polytene chromosomes are giant chromosomes found in salivary gland cells of Drosophila larvae. They form by repeated DNA replication without cell division. They show dark bands and swellings called puffs (Balbiani rings) where genes are active.

Q2. In a test cross, 170 out of 1000 offspring are recombinants. Find the map distance.

RF = (170 ÷ 1000) × 100 = 17%. Map distance = 17 map units (17 cM).

Q3. Explain linkage and its types.

• Tendency of genes on the same chromosome to be inherited together.
• Complete linkage: no recombinants (male Drosophila).
• Incomplete linkage: some recombinants due to crossing over.
• Morgan's Drosophila experiments (body colour and wing size).
• Strength of linkage decreases as distance between genes increases.

Q4. Describe the mechanism of crossing over.

• Synapsis of homologues in zygotene; bivalent formation (synaptonemal complex).
• Pachytene: each bivalent is a tetrad; breakage and exchange between non-sister chromatids (enzyme recombinase).
• Diplotene: chiasmata visible.
• Terminalisation of chiasmata; separation at anaphase I.
• Significance: new combinations, variation, gene mapping.

Q5. Describe structural changes in chromosomes.

• Deletion: loss of a segment (cri-du-chat syndrome – part of chromosome 5).
• Duplication: segment repeated (Bar eye in Drosophila).
• Inversion: segment reversed (paracentric, pericentric).
• Translocation: segment moves to a non-homologous chromosome (reciprocal).
• Draw simple diagrams with letters ABCDEF.

Exam tip

Use lettered diagrams (ABC·DEF) for structural changes. Practise one map-distance numerical.

Quick summary

Molecular biology explains how genes work at the level of DNA, RNA and protein. The central dogma states that DNA makes RNA and RNA makes protein. This topic covers DNA structure, DNA replication, transcription, the genetic code, translation and gene regulation by the lac operon.

Key points

• Central dogma (Francis Crick, 1958): DNA → (transcription) → RNA → (translation) → protein. Reverse transcription (RNA → DNA) occurs in retroviruses.
• DNA double helix: Watson and Crick (1953), using X-ray data of Rosalind Franklin and Maurice Wilkins. Two antiparallel strands; sugar-phosphate backbone; bases inside.
• Base pairing: A = T (two hydrogen bonds), G ≡ C (three hydrogen bonds).
• One turn of the B-DNA helix has about 10 base pairs and is 3.4 nm long; distance between base pairs is 0.34 nm; diameter 2 nm.
• Chargaff's rule: A = T and G = C; total purines = total pyrimidines.
• Replication is semi-conservative – proved by Meselson and Stahl (1958) using ¹⁵N in E. coli.
• DNA polymerase adds nucleotides only in the 5′ → 3′ direction. The leading strand is made continuously; the lagging strand discontinuously as Okazaki fragments, joined by DNA ligase.
• Transcription: RNA polymerase copies the template (antisense) strand. In eukaryotes, the primary transcript (hnRNA) is processed: capping (5′), poly-A tail (3′) and splicing (introns removed, exons joined).
• Genetic code: triplet, non-overlapping, commaless, degenerate, nearly universal; 64 codons – 61 sense codons and 3 stop codons (UAA, UAG, UGA). AUG codes methionine and is the start codon.
• Translation on ribosomes: initiation, elongation, termination. tRNA brings amino acids; its anticodon pairs with the mRNA codon.

Important terms

• Replication fork – Y-shaped region where DNA strands separate.
• Helicase – unwinds DNA; primase – makes RNA primer; topoisomerase (gyrase) – relieves supercoiling.
• Promoter – DNA sequence where RNA polymerase binds.
• Intron / exon – non-coding / coding parts of a eukaryotic gene.
• Operon – group of genes under one promoter and operator (in prokaryotes).
• Wobble hypothesis (Crick) – the third base of a codon can pair loosely, so one tRNA reads more than one codon.

Formulas

• In double-stranded DNA: A + G = T + C, so (A + G)/(T + C) = 1.
• Example: if a DNA has 20% adenine, then T = 20%, A + T = 40%, G + C = 60%, so G = 30% and C = 30%.
• Length of DNA = number of base pairs × 0.34 nm. Example: 1000 bp → 340 nm.

5 important questions

Q1. A DNA sample has 30% guanine. Find the percentage of other bases.

