BA02 — Cell Biology & Genetics

📖 BA02 · AFCAT General Awareness — Biology ★ Moderate-High Yield

The cell is the fundamental unit of life. AFCAT keeps this at Class 9-10 level: organelle functions, mitosis vs meiosis, and DNA base-pairing rules. Master the nicknames and ratios for full marks.

✈ AFCAT Focus: Mitochondria = powerhouse; ribosome = protein factory; lysosome = suicidal bag; Golgi = post office. Mitosis → 2 diploid cells (growth). Meiosis → 4 haploid cells (gametes). DNA: A-T (2 bonds); G-C (3 bonds). Watson & Crick 1953. Monohybrid cross phenotype ratio = 3:1.

PART 1 — CELL TYPES & ORGANELLES

1. Prokaryote vs Eukaryote

Feature	PROKARYOTE	EUKARYOTE
Nucleus	NO membrane-bound nucleus; naked DNA (nucleoid region)	TRUE nucleus with nuclear membrane
Organelles	ABSENT (no membrane-bound organelles)	PRESENT (mitochondria, Golgi etc.)
Cell Wall	PRESENT — Peptidoglycan (bacteria)	Plant: cellulose wall; Animal: no cell wall
Ribosomes	70S (smaller)	80S (larger)
Examples	Bacteria, Blue-green algae (1–10 µm)	Plant, Animal, Fungal cells (10–100 µm)

2. Cell Organelles

Organelle	Function	Nickname	Key Detail
Nucleus	Contains DNA; controls all cell activity	Control centre	Double membrane; nucleolus makes rRNA
Mitochondria	Cellular respiration; produces ATP	Powerhouse of cell	Has own DNA; semi-autonomous; double membrane
Ribosome	Protein synthesis (mRNA translation)	Protein factory	No membrane; 70S prokaryote; 80S eukaryote
Golgi Apparatus	Packaging & secretion of proteins/lipids	Post office of cell	Single membrane; forms lysosomes
Lysosome	Digests worn organelles and foreign material	Suicidal bag	~40 hydrolytic enzymes; causes autolysis
Chloroplast	Photosynthesis	Plants ONLY	Double membrane; contains chlorophyll

PART 2 — CELL DIVISION & GENETICS

3. Mitosis vs Meiosis

♿ MITOSIS — Equational Division

Result: 2 daughter cells; same chromosome count (2n → 2n)
Purpose: Growth, repair, replacement of body cells
Stages: Prophase → Metaphase → Anaphase → Telophase
Cells genetically identical to parent

♿ MEIOSIS — Reductional Division

Result: 4 daughter cells; half chromosome count (2n → n)
Purpose: Formation of gametes (sperm and egg)
Human: 46 chromosomes (2n) → gametes with 23 (n)
Cells genetically varied (crossing over)

4. Genetics — Mendel & DNA

Mendel's Laws:
● Dominance: dominant trait appears in F1; recessive masked (not lost)
● Segregation: alleles separate in gamete formation; each gamete gets one allele
● Monohybrid (Tt × Tt): phenotype ratio = 3 Tall : 1 Dwarf (3:1); genotype = 1:2:1

DNA Double Helix (Watson & Crick, 1953):
● A pairs with T (2 hydrogen bonds) ● G pairs with C (3 hydrogen bonds)
● Central dogma: DNA → mRNA → Protein

📝 AFCAT PYQs — Cell Biology

Q1. "Powerhouse of the cell"? AFCAT PYQ

(a) Nucleus(b) Ribosome(c) Mitochondria(d) Lysosome

✔ (c) Mitochondria

Mitochondria produce ATP via cellular respiration. They have their own DNA (semi-autonomous). Most numerous in metabolically active cells. The most repeated AFCAT cell biology question.

Q2. In DNA, adenine always pairs with: AFCAT PYQ

(a) Guanine(b) Cytosine(c) Thymine(d) Uracil

✔ (c) Thymine

DNA: A-T (2 H-bonds) and G-C (3 H-bonds). In RNA, Thymine is replaced by Uracil (A pairs with U in RNA). AFCAT tests the DNA vs RNA distinction.

🧠 Quick Memory Chart — BA02

💡 Organelle Nicknames

Mitochondria: Powerhouse (ATP)
Ribosome: Protein factory
Lysosome: Suicidal bag
Golgi: Post office
Nucleus: Control centre

♿ Cell Division

Mitosis: 2 cells, same 2n, growth
Meiosis: 4 cells, half n, gametes
Human: 46 chromosomes (2n)
Gamete: 23 chromosomes (n)

📊 DNA

Watson & Crick: 1953
A-T (2 bonds); G-C (3 bonds)
Monohybrid F2: 3:1 phenotype
DNA → mRNA → Protein

📝 Practice Exercise

E1. Which organelle is absent in plant cells but present in animal cells?

(a) Cell wall(b) Chloroplast(c) Centriole(d) Vacuole

E2. If Guanine = 20%, what is Adenine content in that DNA?

(a) 20%(b) 30%(c) 40%(d) 60%

Answers:
E1 → (c) Centriole [forms spindle fibres; absent in plant cells] | E2 → (b) 30% [G=C=20%, so G+C=40%; A+T=60%; A=T, so A=30%]

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