Prokaryotic Cells and Viruses

• Viruses have evolved to exploit the receptor proteins on the host cell. They play a roll in the normal workings of the cell.
• Prokaryotic cells


  Single-celled organisms that are different from eukaryotic cells
• Prokaryotic cells


  • Smaller and simpler than eukaryotic cells
  • Do not have membrane-bound organelles like a nucleus in their cytoplasm
• Bacteria


  Examples of prokaryotes like E. coli
• Structures in a prokaryotic cell


  • Cell wall
  • Cell membrane
  • Circular DNA
  • Plasmids
  • Flagella
• Prokaryotic cells are extremely small, less than 2 μm in diameter
• Eukaryotic cells can be up to 50 times bigger than prokaryotic cells
• Binary fission in prokaryotic cells

  1. Replication of circular DNA and plasmids

  2. DNA loops move to opposite poles of the cell

  3. Cytoplasm begins to divide and new cell walls begin to form

  4. Cytoplasm divides to produce two daughter cells each with one copy of circular DNA and variable copies of plasmids
• Viruses


  Acellular and non-living, consisting of nucleic acid surrounded by protein
• Viruses


  • Have no cell-surface membrane, no cytoplasm and no ribosomes
  • Have a protein coat called a capsid with attachment proteins to cling onto host cells
  • Smaller than bacteria, e.g. HIV is about 0.1 μm across
• Viral replication


  1. Viruses inject their DNA or RNA into the host cell

  2. The host cell's machinery is hijacked to replicate viral particles
• Viruses do not undergo cell division, instead they inject their DNA or RNA into the host cell
• Viruses can only infect certain types of host cells, some can infect many different cell types while others are specific to one species
• Viral attachment proteins allow viruses to attach to and enter host cells
• The cell-surface membrane of a prokaryotic cell is mainly made of lipids and proteins.
• The cell wall of a prokaryotic cell is made of a polymer called murein.
• Murein is a glycoprotein.
• The flagellum is a long hair-like structure that rotates to make the prokaryotic cell move.
• The DNA of a prokaryotic cell is not attached to any histone proteins.
• Plasmids contain genes for things like antibiotic resistance, and can be passed between prokaryotes.
• Some bacteria have a capsule made up of secreted slime. It helps protect the bacteria from attack by cells of the immune system.
• Viruses are nucleic acids surrounded by protein.
• Examples of viruses:

  • HIV (causes AIDs)
  • influenza (causes flu)
  • rhinoviruses (causes colds)
• Host cells are the cells viruses reproduce within.
• Viruses have a protein coat called a caspid with attachment proteins sticking out from it.
• The attachment proteins let viruses cling onto a suitable host cell.
• Viruses use the host cells machinery to make copies of themselves.
• Viruses use attachment proteins to bind to complimentary receptor proteins on the cell-surface membrane.
• Eukaryotic cells


  Larger cells with a nucleus bounded by nuclear membranes (nuclear envelope)
• Prokaryotic cells


  Smaller cells with no nucleus or nuclear envelope
• Bacteria


  • Occur in every habitat in the world
  • Versatile, simple and very successful
  • Small size, normally ranging from 0.1 to 10μm in length
• All bacteria possess a cell wall made up of murein, a polymer of polysaccharides and peptides
• Cell wall


  Physical barrier that excludes certain substances and protects against mechanical damage and osmotic lysis
• Capsule


  Protects bacterium from other cells and helps groups of bacteria to stick together for further protection
• Cell-surface membrane


  Acts as a differentially permeable layer, which controls the entry and exit of chemicals
• Circular DNA


  Possesses the genetic information for the replication of bacterial cells
• Plasmids


  Smaller circular pieces of DNA that may aid the survival of bacteria in adverse conditions, e.g. produce enzymes that break down antibiotics
• Ribosomes


  Smaller (70S) than those in eukaryotic cells (80S), but still synthesise proteins
• Bacteria store food reserves as glycogen granules and oil droplets
• Viruses are acellular non-living particles, smaller than bacteria, ranging in size from 20-300nm