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Biology · AFCAT

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

Nucleus

Prokaryote

NO membrane-bound nucleus; naked DNA (nucleoid region)

Eukaryote

TRUE nucleus with nuclear membrane

Organelles

Prokaryote

ABSENT (no membrane-bound organelles)

Eukaryote

PRESENT (mitochondria, Golgi etc.)

Cell Wall

Prokaryote

PRESENT — Peptidoglycan (bacteria)

Eukaryote

Plant: cellulose wall; Animal: no cell wall

Ribosomes

Prokaryote

70S (smaller)

Eukaryote

80S (larger)

Examples

Prokaryote

Bacteria, Blue-green algae (1–10 µm)

Eukaryote

Plant, Animal, Fungal cells (10–100 µm)

2. Cell Organelles

Nucleus

Function

Contains DNA; controls all cell activity

Nickname

Control centre

Key Detail

Double membrane; nucleolus makes rRNA

Mitochondria

Function

Cellular respiration; produces ATP

Nickname

Powerhouse of cell

Key Detail

Has own DNA; semi-autonomous; double membrane

Ribosome

Function

Protein synthesis (mRNA translation)

Nickname

Protein factory

Key Detail

No membrane; 70S prokaryote; 80S eukaryote

Golgi Apparatus

Function

Packaging & secretion of proteins/lipids

Nickname

Post office of cell

Key Detail

Single membrane; forms lysosomes

Lysosome

Function

Digests worn organelles and foreign material

Nickname

Suicidal bag

Key Detail

~40 hydrolytic enzymes; causes autolysis

Chloroplast

Function

Photosynthesis

Nickname

Plants ONLY

Key Detail

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

Key Point

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)

Key Point

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

(d) Lysosome

✓ Answer: (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

(d) Uracil

✓ Answer: (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

(d) Vacuole

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

(a) 20%

(b) 30%

(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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