G = C = 30%, so G + C = 60%. A + T = 40%, so A = 20% and T = 20%.

Q2. What are Okazaki fragments?

Okazaki fragments are short DNA pieces formed on the lagging strand during replication, because DNA polymerase works only 5′ → 3′. They are later joined by DNA ligase.

Q3. Describe the Meselson–Stahl experiment.

• E. coli grown many generations in ¹⁵NH₄Cl (heavy DNA).
• Shifted to ¹⁴N medium.
• After one generation: all DNA hybrid (intermediate density) in CsCl density gradient.
• After two generations: half hybrid, half light.
• Proves semi-conservative replication.

Q4. Explain the lac operon.

• Proposed by Jacob and Monod (1961) in E. coli; inducible operon.
• Parts: regulator gene (i, makes repressor), promoter, operator, structural genes z (β-galactosidase), y (permease), a (transacetylase).
• No lactose: repressor binds operator; genes off.
• Lactose present: allolactose (inducer) binds repressor; repressor leaves operator; RNA polymerase transcribes genes.
• Example of negative control; also positive control by CAP-cAMP.

Q5. Describe the process of translation.

• Activation: amino acid + tRNA (aminoacyl-tRNA synthetase, ATP).
• Initiation: small ribosomal subunit binds mRNA; initiator tRNA (Met) at AUG; large subunit joins.
• Elongation: next aminoacyl-tRNA at A site; peptide bond (peptidyl transferase); translocation.
• Termination: stop codon; release factor; polypeptide freed.

Exam tip

Learn key experiments with names and years (Watson–Crick 1953, Meselson–Stahl 1958, Jacob–Monod 1961). Draw the lac operon in both "off" and "on" states.

Quick summary

A mutation is a sudden, heritable change in the genetic material. Mutations are the raw material of evolution and of mutation breeding, but they can also cause harm. This topic covers types of gene (point) mutations, physical and chemical mutagens, how they act, and how cells repair damaged DNA.

Key points

• The term mutation was given by Hugo de Vries (1901) from his work on Oenothera lamarckiana. H. J. Muller (1927) first induced mutations with X-rays (in Drosophila); Stadler (1928) did so in barley and maize.
• Mutations may be spontaneous (natural, from replication errors) or induced (by mutagens). They may be somatic (not passed to offspring in animals; can be propagated vegetatively in plants – bud sports) or germinal.
• Base substitution: transition (purine → purine or pyrimidine → pyrimidine, e.g. A→G) or transversion (purine ↔ pyrimidine, e.g. A→T).
• Frameshift: insertion or deletion of bases shifts the reading frame.
• Gene (point) mutations change one or a few bases:
• Effects on protein: silent (same amino acid), missense (different amino acid), nonsense (stop codon; short protein). Frameshifts usually ruin the protein.
• Physical mutagens: ionising radiation (X-rays, gamma rays – break DNA strands) and non-ionising UV light – forms thymine dimers between adjacent thymines.
• Base analogues – 5-bromouracil (mimics thymine), 2-aminopurine.
• Alkylating agents – EMS (ethyl methanesulphonate), mustard gas.
• Deaminating agents – nitrous acid (cytosine → uracil, adenine → hypoxanthine).
• Intercalating agents – acridine dyes, ethidium bromide → frameshifts.
• Chemical mutagens:
• Photoreactivation: enzyme photolyase, activated by visible light, splits thymine dimers directly.
• Excision repair: damaged part is cut out by endonuclease, the gap is filled by DNA polymerase and sealed by ligase (nucleotide excision repair, base excision repair).
• Mismatch repair: corrects wrongly paired bases left after replication.
• SOS / recombination repair: emergency repair of serious damage; error-prone.
• DNA repair systems:
• Use in plants: mutation breeding – e.g. many crop varieties released in India developed with gamma rays.

Important terms

• Mutagen – agent that increases the rate of mutation.
• Transition / transversion – substitution of a base by the same type / the other type.
• Frameshift mutation – addition or loss of bases not in multiples of three.
• Thymine dimer – covalent link between two adjacent thymines caused by UV.
• Photoreactivation – light-dependent repair of thymine dimers.

Key facts

5 important questions

Q1. Distinguish between transition and transversion.

Transition is replacement of a purine by another purine or a pyrimidine by another pyrimidine (A ↔ G, C ↔ T). Transversion is replacement of a purine by a pyrimidine or vice versa (A ↔ T, G ↔ C).

