⚡ Core Concept: Prokaryotic Cell Biology & The Archaeal Paradigm
Prokaryotes encompass two distinct evolutionary domains: Bacteria and Archaea. They are characterized by uncompartmentalized cytoplasm, circular genomes within a nucleoid, coupled transcription-translation, peptidoglycan or pseudomurein cell walls, and extraordinary metabolic plasticity.
1. Bacteria vs. Archaea: Evolutionary & Biochemical Divergence
Carl Woese’s 16S rRNA phylogeny demonstrated that Archaea are genetically closer to Eukarya than to Bacteria:
| Biochemical Feature | Domain Bacteria | Domain Archaea | Domain Eukarya |
|---|---|---|---|
| Membrane Lipids | Ester-linked unbranched fatty acids; glycerol-3-phosphate | Ether-linked branched isoprenoid phytanyl chains; glycerol-1-phosphate; monolayer tetraethers in extremophiles | Ester-linked unbranched fatty acids; glycerol-3-phosphate |
| Cell Wall Peptidoglycan | Present (NAM-NAG with D-amino acid cross-links) | Absent (Pseudomurein with NAT-NAG, S-layer glycoproteins, or methanochondroitin) | Absent (Cellulose, Chitin, or absent) |
| Initiator tRNA | Formylmethionine (fMet) | Methionine (Met) | Methionine (Met) |
| RNA Polymerase | Single simple enzyme (4 subunits: α2ββ’ω + σ) | Complex multi-subunit enzyme (8–12 subunits); TBP and TFB homologs | Three distinct RNA Pols (I, II, III; 12–17 subunits each) |
| Antibiotic Sensitivity | Sensitive to Rifampicin, Ampicillin, Streptomycin | Insensitive to bacterial antibiotics; sensitive to Diphtheria toxin | Insensitive to bacterial antibiotics; sensitive to Diphtheria toxin |
Figure 1: Bacterial Divisome Z-Ring Dynamics & Rotary Flagellar Motor Ultrastructure
Prokaryotic Cytology
Figure 1: Left: FtsZ Z-ring cytokinetic ring assembly regulated by MinCDE pole-to-pole oscillation and nucleoid occlusion. Right: Bacterial flagellar rotary motor driven by MotA/MotB proton motive force channels.
2. Bacterial Division: The FtsZ Divisome Machinery
Bacterial cytokinesis is orchestrated by FtsZ, a structural tubulin homolog that polymerizes into a dynamic contractile ring (the Z-ring) at the mid-cell cortex:
- MinCDE System: MinCD oscillates rapidly from pole to pole, inhibiting FtsZ assembly at cell poles and restricting Z-ring formation strictly to the exact cell midpoint.
- Nucleoid Occlusion (SlmA / Noc): Prevents FtsZ ring constriction across unsegregated chromosomes, preventing DNA bisection.
- Divisome Recruitment: The Z-ring recruits FtsA, ZipA, and peptidoglycan transglycosylases/transpeptidases (FtsI / PBP3) to synthesize the dividing septum.
3. Bacterial Motility: The Flagellar Rotary Engine
The bacterial flagellum is a true rotary nanomotor driven by proton motive force ($H^+$) or sodium motive force ($Na^+$) across the plasma membrane:
- Basal Body: Contains MS and C rings embedded in the inner membrane, surrounded by MotA/MotB stator proton channels that generate torque.
- Hook & Filament: Hook acts as a flexible universal joint; the long helical filament is composed of thousands of copies of flagellin.
- Chemotaxis (CheA/CheY): Counter-clockwise (CCW) rotation bundles filaments for directional swimming (“runs”); clockwise (CW) rotation disperses filaments, causing random reorientation (“tumbles”).
Bacterial Endospores
Under severe nutrient starvation, Gram-positive Firmicutes (e.g., Bacillus, Clostridium) initiate asymmetric sporulation, producing metabolically dormant endospores dehydrated to ~10–25% water and saturated with calcium dipicolinate and SASPs (Small Acid-Soluble Proteins). Endospores withstand boiling, desiccation, UV radiation, and autoclaving at 121°C for 20 minutes.
