3.2.1.2 Structure of prokaryotic cells and of viruses

• Prokaryotes


  are single celled organisms — e.g bacteria like E. coli
• Prokaryotic cells are

  much smaller and simpler than eukaryotic cells — don't have any membrane bound organelles like a nucleus in their cytoplasm
• Just like in a eukaryotic cell the cell surface membrane is

  ● mainly made of lipids and proteins ● It controls the movement of substances into and out of the cell

  
• The cell wall supports


  ● the cell and prevents it from changing shape● It's made of polymer called murein — murein is a glycoprotein

  
• Glycoprotein


  a protein with carbohydrate attached
• Cytoplasm contains ribosomes


  but they're smaller than those in eukaryotic cell

  
• The flagellum (plural flagella)


  ● a long , hair like structure that rotates to make the prokaryotic cell move ● Not all prokaryotes have a flagellum ● Some have more than one

  
• Unlike a eukaryotic cell , a prokaryotic cell doesn't have

  ● a nucleus● the DNA floats free in the cytoplasm — It's circular DNA present as one long coiled - up strand not attached to any histone proteins

  
• Some prokaryotes also have

  ● a capsule made up of secreted slime ● It helps to protect the bacteria from attack by cells of the immune system— e.g bacteria

  
• Plasmids are small


  loops of DNA that aren't part of the main circular DNA molecule Plasmids contain genes for things like antibiotic resistance and can be passed between prokaryotes . Plasmids are not always present in prokaryotic cells . Some prokaryotic cells have several

  
• All prokaryotic cells contain

  ●Cell Wall (containing peptidoglycan) ●Cell surface membrane ●Cytoplasm ●Circular DNA ●Ribosomes

  
• Some prokaryotic cells contain

  ● Flagella — for motion● Capsule — for protection● Infolding of cell surface membrane — may allow photosynthesis or carry out nitrogen fixation● Plasmid — small circle of DNA● Pili — for attatchment to other cells or surfaces, involved in sexual reproduction

  
• Prokaryotic cells are extremely


  small — less than 2 µm in diameter
• Prokaryotic cells replicate by

  the process called binary fission

  
• In binary fission the cell

  replicates (makes copies of) its genetic material before physically splitting into two daughter cells

  
• Replicate


  make copies of
• Step 1 : The process of binary fission

  The circular DNA and plasmids replicate— The main DNA loop only replicated once — plasmids can be replicated loads of times

  
• Step 2 : The process of binary fission


  The cell gets bigger and the DNA loops move to opposite poles (ends) of the cell

  
• Poles

  ends
• Step 3 : The process of binary fission


  ● The cytoplasm begins to divide — new cell walls begin to form

  
• Step 4 : The process of binary fission

  ● 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 the plasmid(s)

  
• DNA and RNA are


  are nucleic acids
• There are lots of different viruses


  ● they can look very different from each other● Some viruses e.g. (HIV) also have an envelope which surrounds the capsid
• Viruses are

  ● acellular — they're not cells● nucleic acids surrounded by protein● not alive

  
• Examples of viruses include

  ● HIV — causes AIDS● influenza — causes the flu ● rhinoviruses — causes colds

  
• Unlike bacteria viruses

  have no cell surface membrane, cytoplasm or ribosomes

  
• All viruses


  invade and reproduce inside the cells of other organisms

  
• Host cells


  the cells of other organisms that the viruses invade and reproduce inside

  
• Viruses have

  ● core of genetic material — either DNA or RNA● capsid — protein coat● attachment proteins

  
• the basic structure of a virus

  ● have protein coat , called a capsid — with attachment proteins sticking out from it● attachment proteins let the virus cling onto suitable host cell ● Viruses are even smaller than bacteria — e.g. HIV is about 0.1 μm across

  
• Viruses don't undergo cell division because

  they're not alive

  
• Instead of cell division viruses

  inject their DNA or RNA into the host cell — this hijacked cell then uses its own ' machinery ' (e.g. enzymes, ribosomes) to replicate the viral particles— process is called viral replication

  
• The process of viral replication

  ● Virus attaches to host cell receptor proteins● Genetic material is released into the host cell● Genetic material and proteins are replicated by host cell 'machinery'● Viral components assemble● Replicated viruses released from host cell

  
• In order to inject their DNA or RNA viruses first have to

  attatch to the host cell surface — to do this they use their attachment proteins to bind complementary receptor proteins on the cell-surface membrane of the host cells

  
• Different viruses have

  ● different attachment proteins — therefore require different receptor proteins on host cells ● As a result some viruses can only infect one type of cell — e.g. only infect one species of bacteria while others can infect lots of different cells — e.g. influenza

  
• The receptor proteins on the host cells


  ● aren't just there to let viruses attach ● they're actually proteins that play a role in the normal workings of the cell — Viruses have just evolved to exploit them

  
• Different viruses enter and leave the cell in


  different ways — are replicated by the host cell in slightly different ways— e.g. HIV releases its capsid into the cell as well as genetic material
• Size Comparison

  ● Prokaryotes — 0.5-5 μm diameter● Eukaryotes — up to 100 μm diameter
• Genome Comparison

  ● Prokaryotes — DNA is circular with no proteins in the cytoplasm ● Eukaryotes — DNA is associated with histones
• Histones


  protiens formed into chromosomes