Biology Basics – Assam Police Sub‑Inspector (SI) Exam
Prepared for competitive exams – IIT‑JEE style depth with derivations, key formulae, and a hard practice question.
1. Cellular Biology
| Aspect | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | No membrane‑bound nucleus | Membrane‑bound nucleus |
| DNA | Circular, single chromosome | Linear, multiple chromosomes |
| Organelle | Few (ribosomes, plasmids) | Many (mitochondria, ER, Golgi, etc.) |
| Cell size | 1–10 µm | 10–100 µm |
| Replication | Simple, oriC → oriC | Complex, origin of replication per chromosome |
1.1 Cell Cycle & Division
| Phase | Key Events | Outcome |
|---|---|---|
| G1 | Cell growth, protein synthesis | Prep for DNA synthesis |
| S | DNA replication (each chromosome → sister chromatids) | 2× DNA |
| G2 | Further growth & organelle synthesis | Ready for mitosis |
| M | Mitosis (metaphase → anaphase → telophase) + cytokinesis | 2 diploid daughter cells |
Tricky point – Mitosis = somatic division, Meiosis = gamete formation (2 rounds of division ⇒ 4 haploid cells).
1.1.1 Mitosis vs Meiosis (Diagrammatic Flow)
Mitosis: 2n ──► 2n (no reduction)
Meiosis: 2n ──► 2n → 4n → 4n → 4n (reduction)
1.2 Central Dogma
DNA ─► RNA ─► Protein
- Transcription – RNA polymerase synthesises mRNA from DNA template.
- Translation – Ribosomes read mRNA codons → tRNA anticodons → amino acid chain.
Key formula – mRNA codon = 3 nucleotides → 1 amino acid.
| Process | Enzymes | Location |
|---|---|---|
| Transcription | RNA polymerase | Nucleus |
| Translation | Ribosomes | Cytoplasm |
2. Genetics & Inheritance
2.1 Mendelian Laws
| Law | Statement | Example |
|---|---|---|
| Law of Segregation | Alleles separate during gamete formation | Aa → A or a |
| Law of Independent Assortment | Alleles of different genes assort independently | A/a × B/b → 9:3:3:1 |
| Law of Dominance | Dominant allele masks recessive | Aa phenotype = A |
2.2 Punnett Square Basics
| Gamete 1 | Gamete 2 | |
|---|---|---|
| Gamete 1 | ||
| Gamete 2 |
Trick – For dihybrid cross × 9:3:3:1 only if loci assort independently.
2.3 Genetic Notations
| Symbol | Meaning |
|---|---|
A |
Dominant allele |
a |
Recessive allele |
Aa |
Heterozygous |
aa |
Homozygous recessive |
2.4 Chromosomal Inheritance
| Chromosome | Ploidy | Example |
|---|---|---|
| Autosome | 2n (diploid) | 46 chromosomes in humans |
| Sex chromosome | X or Y | XX (female), XY (male) |
3. Human Anatomy & Physiology
| System | Key Organs | Primary Function |
|---|---|---|
| Circulatory | Heart, blood vessels | Transport O₂, nutrients |
| Respiratory | Lungs, trachea | Gas exchange |
| Digestive | Stomach, intestines | Nutrient absorption |
| Nervous | Brain, spinal cord | Control & coordination |
| Endocrine | Thyroid, pancreas | Hormone secretion |
| Immune | Lymph nodes, spleen | Defense against pathogens |
3.1 Homeostasis
| Component | Negative Feedback | Positive Feedback |
|---|---|---|
| Example | Thermoregulation | Childbirth |
| Mechanism | ↓ stimulus → ↓ response | ↑ stimulus → ↑ response |
Important – All physiological processes depend on feedback loops for stability.
4. Ecology & Environmental Biology
| Term | Definition | Example |
|---|---|---|
| Ecosystem | Community + abiotic environment | Forest ecosystem |
| Food Chain | Linear transfer of energy | Grass → Rabbit → Fox |
| Food Web | Interconnected food chains | Complex web in lake ecosystem |
| Biodiversity | Variety of life | Species richness in Himalayas |
4.1 Ecological Interactions
| Interaction | Description | Example | |-------------