Q2. How does UV light cause mutation?

UV light joins two adjacent thymine bases on the same strand to form a thymine dimer. This distorts DNA and blocks replication, causing errors if not repaired.

Q3. Classify mutations and describe point mutations with examples.

Spontaneous and induced, somatic and germinal, gene and chromosomal; substitution and frameshift; silent, missense and nonsense effects with codon examples.

Q4. Describe the action of chemical mutagens.

Base analogues, alkylating, deaminating and intercalating agents with one example and mechanism each.

Q5. Explain the mechanisms of DNA repair.

Photoreactivation, excision repair (steps), mismatch repair and SOS repair; significance for keeping the genome stable.

Exam tip

Make a mutagen table and learn one repair process in detail with a diagram – excision repair is the most often asked.

Quick summary

Plant breeding is the art and science of improving the genetic make-up of crop plants to get better varieties – higher yield, better quality, and resistance to diseases, pests and stress. This topic covers aims, methods (introduction, selection, hybridisation, mutation and polyploidy breeding), heterosis, and India's achievements such as the Green Revolution.

Key points

• Aims: higher yield, better quality (protein, oil, taste), disease and insect resistance, tolerance to drought, salinity and heat, early maturity, suitability for machines.
• Plant introduction – bringing plants from another region or country. Primary introduction: used directly; secondary: used after selection or crossing. In India, the National Bureau of Plant Genetic Resources (NBPGR), New Delhi, handles introduction and germplasm.
• Germplasm – the total collection of genes (varieties, wild relatives) of a crop.
• Selection – choosing superior plants from a mixed population. Types: mass selection (many plants with good features; seeds mixed), pure line selection (progeny of a single self-pollinated plant; W. Johannsen), clonal selection (in vegetatively propagated crops like potato, sugarcane).
• Hybridisation – crossing two genetically different plants to combine useful characters. Steps: choice of parents, emasculation (removing anthers from bisexual flowers before they shed pollen), bagging, artificial pollination, labelling, collection of seeds, and selection of desirable progeny over generations.
• Types of crosses: intravarietal, intervarietal, interspecific, intergeneric (e.g. Triticale = wheat × rye).
• Heterosis (hybrid vigour) – F₁ hybrid is superior to both parents (term by G. H. Shull). Used in hybrid maize, bajra, jowar, cotton.
• Mutation breeding – inducing mutations with radiation (X-rays, gamma rays) or chemicals (e.g. EMS, colchicine for polyploidy) and selecting useful mutants.
• Polyploidy breeding – doubling chromosomes with colchicine; gives larger organs or seedless fruit (triploid watermelon, banana).

Important terms

• Emasculation – removal of stamens from a flower to prevent self-pollination.
• Bagging – covering emasculated flowers with butter-paper bags to prevent unwanted pollen.
• Pure line – progeny of a single homozygous self-pollinated plant.
• Inbreeding depression – loss of vigour on repeated inbreeding of cross-pollinated plants.
• Back-cross method – repeated crossing with a parent to transfer one gene (e.g. disease resistance) into a good variety.
• Biofortification – breeding crops with higher vitamins and minerals.
• Male sterility – inability to produce viable pollen; used to make hybrid seed without emasculation.

Key facts

• Green Revolution (1960s): high-yielding semi-dwarf wheat (from Norman Borlaug, CIMMYT, Mexico – e.g. Sonalika, Kalyan Sona) and rice varieties (IR-8 from IRRI, Philippines; Jaya and Ratna in India). Dr. M. S. Swaminathan is called the father of the Green Revolution in India.
• Sugarcane: Saccharum barberi (north India) was crossed with Saccharum officinarum (south India) to combine thick stems and high sugar with better adaptation.
• Disease-resistant varieties (examples taught in India): Himgiri (wheat; resistant to leaf and stripe rust, hill bunt), Pusa Komal (cowpea; bacterial blight), Pusa Sadabahar (chilli; chilli mosaic virus, tobacco mosaic virus and leaf curl), Pusa Shubhra (cauliflower; black rot and curl blight black rot).
• Insect-resistant examples: Pusa Gaurav (Brassica; aphids), Pusa Sawani and Pusa A-4 (okra; shoot and fruit borer).

5 important questions

Q1. What is heterosis?

