Prokaryotic Cells and Viruses


Differences between a prokaryote and a eukaryote

  • The cytoplasm of a prokaryote has no membrane-bound organelles (unlike a eukaryote)
    • It has ribosomes, but these are slightly smaller (70S) than eukaryotic ribosomes (80S)
  • Some prokaryotes have a flagellum, which is a long, hair-like structure that rotates to make the cell move. Not all have one and some have a few
  • Doesn't have a nucleus - instead has a strand of circular DNA. Not attached to any histone proteins
  • Small loops of DNA called plasmids (similar to flagella, not always present) contain genes for antibiotic resistance and can be passed between other prokaryotes
  • Still has a plasma membrane
  • A cell wall is present and is made with murein, a glycoprotein
  • Some prokaryotes have a slime capsule to protect them from attack from the immune system
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Binary Fission

  • Used by prokaryotes
    • The circular DNA and plasmid(s) replicate. The main DNA loop is only replicated once, but plasmids can be replicated loads of times
    • The cell gets bigger and the DNA loops move to opposite poles of the cell
    • The cytoplasm begins to divide (and new cell walls begin to form)
    • The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but can have a variable number of copies of plasmids
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  • Acellular - not a cell
  • Simply just nucleic acids surrounded by protein
  • Much smaller than bacteria, HIV is just 1um across
  • No plasma membrane, no cytoplasm and no ribosomes
  • They cannot reproduce on their own and need to reproduce inside cells
    • They use attachment proteins to bind to complementary receptor proteins on the surface of host cells
    • Different viruses have different attachment proteins and therefore can only hook onto one type of cell (some can infect a lot of different cells)
    • Because they're not alive, viruses don't undergo cell division - instead they inject their DNA or RNA into the host cell, then the cell uses its own organelles to replicate the viral particles
  • Viruses contain a core of genetic material - either DNA or RNA
  • The protein coat around the core is called the capsid
  • Attachment proteins stick out from the edge of the capsid
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