Biology: Cell Biology and Genetics

Biology: Cell Biology and Genetics

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• Cell theory states all organisms consist of cells, the cell is the structural and functional unit of life, and every cell originates from a pre-existing cell (Rudolf Virchow, 1855).
• Prokaryotic vs. eukaryotic: prokaryotes (bacteria, archaea) lack a membrane-bound nucleus and organelles; eukaryotes (animals, plants, fungi, protists) possess a true nucleus plus mitochondria, ER, Golgi, and (in plants) chloroplasts and a cellulose cell wall.
• Hardy–Weinberg equilibrium is described by p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies), where p = dominant allele frequency and q = recessive allele frequency.
• Mitosis (PMAT: prophase, metaphase, anaphase, telophase) yields two identical diploid cells for growth; meiosis yields four haploid gametes with crossing-over in prophase I.
• NAT-I typically frames 1–2 questions from this cluster as MCQs testing Mendelian ratios, organelle function, or base-pairing rules.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Cell Structure and the Fluid Mosaic Membrane

Every eukaryotic cell is bounded by a plasma membrane described by the fluid mosaic model — a phospholipid bilayer in which proteins drift laterally. Phospholipids orient hydrophilic heads outward and hydrophobic tails inward; cholesterol modulates fluidity. Surface-area-to-volume ratio (SA/V = 6/s for a cube of side s) explains why large organisms need specialised exchange surfaces.

Organelle Functions

Organelle	Primary Role
Nucleus	Stores DNA; site of transcription
Mitochondrion	Aerobic respiration; ATP production; own circular DNA
Ribosome	Protein synthesis (translation)
Rough ER	Folds and modifies secretory proteins
Golgi apparatus	Packages, sorts, ships vesicles
Lysosome	Hydrolytic digestion (animal cells)
Chloroplast	Photosynthesis (plant cells)
Centriole	Organises spindle in animal mitosis

Cell Division

Mitosis is equational (2n → 2n): one round of division after one round of DNA replication, producing genetically identical daughter cells for growth and tissue repair. Meiosis is reductional (2n → n): two successive divisions (Meiosis I and II) preceded by one S-phase, producing four haploid gametes. Crossing over during prophase I (chiasmata formation between non-sister chromatids of homologous bivalents) and independent assortment of homologous pairs at metaphase I generate the genetic variation that Mendel’s second law describes.

Molecular Genetics and the Central Dogma

DNA is a double helix of antiparallel strands with complementary base pairing: A = T (2 H-bonds) and G ≡ C (3 H-bonds), so Chargaff’s rule (A = T, G = C) holds. The central dogma flows DNA → mRNA → protein via replication, transcription, and translation. A codon is a 3-nucleotide mRNA triplet read by a complementary anticodon on tRNA; 61 codons specify amino acids, 3 are stop codons (UAA, UAG, UGA).

Mendelian Inheritance

The Law of Segregation states each diploid carries two alleles per gene that separate into different gametes; the Law of Independent Assortment applies to genes on different chromosomes. A monohybrid cross of two heterozygotes (Aa × Aa) yields a 1:2:1 genotypic ratio and 3:1 phenotypic ratio; a dihybrid cross (AaBb × AaBb) yields the classic 9:3:3:1 ratio.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Non-Mendelian Patterns Frequently Tested

• Incomplete dominance: heterozygote shows an intermediate phenotype (e.g. red × white snapdragon → pink F₁); phenotypic ratio 1:2:3 in F₂ instead of 3:1.
• Codominance: both alleles fully expressed (AB blood group, roan cattle).
• Multiple alleles: ABO blood group has three alleles (I^A, I^B, i) with six possible genotypes but four phenotypes.
• Sex-linked inheritance: X-linked recessive traits (haemophilia, colour blindness) appear more often in males (XY) because a single recessive allele is hemizygous; a carrier mother (X^H X^h) transmits to 50 % of sons.
• Linkage: genes on the same chromosome tend to inherit together, deviating from the 9:3:3:1 expectation — measured by recombination frequency, where 1 % recombinants ≈ 1 centimorgan (Morgan unit).

Population Genetics with Hardy–Weinberg

For a population in equilibrium (no mutation, migration, selection, random mating, large size), p² = homozygous dominant frequency, 2pq = heterozygous carrier frequency, q² = homozygous recessive frequency. If 9 % of a population shows a recessive phenotype, q² = 0.09, so q = 0.30 and p = 0.70, giving carrier frequency 2pq = 0.42 (42 %). Note that allele frequency is NOT the same as genotype frequency — a common NAT-I trap.

Common Mistakes and Traps

1. Confusing allele frequency (p, q) with genotype frequency (p², 2pq, q²).
2. Forgetting that meiosis II resembles mitosis but starts with n, not 2n — sister chromatids separate, no homologues.
3. Believing mitochondria and chloroplasts have 46 chromosomes — they carry their own small circular DNA, independent of the nuclear genome (endosymbiotic theory, Lynn Margulis).
4. Mixing up anticodon directionality — tRNA anticodons pair antiparallel to mRNA codons.

Exam Strategy for NAT-I (Subject Knowledge, ~4 % weight)

Expect 1–2 MCQs: one on organelle function or mitosis stage, one on Mendelian ratios or a Hardy–Weinberg calculation. Read phenotypes carefully (dominant vs. recessive in stem wording) and check whether “frequency” refers to alleles or genotypes.

Practice Prompts

1. In a population, 16 % show a recessive trait. Calculate the carrier frequency and the homozygous dominant frequency.
2. A woman heterozygous for colour blindness (X^C X^c) marries a normal man (X^C Y). What proportion of sons will be colour-blind?

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