Heterosis or hybrid vigour is the superiority of an F₁ hybrid over its parents in yield, size or vigour. It is used to produce hybrid seeds of crops like maize and bajra.

Q2. What is emasculation? Why is it necessary?

Emasculation is the removal of anthers from a bisexual flower before they burst. It prevents self-pollination so that only the pollen of the chosen male parent fertilises the flower.

Q3. Describe the steps of hybridisation.

• Choose parents with desired characters.
• Emasculate female parent flowers (bud stage).
• Bag the flowers.
• Dust pollen from male parent on stigma; re-bag; label.
• Collect F₁ seed; grow; select desirable plants in F₂ and later generations.
• Test in trials; release as a new variety.

Q4. Compare mass selection, pure line selection and clonal selection.

• Mass selection: many good plants; seeds bulked; quick but mixed.
• Pure line: single self-pollinated plant's progeny; uniform and stable.
• Clonal: in vegetatively propagated crops; selection among clones; keeps hybrid vigour.
• Give one crop example for each.

Q5. Write a note on the Green Revolution in India.

• Period: mid-1960s onwards.
• Semi-dwarf HYV of wheat and rice; Borlaug and Swaminathan.
• Inputs: irrigation, fertilisers, pesticides, mechanisation.
• Results: India became self-sufficient in food grains.
• Problems: soil and water damage, regional imbalance, loss of traditional varieties.

Exam tip

Write the hybridisation steps in order with the words emasculation and bagging. Name one disease-resistant variety and its crop.

Quick summary

Plant tissue culture is the growing of plant cells, tissues or organs on an artificial nutrient medium under sterile conditions. It is possible because plant cells are totipotent. Biotechnology uses living cells and genetic engineering to make useful products. This topic covers the technique of tissue culture, its types and uses, and recombinant DNA technology including transgenic plants like Bt cotton.

Key points

• G. Haberlandt (1902) is called the father of plant tissue culture. He first proposed culturing isolated plant cells.
• Totipotency – the ability of a single plant cell to grow into a complete plant.
• Explant – any part of a plant (leaf, stem, root tip, anther) used to start a culture.
• MS medium (Murashige and Skoog, 1962) is the most common medium: inorganic salts, sucrose (carbon source), vitamins, amino acids, growth regulators (auxin and cytokinin) and agar (for solid medium).
• Aseptic conditions are essential: work in a laminar air flow cabinet; medium sterilised in an autoclave (121 °C, 15 lb/in² pressure, about 15–20 minutes); explants surface-sterilised (e.g. with sodium hypochlorite or mercuric chloride).
• Callus – unorganised mass of dividing cells formed on the explant.
• Organogenesis – formation of shoots or roots from callus; high cytokinin → shoots; high auxin → roots (Skoog and Miller).
• Somatic embryogenesis – formation of embryo-like structures from somatic cells; used to make artificial (synthetic) seeds.
• Micropropagation – rapid multiplication of thousands of identical plants (somaclones) in a short time, e.g. banana, sugarcane, orchids.
• Meristem culture – culturing shoot tips to get virus-free plants (e.g. potato, banana).
• Anther culture gives haploid plants (Guha and Maheshwari, 1964, Datura).
• Protoplast fusion (somatic hybridisation) produces somatic hybrids, e.g. pomato (potato + tomato).

Important terms

• Recombinant DNA (rDNA) – DNA made by joining DNA from two different sources.
• Restriction enzymes – "molecular scissors" that cut DNA at specific sequences (e.g. EcoRI recognises GAATTC).
• DNA ligase – "molecular glue" that joins DNA pieces.
• Vector – carrier that takes foreign DNA into a host (plasmids like pBR322; Ti plasmid of Agrobacterium tumefaciens for plants).
• Transgenic plant – plant carrying a foreign gene.
• PCR – polymerase chain reaction (Kary Mullis) for making many copies of DNA; uses Taq polymerase.

Key facts

• Steps of rDNA technology: isolate DNA → cut with restriction enzyme → join gene to vector with ligase → introduce into host (transformation) → select transformed cells (antibiotic markers) → grow and get product.
• Bt cotton: carries cry genes from the bacterium Bacillus thuringiensis; the Cry protein kills bollworms. Approved in India in 2002.
• Golden rice: rice engineered to make β-carotene (provitamin A).
• Flavr Savr tomato: first commercial GM food (1994), with delayed softening.

