Key Points: Prokaryotic Cells

Definition and Examples: Prokaryotes are organisms whose cells lack a membrane-bound nucleus. Major groups: Bacteria (Eubacteria) and Archaea; also includes Cyanobacteria (blue-green algae). Size range: 1–10 µm.

Genetic Material: The prokaryotic chromosome is a single, circular, double-stranded DNA molecule. It is "naked" — not associated with histone proteins. It occupies the nucleoid region of the cytoplasm with no surrounding membrane.

Ribosomes: 70S ribosomes are the sole protein-synthesizing machinery (50S large subunit + 30S small subunit). These are the targets of antibiotics: streptomycin/tetracycline target the 30S subunit; erythromycin/chloramphenicol target the 50S subunit. NOTE: Svedberg units are not additive.

Cell Wall: Composed of peptidoglycan (murein) — a unique polymer of amino sugars crosslinked by short peptides. Target of penicillin (inhibits peptidoglycan cross-linking). Gram-positive bacteria have thick walls; Gram-negative bacteria have thin walls plus an outer membrane. Mycoplasma lacks any cell wall.

Mesosomes: Infoldings of the plasma membrane, unique to prokaryotes. Functions: (1) increase surface area for cellular respiration (compensate for lack of mitochondria); (2) serve as attachment sites for circular DNA during replication; (3) assist in cell wall formation and septum formation during binary fission.

Plasmids: Small (1–200 kb), circular, double-stranded DNA molecules that replicate autonomously, independent of the main chromosome. Carry accessory genes (antibiotic resistance, virulence factors, metabolic capabilities). Essential tools in biotechnology as gene-cloning vectors.

Flagella: Made entirely of the protein flagellin (not tubulin). Rotate like a propeller powered by the proton motive force ($H^{+}$ gradient). No membrane covering. No 9+2 microtubule arrangement. Structurally simpler than eukaryotic flagella.

Cell Division: Binary fission: circular chromosome replicates → cell elongates → septum (cross-wall) forms dividing the cell into two equal daughters. No spindle apparatus, no centrioles, no chromosome condensation. Rapid (~20 minutes in E. coli under optimal conditions).

Absent Structures (NEET Trap): No nuclear membrane, no membrane-bound organelles, no mitochondria, no chloroplasts, no ER, no Golgi, no lysosomes, no centrioles, no 9+2 cilia/flagella, no cytoskeleton (or primitive), no histones associated with DNA, no linear chromosomes.

Key NEET Point: Mitochondria and chloroplasts in eukaryotic cells contain 70S ribosomes (same as prokaryotes) because of their prokaryotic endosymbiotic origin — this is the most tested concept in this chapter.