Biology

• Structure of a typical bacterial cell


  • Small and unicellular
  • Peptidoglycan cell wall
  • Circular DNA
  • 70S ribosomes
  • Lack of membrane-bound organelles
• Gene expression regulation in prokaryotes

  1. Concept of simple operons (including lac and trp operons)

  2. Role of regulatory genes

  3. Distinguishing between inducible and repressible systems
• Prokaryotes are divided into two domains: archaea and bacteria
• Endosymbiont theory provides a plausible explanation for the development of membranous organelles in eukaryotes
• Asexual reproduction by binary fission in a typical prokaryote

  Mechanism of binary fission
• Transformation, transduction, and conjugation

  Role of F plasmids in giving rise to variation in prokaryotic genomes
• Structure and organisation of prokaryotic genome

  • DNA is double-stranded
  • Number of nucleotides
  • Packing of DNA
  • Circularity
  • Absence of introns
• Endosymbiont theory suggests that organelles like mitochondria and chloroplasts were formerly free-living prokaryotes taken up by another cell, leading to a symbiotic relationship
• Prokaryotes generally lack membrane-bound organelles and endomembrane systems
• 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
• Cell Surface Membrane

  • Comprised of a phospholipid bilayer
  • Site of electron transport chains and enzymes for respiration and photosynthesis
• Storage granules

  Nutrients and chemical reserves stored in the cytoplasm
• 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
• Ribosomes

  • Sites of protein synthesis
  • 70S in bacteria, made up of 50S and 30S subunits
• Bacterial cells are very small, about 0.2 µm in diameter and 2-8 µm in length, and are unicellular
• Internal structures of bacteria

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


  • Small, circular, autonomously replicating DNA molecules
  • May contain genes conferring advantages like antibiotic resistance
  • Used in genetic engineering as vectors for introducing foreign DNA
  • Bacterial genome includes both chromosomal and plasmid DNA
• In Gram-positive bacteria, the cell wall comprises a thick peptidoglycan layer, while in Gram-negative bacteria, the cell wall includes a thin peptidoglycan layer followed by an additional outer membrane
• The glycocalyx can be a distinct layer called the capsule or exists as a diffused mass known as the slime layer
• Electron transport chains in bacterial cell membrane

  Enzymes involved in respiration and photosynthesis (including ATP synthase) are embedded in the membrane to produce ATP
• Components of the cell wall

  • Peptidoglycan
• Fimbriae are short, bristle-like fibres for attachment to surfaces or other bacteria/organisms
• Functions of the glycocalyx include immune evasion and cell/surface adhesion
• Bacteria may be classified as Gram-positive or Gram-negative depending on the color they are stained by using the Gram stain
• Structural features of prokaryotic (bacterial) cells include cell wall composed of peptidoglycan, location of chromosome in nucleoid region, and a single, circular, double helix DNA chromosome
• Sugar components of prokaryotic cell surface membrane

  • Alternating units of NAG (N-acetylglucosamine) & NAM (N-acetylmuremic acid)
• Cell membrane folding to form mesosome

  Increases the surface area for embedded enzymes to carry out respiration
• Cell wall protects the cell from osmotic lysis
• Some bacteria have a layer of polysaccharides known as the glycocalyx to the exterior of the cell wall
• The capsule may contain proteins for recognition and adhesion
• Flagella are long appendages for motility in bacteria
• Pili are longer and fewer in number than fimbriae and are involved in motility and DNA transfer
• Chromosome


  • A single, circular, double helix DNA – supercoiled
• Ribosomes


  • 70S (vs 80S in eukaryotes)
• Organelles


  • No membrane-bound organelles
• Plasmids


  • Extra-chromosomal DNA that replicates autonomously; quantity can range from five to several hundred
• Table 3 – Structural differences between prokaryotic and eukaryotic cells
• Appendages

  1. Flagella: Swimming movement; propulsion

  2. Fimbriae: Attachment to surfaces and to other bacteria/organisms

  3. Pili: (i) Mediates DNA transfer during conjugation (sex pilus) (ii) Motility by retraction
• Slime layer


  • Composed of polysaccharides and sometimes polypeptides; enables attachment to surfaces and formation of biofilms