5 important questions

Q1. What is totipotency?

Totipotency is the inherent ability of a living plant cell to divide, differentiate and grow into a whole plant. It is the basis of plant tissue culture.

Q2. Why is meristem culture used to obtain virus-free plants?

The apical meristem divides very fast and has no vascular connection, so viruses usually do not reach it. Plants grown from cultured meristems are therefore free of viruses.

Q3. Describe the general technique of plant tissue culture.

• Preparation of MS medium; sterilisation in autoclave.
• Selection and surface sterilisation of explant.
• Inoculation in laminar air flow.
• Incubation at about 25 °C with light/dark cycle.
• Callus formation; sub-culturing; regeneration of shoots and roots.
• Hardening of plantlets and transfer to soil.

Q4. Give the applications of plant tissue culture.

• Micropropagation of elite plants.
• Virus-free plants by meristem culture.
• Haploids by anther culture (fast homozygous lines).
• Somatic hybrids by protoplast fusion.
• Synthetic seeds; germplasm conservation (cryopreservation).
• Secondary metabolites (e.g. shikonin).

Q5. Explain the production of Bt cotton.

• cry gene from Bacillus thuringiensis isolated.
• Inserted into a vector (Ti plasmid of Agrobacterium).
• Cotton cells transformed; transformed cells selected and regenerated by tissue culture.
• Bt toxin is inactive protoxin; becomes active in the alkaline gut of insect larvae and damages gut cells.
• Benefits: less insecticide use; concerns: resistance in pests, biosafety.

Exam tip

Mention Haberlandt (father of tissue culture), MS medium and the auxin–cytokinin ratio in every long answer. A flow chart of rDNA steps earns full marks.

Quick summary

Modern biology produces huge amounts of DNA and protein sequence data. Bioinformatics uses computers, databases and software to store, search and analyse these data. Genomics is the study of whole genomes – their sequencing, mapping and function. This topic covers biological databases, sequence alignment and BLAST, genome sequencing methods, plant genome projects and applications in crop improvement.

Key points

• The term bioinformatics was used by Paulien Hogeweg and Ben Hesper (1970). It joins biology, computer science and statistics.
• Primary (nucleotide) databases: GenBank (NCBI, USA), ENA (EMBL, Europe) and DDBJ (Japan). These three share data daily (INSDC).
• Protein databases: UniProt (sequences and functions) and PDB (Protein Data Bank – 3-D structures).
• Secondary databases (patterns, motifs, families): Pfam, PROSITE. Plant-specific: TAIR (Arabidopsis), Gramene (cereals).
• Sequence alignment compares two or more sequences to find similarity: pairwise or multiple (Clustal); global (whole length; Needleman–Wunsch) or local (similar regions; Smith–Waterman).
• BLAST (Basic Local Alignment Search Tool, Altschul et al., 1990) quickly finds similar sequences in a database. Types: blastn (DNA vs DNA), blastp (protein vs protein), blastx (translated DNA vs protein).
• Phylogenetic trees built from aligned sequences show evolutionary relationships (e.g. MEGA software).
• Genomics branches: structural (sequencing and mapping), functional (what genes do – transcriptomics, proteomics) and comparative (comparing genomes of species).
• Sanger (dideoxy chain-termination) sequencing (Frederick Sanger, 1977) uses ddNTPs that stop chain growth. Next-generation sequencing (NGS) reads millions of fragments in parallel, making genomes cheap and fast.
• Plant genomes sequenced: Arabidopsis thaliana – first plant genome (2000, about 135 Mb, 5 chromosome pairs); rice (Oryza sativa, 2002/2005; India contributed to the International Rice Genome Sequencing Project); later tomato, maize, wheat and others.
• Applications: finding genes for disease and drought resistance, marker-assisted selection (MAS) in breeding, DNA barcoding, drug discovery, studying evolution.

Important terms

• Database – organised collection of data that can be searched by computer.
• Accession number – unique ID of a sequence entry in a database.
• Sequence alignment – arranging sequences to show matching regions.
• Genome – complete set of DNA of an organism.
• Proteomics – study of all proteins of a cell or organism.
• Molecular marker – DNA sequence used to tag a gene or trait (RFLP, RAPD, SSR, SNP).

Key facts

5 important questions

Q1. What is BLAST?

