2.1.2 - Prokaryotic Cells & Viruses

• Prokaryotes are single celled organism
• bacteria are examples of prokaryotes
• prokaryotes are smaller and more simple than eukaryotes
• prokaryotes don’t have membrane bound organelles in their cytoplasm
• prokaryotes components:

  • cell surface membrane
  • cell wall (made of murein)
  • cytoplasm
  • ribosomes (smaller than eukaryotes)
  • flagella
  • circular DNA loop
  • plasmid DNA
  • slime capsule
• the flagellum (plural flagella) is a long hair like structure that rotates to allow the cell to move
• some prokaryotes have no flagella, while some have multiple flagella
• prokaryotes are extremely small - less than 2 micrometres in diameter
• eukaryotes can be up to 50 times larger than prokaryotes
• Prokaryotes replicate by binary fission
• in binary fission, the cell replicates it’s genetic material before splitting into 2 daughter cells
• process of binary fission

  1. circular DNA and plasmid DNA replicates (plasmids can be replicated many times, circular DNA can only be replicated once)
  2. cell gets bigger and DNA moves to opposite poles of the cell
  3. cytoplasm begins to divide (cytokinesis) and new cell walls begin to form
• each daughter cell produced by binary fission has one copy of the circular DNA but a variable number of copies of plasmids
• viruses are acellular (they are not cells)
• viruses are just nucleic acids surrounded by protein
• examples of viruses include HIV (human immunodeficiency virus), influenza (flu) and rhinoviruses (cause colds)
• all viruses invade and reproduce inside the cells of other organisms (host cells)
• viruses are comprised of:

  • genetic material (either DNA or RNA)
  • capsid (protein coat)
  • attachment proteins
• viruses are even smaller than bacteria - e.g. HIV is about 0.1 micrometres in diameter
• viruses are not living so don’t undergo cell division, instead they replicate by hijacking the reproductive system of the host cell and using it to replicate virus particles
• viral replication:

  1. virus attaches to host cell receptor proteins
  2. genetic material released into host cell
  3. genetic material and proteins are replicated by the host cell’s reproductive system
  4. viral components assemble
  5. replicated virus particle is released from the cell