1. Cell Theory Foundations & Cell Organelle Directory
The cell is the structural and functional unit of all living organisms. Robert Hooke observed cork cells under a crude microscope in 1665. The classical Cell Theory was formulated by botanist Matthias Schleiden (1838) and zoologist Theodor Schwann (1839), later expanded by Rudolf Virchow (1855) with the doctrine "Omnis cellula-e cellula" (all cells arise from pre-existing cells).
| Organelle | Exam Epithet / Popular Name | Structural & Biochemical Function |
|---|---|---|
| Mitochondria | Powerhouse of the Cell | Double-membrane bound organelle; site of aerobic cellular respiration (Krebs Cycle) producing ATP (Adenosine Triphosphate). Possesses its own circular DNA and 70S ribosomes (Endosymbiotic origin). |
| Lysosomes | Suicidal Bags of the Cell | Membrane-bound vesicles formed by Golgi apparatus containing hydrolytic digestive enzymes (acid hydrolases optimal at pH 5.0). Digests worn-out cellular components via autolysis. |
| Ribosomes | Protein Factory of the Cell | Non-membrane bound ribonucleoprotein granules composed of rRNA and proteins. Sites of protein translation. Discovered by George Palade. (70S in prokaryotes, 80S in eukaryotes). |
| Golgi Complex | Packaging & Dispatch Unit | Discovered by Camillo Golgi. Consists of flattened cisternae; modifies, packages, and sorts proteins and lipids into secretory vesicles. Responsible for acrosome formation in mammalian sperm. |
| Endoplasmic Reticulum (ER) | Intracellular Highway | Rough ER (RER): Studded with ribosomes; synthesizes export proteins. Smooth ER (SER): Devoid of ribosomes; synthesizes lipids/steroids and detoxifies drugs/poisons in liver cells. |
| Centrosome / Centrioles | Spindle Fiber Organizer | Present in animal cells; absent in higher plant cells. Coordinates spindle apparatus during mitotic and meiotic chromosome separation. |
2. Cell Division: Mitosis (Equational) vs Meiosis (Reductional)
Mitosis (Somatic Cell Division)
- Occurs in somatic (body) cells for growth, repair, and regeneration.
- Produces two genetically identical diploid (2n) daughter cells.
- Sequential Stages: Prophase → Metaphase (chromosomes align at equatorial plate; best stage to count/study chromosome morphology) → Anaphase (centromeres split) → Telophase.
Meiosis (Germ Cell / Gamete Division)
- Occurs in reproductive cells (testes/ovaries) to produce gametes (sperm and ova).
- Produces four genetically diverse haploid (n) daughter cells.
- Crossing Over: Exchange of genetic segments between non-sister chromatids of homologous chromosomes occurs during the Pachytene stage of Prophase I via enzyme recombinase. Generates biological variation.
3. Mendelian Genetics & High-Frequency Genetic Syndromes
Gregor Johann Mendel established the three classical laws of inheritance working on garden peas (Pisum sativum): Law of Dominance, Law of Segregation, and Law of Independent Assortment.
| Disorder | Chromosomal Nature | Clinical Manifestations & Exam Trap |
|---|---|---|
| Down Syndrome | Trisomy of 21st Chromosome (47, XX / XY) | Autosomal aneuploidy. Characteristic broad flat face, epicanthic eye folds, protruding tongue, short stature, and congenital heart defects. |
| Turner Syndrome | Monosomy of X Chromosome (45, X0) | Occurs only in females. Sterile ovaries, webbed neck, short stature, lack of secondary sexual characteristics. |
| Klinefelter Syndrome | Extra X Chromosome in Males (47, XXY) | Affects males. Tall stature, sterile testes, gynecomastia (development of breast tissue in males). |
| Sickle Cell Anemia | Autosomal Recessive Point Mutation | Substitution of Glutamic acid by Valine at the 6th position of the beta-globin chain. Under low oxygen, RBCs transform from biconcave disc to sickle shape. Confers resistance against malaria. |
| Hemophilia (Royal Disease) | X-Linked Recessive Genetic Defect | Deficiency of Blood Clotting Factor VIII (Hemophilia A) or Factor IX (Hemophilia B). Minor cuts result in uncontrolled non-stop bleeding. Transmitted through carrier mothers to male offspring. |