Biology (PGT) — HTET Level 3 Study Notes

Overview

Biology at the PGT level for HTET covers the entire Class XI–XII NCERT syllabus across Botany, Zoology, Genetics, and Biotechnology. This is a content-heavy subject where factual accuracy and conceptual clarity determine scores. Expect questions on plant and animal physiology, cell biology, ecology, human reproduction, genetics problems, and applications of biotechnology.

The PGT paper tests your depth of knowledge — not just definitions but mechanisms, processes, and their significance. Questions often involve diagrams (though you must visualize them mentally), numerical genetics problems, and application-based scenarios. Mastering the NCERT textbooks thoroughly is non-negotiable; most questions are directly or indirectly sourced from these texts.

Prioritize high-yield areas: Genetics and Evolution, Human Physiology, Plant Physiology, and Biotechnology. These sections consistently carry more weight and offer scoring opportunities if concepts are clear.

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Key Concepts

• **Cell as the structural and functional unit**: Prokaryotic vs eukaryotic cells, cell organelles and their functions, cell cycle phases (G1, S, G2, M), and differences between mitosis and meiosis.

• **Biomolecules**: Structure and function of carbohydrates, proteins, lipids, nucleic acids; enzyme action follows lock-and-key or induced-fit model; Michaelis-Menten kinetics concept.

• **Plant Physiology essentials**: Photosynthesis (light and dark reactions, C3 vs C4 vs CAM pathways), respiration (glycolysis, Krebs cycle, ETC), transpiration pull theory, mineral nutrition and nitrogen cycle.

• **Human Physiology systems**: Digestive (enzymatic action), respiratory (oxygen-haemoglobin dissociation curve), circulatory (cardiac cycle, double circulation), excretory (nephron function, counter-current mechanism), nervous (synapse, reflex arc) and endocrine (hormones and feedback).

• **Genetics foundations**: Mendelian inheritance, deviations from Mendelism (incomplete dominance, codominance, multiple alleles, polygenic inheritance), linkage and crossing over, sex determination (XX-XY, ZW-ZZ), chromosomal disorders.

• **Molecular Biology**: DNA structure (Watson-Crick model), replication (semi-conservative), transcription, genetic code properties, translation, lac operon regulation.

• **Biotechnology applications**: Recombinant DNA technology, PCR, gel electrophoresis, transgenic organisms, gene therapy, Bt crops, golden rice, DNA fingerprinting.

• **Ecology and Environment**: Ecosystem components, food chains and webs, ecological pyramids, biogeochemical cycles, ecological succession, biodiversity and conservation strategies.

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Formulas / Key Facts

**Genetics Calculations**:

• Monohybrid ratio: 3:1 (phenotypic), 1:2:1 (genotypic)
• Dihybrid ratio: 9:3:3:1
• Test cross ratio: 1:1 (monohybrid), 1:1:1:1 (dihybrid)
• Map distance = (Recombinant offspring / Total offspring) × 100 cM

**Biochemistry Constants**:

• ATP yield from one glucose (aerobic): 36–38 ATP
• Net ATP from glycolysis: 2 ATP
• Krebs cycle per acetyl-CoA: 1 ATP, 3 NADH, 1 FADH2

**Must-Remember Facts**:

• Mitochondria and chloroplasts have 70S ribosomes (like prokaryotes)
• Human chromosome number: 46 (23 pairs)
• Blood groups: ABO system shows codominance and multiple allelism
• Haemophilia and colour blindness are X-linked recessive
• Restriction enzymes cut at palindromic sequences
• First transgenic plant: Tobacco; first transgenic animal: Mouse
• Dolly the sheep: First mammal cloned from adult somatic cell (1996)
• Bt toxin gene source: Bacillus thuringiensis
• Hardy-Weinberg equation: p² + 2pq + q² = 1

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Worked Examples

**Example 1: Monohybrid Cross**

Problem: In pea plants, tall (T) is dominant over dwarf (t). Cross a heterozygous tall plant with a dwarf plant. Find the phenotypic ratio.

Solution:

• Parents: Tt × tt
• Gametes: T, t from Tt; t from tt
• Punnett square:
• Tt (tall), Tt (tall), tt (dwarf), tt (dwarf)
• Wait, correction — gametes from tt are only t
• Offspring: Tt, Tt, tt, tt → 2 Tt : 2 tt
• Phenotypic ratio: 1 Tall : 1 Dwarf (50% each)

This is a test cross confirming the heterozygous nature of the tall parent.

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**Example 2: Dihybrid Cross**

Problem: In guinea pigs, black coat (B) is dominant over white (b), and rough coat (R) is dominant over smooth (r). Cross BbRr × BbRr. What fraction will be black and smooth?

Solution:

• For black: Bb × Bb gives 3 black : 1 white → P(black) = 3/4
• For smooth: Rr × Rr gives 3 rough : 1 smooth → P(smooth) = 1/4
• P(black AND smooth) = 3/4 × 1/4 = 3/16

Answer: 3/16 of offspring will be black and smooth.

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**Example 3: Hardy-Weinberg Application**

Problem: In a population, 16% show the recessive phenotype. Find the frequency of heterozygous individuals.

Solution:

• q² = 0.16, so q = 0.4
• p = 1 - q = 0.6
• Heterozygous frequency (2pq) = 2 × 0.6 × 0.4 = 0.48 = 48%

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Common Mistakes

• **Confusing mitosis and meiosis outcomes**: Mitosis produces 2 identical diploid cells; meiosis produces 4 genetically different haploid cells. Students often mix up where crossing over occurs (meiosis I only).

• **Mixing C3 and C4 pathways**: C3 plants fix CO2 directly via RuBisCO in mesophyll; C4 plants first fix CO2 into 4-carbon compound (OAA) in mesophyll, then release it in bundle sheath for Calvin cycle. C4 avoids photorespiration.

• **Forgetting codominance vs incomplete dominance**: In codominance, both alleles express fully (ABO blood groups); in incomplete dominance, there's a blended phenotype (pink snapdragons from red × white).

• **Errors in genetic ratios**: Always verify if the question asks phenotypic or genotypic ratio. Also check for sex-linkage — X-linked traits show different ratios in reciprocal crosses.

• **Confusing DNA replication enzymes**: Helicase unwinds, primase adds RNA primer, DNA polymerase III synthesizes new strand (5' to 3'), DNA polymerase I removes primers, ligase joins Okazaki fragments.

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Quick Reference

• **Photosynthesis equation**: 6CO2 + 6H2O + light → C6H12O6 + 6O2

• **Respiration equation**: C6H12O6 + 6O2 → 6CO2 + 6H2O + 36-38 ATP

• **Central Dogma**: DNA → RNA → Protein (Replication, Transcription, Translation)

• **Genetic code**: Universal, degenerate, non-overlapping, comma-less; AUG = start codon; UAA, UAG, UGA = stop codons

• **PCR steps in order**: Denaturation (94°C) → Annealing (55–60°C) → Extension (72°C)

• **Ecological succession**: Primary (bare rock) vs Secondary (disturbed area with soil); climax community is stable end point