14 Bacteria

• Asexual reproduction by binary fission in a typical prokaryote

  Mechanism of binary fission
• Gene expression regulation in prokaryotes

  1. Regulation through simple operons (including lac and trp operons)

  2. Role of regulatory genes

  3. Distinguishing between inducible and repressible systems
• Most bacterial cells are very small (about 0.2 µm in diameter and 2-8 µm in length) and are unicellular
• Structure and organisation of prokaryotic genome

  • DNA is double-stranded
  • Number of nucleotides
  • Packing of DNA
  • Circularity
  • Absence of introns
• Transformation, transduction, and conjugation give rise to variation in prokaryotic genomes
• Endosymbiont theory explains the development of membranous organelles in eukaryotes
• Transformation, transduction, and conjugation in prokaryotes

  Role of F plasmids in transformation, transduction, and conjugation
• Structure of a typical bacterial cell


  • Small and unicellular
  • Peptidoglycan cell wall
  • Circular DNA
  • 70S ribosomes
  • Lack of membrane-bound organelles
• Internal structures of bacteria

  • Nucleoid
  • Ribosomes
  • Storage granules
  • Plasmid(s)
• Ribosomes

  • Sites of protein synthesis
  • 70S in bacteria made up of 50S and 30S subunits
• Classification of bacteria based on morphology

  • Coccus (spherical)
  • Bacillus (rod-like)
  • Spirochete (spiral)
  • Filamentous (elongated)
• Prokaryotic genome structure and organization

  • DNA is double-stranded
  • Number of nucleotides varies
  • DNA is packed into a dense region called the nucleoid
  • DNA is circular
  • Absence of introns
• Storage granules

  Nutrients and chemical reserves stored in the cytoplasm
• Cell Surface Membrane

  • Comprised of a phospholipid bilayer
  • Site of electron transport chains and enzymes for respiration and photosynthesis
• Bacterial cells are very small, about 0.2 µm in diameter and 2-8 µm in length, and are unicellular
• Plasmid(s)


  • Small, circular, autonomously replicating DNA molecules
  • May contain genes conferring advantages to bacteria
  • Used in genetic engineering as vectors for introducing foreign DNA
  • Bacterial genome includes both chromosomal and plasmid DNA
• Nucleoid

  • Dense region in the bacterial cell where chromosomal DNA is generally confined
  • Main component of the genome is a double-stranded, circular DNA molecule associated with DNA-binding proteins
  • DNA forms loop domains anchored by proteins and undergoes supercoiling
  • Bacterial chromosome has no introns
• Sugar components of prokaryotic cell surface membrane

  • Alternating units of NAG (N-acetylglucosamine) & NAM (N-acetylmuremic acid)
• Electron transport chains in bacterial cell membrane

  Enzymes involved in respiration and photosynthesis (including ATP synthase) are embedded in the membrane to produce ATP
• Cell wall protects the cell from osmotic lysis
• Flagella are long appendages for motility in bacteria
• Some bacteria have a layer of polysaccharides known as the glycocalyx to the exterior of the cell wall
• Sex pilus allows genetic material transfer between bacterial cells in conjugation
• Cell membrane folding to form mesosome

  Increases the surface area for embedded enzymes to carry out respiration
• Prokaryotic cell wall is composed of peptidoglycan, preventing osmotic lysis and conferring rigidity and shape to cells
• Fimbriae are short, bristle-like fibres for attachment to surfaces or other bacteria/organisms
• In Gram-positive bacteria, the cell wall comprises a thick peptidoglycan layer, which retains the dye
• Bacteria may be classified as Gram-positive or Gram-negative based on staining by the Gram stain
• Glycocalyx can be a distinct layer (capsule) or a diffused mass (slime layer)
• In Gram-negative bacteria, the cell wall includes a thin peptidoglycan layer, followed by an additional outer membrane
• Components of the cell wall

  • Peptidoglycan
• Fluid mosaic model


  Applies to prokaryotic cell surface membrane as well
• Pili are longer structures involved in motility and DNA transfer in bacteria
• Capsule may protect bacteria from being engulfed via phagocytosis and enable adhesion to surfaces
• Chromosome in prokaryotic cells is a single, circular, double helix DNA located within the nucleoid region without a membrane-bound nucleus
• Capsule


  • Composed of polysaccharides and sometimes polypeptides; provides protection against phagocytic engulfment; enables attachment to surfaces; contains water to prevent desiccation
• Organelles

  • No membrane-bound organelles
• Cell wall


  • Composed of peptidoglycan; prevents osmotic lysis of cell protoplast and confers rigidity and shape to cells
• Appendages: Fimbriae


  Attachment to surfaces and to other bacteria/organisms
• Binary fission is the asexual means by which bacterial cells produce genetically identical offspring. This can be a selective advantage in a stable, favourable environment as it allows successful genotypes to rapidly reproduce and colonise a habitat. Before one bacterial cell splits into two independent cells, the bacterial chromosome must first replicate. Each daughter cell receives one copy of the chromosome as the cell membrane pinches inward to divide the parent cell into