BLAST (Basic Local Alignment Search Tool) is a computer program that compares a query DNA or protein sequence with sequences in a database and lists the most similar ones with scores.

Q2. Name the first plant whose genome was sequenced.

Arabidopsis thaliana (thale cress), in the year 2000. Its genome is small (about 135 Mb) with 5 chromosome pairs.

Q3. Describe biological databases with examples.

Primary and secondary databases; nucleotide (GenBank, ENA, DDBJ), protein (UniProt, PDB), specialised plant databases; how data are retrieved by accession number.

Q4. Explain sequence alignment and its types.

Purpose; pairwise vs multiple; global vs local; scoring with matches, mismatches and gaps; tools (BLAST, Clustal); use in phylogeny.

Q5. Write a note on plant genome projects and applications of genomics.

Sanger and NGS methods; Arabidopsis and rice projects; structural, functional and comparative genomics; marker-assisted breeding, gene discovery.

Exam tip

Learn the full forms (BLAST, NCBI, PDB, EMBL, DDBJ) – they are common one-mark questions. Mention Arabidopsis 2000 and rice in any genomics answer.

Quick summary

Biostatistics is the use of statistics to collect, present and analyse biological data, such as leaf length, plant height, seed yield or results of genetic crosses. This topic covers sampling, frequency distribution, measures of central tendency (mean, median, mode), measures of dispersion (range, standard deviation, coefficient of variation) and the chi-square test, with worked examples.

Key points

• Population – the whole group under study; sample – a part chosen to represent it. Random sampling gives every unit an equal chance.
• Variables: continuous (height, weight) and discrete (number of petals, seeds per pod).
• Data are presented as tables, frequency distributions, histograms, frequency polygons, bar diagrams and pie charts.
• Mean (arithmetic average) = sum of observations ÷ number of observations.
• Median – middle value when data are arranged in order; for an even number, average of the two middle values.
• Mode – the value that occurs most often.
• Range = highest value − lowest value (simple but crude).
• Standard deviation (σ) – square root of the mean of squared deviations from the mean; the best measure of spread (Karl Pearson, 1893).
• Coefficient of variation (CV) compares variability of data in different units or with different means.
• In a normal distribution, mean = median = mode; about 68% of values lie within mean ± 1σ and 95% within mean ± 2σ.
• Chi-square (χ²) test (Karl Pearson, 1900) checks whether observed results fit expected ratios (e.g. Mendelian 3 : 1). If calculated χ² is less than the table value at 5% level, the difference is due to chance and the hypothesis is accepted.
• Degrees of freedom (df) = number of classes − 1.

Important terms

• Frequency – number of times a value occurs.
• Deviation – difference between an observation and the mean.
• Null hypothesis – assumption that there is no real difference between observed and expected values.
• Level of significance – probability (usually 5%, P = 0.05) used to accept or reject the hypothesis.

Formulas

• Mean: x̄ = Σx / n
• Standard deviation: σ = √[Σ(x − x̄)² / n] (for a small sample many books use n − 1 in place of n)
• Coefficient of variation: CV = (σ / x̄) × 100
• Chi-square: χ² = Σ (O − E)² / E, where O = observed, E = expected frequency
• Table χ² at 5% level: df 1 = 3.84, df 2 = 5.99, df 3 = 7.82

5 important questions

Q1. Define standard deviation.

Standard deviation is the square root of the arithmetic mean of the squared deviations of observations from their mean. It shows how widely values are spread around the mean.

Q2. What is the null hypothesis in the chi-square test?

It assumes that there is no significant difference between observed and expected frequencies and that any difference is due to chance.

Q3. Find the mean, median, mode, SD and CV of leaf lengths (cm): 10, 12, 12, 14, 17.

1. Mean = 65 / 5 = 13 cm.
2. Median (3rd value of 5 in order) = 12 cm; Mode = 12 cm.
3. σ = √(28 / 5) = √5.6 = 2.37 cm.
4. CV = (2.37 / 13) × 100 = 18.2%.

Q4. In a monohybrid cross, 400 F₂ plants gave 290 tall and 110 dwarf. Test the fit to 3 : 1.

1. Expected: tall = 400 × 3/4 = 300; dwarf = 400 × 1/4 = 100.
2. χ² = (290 − 300)²/300 + (110 − 100)²/100 = 100/300 + 100/100 = 0.33 + 1.00 = 1.33.
3. df = 2 − 1 = 1; table value at 5% = 3.84.
4. 1.33 < 3.84, so the difference is due to chance – the data fit the 3 : 1 ratio.

