• Viruses are the smallest living organism   10nm-300nm
  • they are v.hard to term dead or alive
  • All viruses have a protein coat (capsid) that surrounds the nucleic acid 
  • the nucleic acid can be RNA or DNA
  • there are many types of viruses including: HIV and bacteriophages

Viruses only replicate within a host cell (and thus destroying the host)

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  • A bacteriophage (phage) is a virus that has a DNA core
  • a phage attacks bacterial cells.
  • They contain DNA bounded by a protein coat
  • Phages invade the bacteria, inwhich they inject viral DNA 
  • DNA replicates and is packed into protein coats 
  • bacterial cells destroyed (as new phages released)


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Human Immunodeficiency Virus (HIV)


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structure - HIV consists of a core of RNA bounded by a protein coat and a lipid bilayer containing glycoprotein.

HIV is retrovirus - 'any group of RNA viruses which insert a DNA copy of their                                        genome into the host cell in order to replicate.'


  • HIV viruse deliver the RNA with the enzyme reverse transcriptase into the host cell.
  • The enzyme catalyses the synthesis of DNA from RNA
  • host cell makes copies of viruses
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What does HIV do to the body?

HIV invades a type of lymphocyte (T-helper cells) thus weakening the immune system.


HIV destroys T-cells thus damaging the body's ability to fight off invading germs and diseases. The weakened immune system is known as AIDs.


  • AIDs is the result of an HIV infection, but not every1 with HIV has AIDs.
  • people with HIV don't show symtoms
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