#prokaryotes

Glycolipid and Phospholipid

Which lipid is absent in most prokaryotic and plant membranes?

Evolutionary lipidomics reveals sterol distribution discontinuous across domains life reflecting oxygen dependent biosynthesis pathway complexity. Eukaryotes animals synthesize cholesterol via HMGCoA reductase squalene monooxygenase epoxidase lanosterol fourteen demethylase CYP51 DHCR enzymes pathway requiring molecular oxygen maintaining thirty to forty percent plasma membrane buffering fluidity organizing rafts signaling. Fungi use ergosterol additional double bonds methyl group phytosterol plants stigmasterol sitosterol campesterol from cycloartenol SMT sterol methyltransferase modification. Most bacteria lack sterol biosynthetic enzymes squalene cyclization and sterol auxotrophy; membranes composed phosphatidylethanolamine phosphatidylglycerol cardiolipin hopanoids pentacyclic triterpenoids surrogate sterol function rigidify Methylococcus planctomycetes providing order stability. Mycoplasma pneumoniae lacking cell wall scavenges cholesterol host serum incorporating membrane requiring exogenous cholesterol growth. Sphingomyelin also predominantly animal glycosphingolipid rare prokaryotes. Therefore cholesterol truly absent most prokaryotic plant membranes explaining differential susceptibility cholesterol binding toxins perfringolysin O Streptolysin O polyenes amphotericin B selective antimicrobial strategies. Phylogeny underlies membrane evolution topics microbiology textbooks and cell biology comparisons across kingdoms for structure function diversity.

Ref: Nes WD, Chem Rev 2011, Sterol biosynthesis diversity bacterial absence cholesterol and hopanoids.

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Cytoskeleton

The actin cytoskeleton in prokaryotic cells is functionally similar to which of the following eukaryotic proteins?

Prokaryotic cells maintain rod shape and organize cell wall synthesis without eukaryotic compartments using cytoskeletal homologs sharing ancient ATPase fold. MreB, belonging to actin Hsp70 superfamily, shares structural core of five conserved sequence motifs forming ATP binding pocket between domains IA and IIA. Purified MreB polymerizes into ATP dependent antiparallel double filaments that rotate circumferentially beneath cytoplasmic membrane driven by cell wall elongation machinery. It scaffolds elongasome complex containing RodA glycosyltransferase, PBP2 transpeptidase and MreC MreD, directing insertion of new peptidoglycan hoops to maintain lateral wall. Depletion or inhibition by small molecule A22 causes rod to sphere transition confirming morphogenetic role, similar to actin controlling shape in eukaryotes. Keratin and vimentin are metazoan intermediate filament proteins absent in most bacteria and archaea, while ActA is Listeria monocytogenes surface protein that activates host Arp2/3 complex to form actin comet tails for motility, not bacterial cytoskeleton. Thus prokaryotic actin analog best matching functional criteria is MreB protein.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Cytoskeleton and Bacterial Homologs MreB.

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Microbial Physiology

Which domain of life do Archaea belong to?

Archaea represent a distinct domain of life alongside Bacteria and Eukarya, established by Carl Woese through comparative analysis of 16S ribosomal RNA sequences. Morphologically they resemble bacteria as prokaryotes lacking membrane-bound nucleus, typically possessing circular chromosomes and 70S-like ribosomes, yet they exhibit unique molecular features in transcription, translation, and membrane composition that separate them from true bacteria. For educational simplification, archaea are often grouped under broad term prokaryotes to contrast with eukaryotes that contain nuclear envelope, linear chromosomes, and spliceosomal introns. This informal grouping reflects absence of nucleus and size range similar to bacteria, despite deep evolutionary divergence. Some textbooks still use prokaryote as umbrella term encompassing both Bacteria and Archaea, while acknowledging three-domain classification. Thus when choices restrict to prokaryote versus eukaryote, archaea are classified among prokaryotes, recognizing their lack of nuclear membrane and mitotic apparatus, though modern systematics emphasizes independent domain status with distinct information processing machinery. This informal umbrella usage persists in teaching materials contrasting prokaryotic organization with eukaryotic traits like mitosis, splicing, and membrane-bound organelles, even though molecular phylogeny supports three distinct domains with Archaea closer to Eukarya in information processing enzymes.

Ref: Woese & Fox, PNAS 1977, Phylogenetic Structure of Prokaryotes; Madigan et al., Brock Biology, Archaea Domain.

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Microbial Physiology

The bacterial reproduction process of binary fission differs from mitosis in that:

Binary fission represents prokaryotic division program fundamentally distinct from eukaryotic mitosis in mechanism and regulation. Initiation begins with replication of single circular chromosome from unique origin oriC proceeding bidirectionally via replisomes containing DNA polymerase III. Segregation of origins is mediated by ParABS partitioning system and SMC condensin complexes moving DNA toward opposite poles as cell elongates via MreB-guided peptidoglycan insertion. Division site selection uses precise regulatory systems: oscillating MinCDE proteins concentrate MinC inhibitor at poles preventing polar Z-ring assembly, and nucleoid occlusion proteins SlmA in E. coli and Noc in B. subtilis bind chromosomal DNA blocking FtsZ polymerization over nucleoids, ensuring tubulin homolog FtsZ assembles into contractile Z-ring only at mid-cell between separated chromosomes. FtsZ then recruits FtsA, ZipA, FtsQ, FtsL, FtsW and FtsI transpeptidase that synthesize septal wall and drive constriction via GTP hydrolysis. No microtubule spindle, kinetochores, centrosomes or chromosome condensation occurs, and daughters are monoploid identical cells, making absence of spindle the defining difference from mitosis.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 4: Binary Fission and Absence of Spindle Apparatus.

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Regulation of Gene Expression - Lac Operon

Which of the following correctly describes an inducible operon?

An inducible operon (such as the lac operon) is normally off but can be activated by an inducer like lactose.