Q5. Explain measures of central tendency and dispersion with their merits.

Mean, median, mode – definitions, formulas, merits and demerits; range, mean deviation, standard deviation, CV; normal curve diagram.

Exam tip

In numericals, always draw the table of x, deviation and squared deviation, show each step and write the unit. In χ², state the conclusion clearly – "accepted" or "rejected" at 5% level.

Quick summary

Organic evolution is the slow, continuous change of living organisms over many generations, by which new species arise from earlier ones. This topic covers the evidences of evolution (with plant examples), the theories of Lamarck, Darwin and de Vries, the modern synthetic theory, and the ideas of speciation and isolation.

Key points

• Palaeontological – fossils show a sequence from simple to complex (algae → bryophytes → pteridophytes → gymnosperms → angiosperms) and connecting links such as seed ferns (Pteridospermae).
• Morphological and anatomical – homologous organs (same origin, different function: thorn of Bougainvillea and tendril of Passiflora, both modified stems) and analogous organs (different origin, same function: tendril of pea – leaflet – and tendril of grapevine – stem).
• Connecting links – Gnetum (between gymnosperms and angiosperms), Psilotum among pteridophytes.
• Embryological, biochemical (same genetic code, similar cytochrome c) and biogeographical evidences.
• Evidences of evolution:
• Lamarckism (Jean-Baptiste Lamarck, Philosophie Zoologique, 1809): use and disuse of organs and inheritance of acquired characters. Rejected after Weismann's experiments (germplasm theory).
• Darwinism (Charles Darwin, On the Origin of Species, 1859; Wallace proposed the same idea): overproduction, struggle for existence, variation, survival of the fittest (Herbert Spencer's phrase) and natural selection, leading to new species.
• Limits of Darwinism: could not explain the origin and inheritance of variation; did not distinguish heritable from non-heritable variation.
• Mutation theory (Hugo de Vries, 1901) from Oenothera lamarckiana: new species arise by sudden large heritable changes (mutations) in one step.
• Modern synthetic theory (Neo-Darwinism) (Dobzhansky, Huxley, Mayr, Stebbins): gene mutation, chromosomal changes, recombination, natural selection and reproductive isolation together cause evolution. Genetic drift and gene flow also act.
• Hardy–Weinberg principle (1908): allele frequencies stay constant in a large, randomly mating population without mutation, migration, selection or drift: p² + 2pq + q² = 1.
• Speciation: allopatric (geographically separated) and sympatric (in the same area). In plants, polyploidy gives instant sympatric speciation (e.g. bread wheat, Primula kewensis).
• Isolating mechanisms: pre-zygotic (seasonal, habitat, mechanical, gametic) and post-zygotic (hybrid inviability, hybrid sterility).

Important terms

• Homologous organs – organs of same origin and structure but different functions.
• Analogous organs – organs of different origin but similar function.
• Natural selection – survival and reproduction of individuals best suited to the environment.
• Speciation – formation of new species.
• Genetic drift – random change in allele frequency in small populations.

Key facts

5 important questions

Q1. Give one plant example each of homologous and analogous organs.

Homologous: thorn of Bougainvillea and tendril of Cucurbita (both modified stem/branch). Analogous: tendril of pea (modified leaflet) and tendril of grapevine (modified stem).

Q2. What is the Hardy–Weinberg principle?

It states that in a large, random-mating population with no mutation, migration, selection or drift, allele and genotype frequencies remain constant from generation to generation (p² + 2pq + q² = 1).

Q3. Explain Darwin's theory of natural selection and its limitations.

Five main points with plant examples; criticism – source of variation unknown, overemphasis on small variations; how modern genetics filled the gap.

Q4. Discuss evidences of evolution with special reference to plants.

Fossils and geological record; connecting links; homologous and analogous organs; comparative biochemistry and DNA sequences.

Q5. Describe the modern synthetic theory of evolution.

Factors: gene mutation, chromosomal mutation, recombination, natural selection, isolation, genetic drift; speciation by polyploidy in plants.

Exam tip

Use the table to compare theories. Plant examples (Oenothera, wheat polyploidy, Gnetum as connecting link) score more marks in a botany paper than animal examples.