Cell Biology

Prokaryotic cells (bacteria, cyanobacteria) lack a membrane-bound nucleus and organelles — genetic material floats freely as a nucleoid. Eukaryotic cells (plants, animals, fungi) have a true nucleus enclosed in a double membrane and membrane-bound organelles like mitochondria, chloroplasts, and ER. For exams: all crop plants are eukaryotes; Rhizobium and Azotobacter (soil bacteria) are prokaryotes.

Mitosis has 4 stages — Prophase, Metaphase, Anaphase, Telophase (PMAT) — producing 2 genetically identical daughter cells with the same chromosome number as the parent. Meiosis has 2 divisions (Meiosis I and II) with 8 sub-stages, producing 4 haploid gametes. Meiosis I is the reductional division (homologs separate); Meiosis II is the equational division (chromatids separate).

Key enzymes: DNA Helicase (unwinds double helix), Primase (synthesises RNA primer), DNA Polymerase III (main enzyme, adds nucleotides 5'→3'), DNA Polymerase I (removes RNA primer), DNA Ligase (joins Okazaki fragments on lagging strand). Topoisomerase relieves supercoiling ahead of the replication fork. for exams: DNA Ligase is the enzyme that seals nicks — tested frequently.

The operon model (Jacob-Monod, 1961) explains gene regulation in prokaryotes. A structural operon consists of: Promoter (RNA polymerase binding), Operator (repressor binding site), and Structural genes (coding sequences). In the Lac operon (E. coli), lactose acts as an inducer — it inactivates the repressor protein, allowing RNA polymerase to transcribe lac genes. This model is the basis of inducible and repressible operons.

Chromosomal aberrations are changes in chromosome number (aneuploidies) or structure. Structural: deletion, duplication, inversion, translocation. Numerical: monosomy (2n-1), trisomy (2n+1), nullisomy (2n-2). Colchicine induces polyploidy by inhibiting spindle fibre formation (prevents chromosome separation). Important for exams: Down syndrome = trisomy 21; Turner syndrome = 45,X (monosomy X).

Rice: 2n=24; Wheat (hexaploid): 2n=42; Maize: 2n=20; Cotton (tetraploid): 2n=52; Sugarcane: 2n=80 (highly polyploid); Groundnut (tetraploid): 2n=40; Tomato: 2n=24; Potato (tetraploid): 2n=48; Banana: 2n=22 (usually triploid in cultivated forms). These numbers are tested directly in exams and exams.