A 2.3 - VIRUSES

• What are viruses?

  non-cellular agents that infects cells and reproduces inside them
• Small size viruses

  between 20 - 300nm (smaller than almost all bacteria) and must be smaller than host cells to allow entry into them (are small as they lack cytoplasm and other structural features)
• fixed size

  do not grow or increase in size and is assembles in a host cell (fixed number of components)
• nucleic acid as genetic material


  all viruses have genes made of DNA or RNA which synthesize proteins (nucleic acid-to-polypeptide translation)
• capsid made of protein


  before viruses are released, their genetic material is enclosed in a protein coat (capsid) which gives viruses a symmetrical structure and protect genetic material
• what is a capsid made of

  repeating protein subunits
• metabolism of viruses

  viruses use host cell's metabolism and thus have no cytoplasm and contain few/no enzymes
• what are viruses structures like?

  diverse in shape and structure - no genes occur in all viruses
• what are the 2 evolutionary origins that scientists have deduced that viruses have

  diversity of genetic material and enveloped and non-enveloped viruses
• diversity of genetic material in a virus

  either DNA or RNA and could be single or double stranded; variation in length of nucleic acid and may be circular (no ends) or linear (2 ends)
• what are the 3 methods single-stranded RNA viruses use

  positive-sense RNA, negative-sense RNA and retrovirus
• positive sense RNA

  Single-stranded RNA genomes ready for immediate translation into proteins (use genes directly as mRNA)
• negative sense RNA


  RNA genomes that need to be converted into the proper form to be made into proteins (transcribe their genes to make mRNA)
• Retrovirus


  make double stranded DNA copies of their RNA genes and then transcribe the negative-sense strand of the DNA to produce mRNA
• what must happen for viruses to be released from their host cell?

  viruses must burst in process called lysis or can be released by budding (where they become covered in membrane)
• What is a non-enveloped virus?

  -lacks a bilayer lipid membrane-heat resistant
• What is an enveloped virus?

  virus that contains additional layers around the nucleocapsid
• What is the lytic cycle?

  phage causes lysis and death of host cell
• What is a bacteriophage?

  A virus that infects bacteria
• structure of bacteriophage?

  proteins at the tips of its tails which bind to the outer surface of its host
• how does DNA enter host cell from bacteriophage

  through its tubular tail (viral DNA has a single stranded coil)
• what happens to DNA when in host?

  link by base pairings to convert the molecule from a linear to circular form
• lysogenic cycle


  the viral DNA becomes integrated into the bacterial DNA molecule, so new whole virus particles are not produced
• lytic cycle


  reproduces and then bursts out of the host cell, killing the host cell
• what cycle do most viruses follow (in animal and plant cells)

  lytic cycle
• what type of virus is bacteriophage lambda

  DNA virus that uses either bacterium or an Archean as its host
• is bacteriophage enveloped?

  it is non-enveloped
• genetic material in a bacteriophage

  one double-stranded DNA molecule with positive and negative sense strands and over 48,000 base pairs
• distinctive features of bacteriophage

  follow either a lytic (reproduces and kills host) or lysogenic cycle (integrate DNA and not kill host)
• host of bacteriophage

  Escherichia coli (E-Coli)
• what type of virus is COVID-19

  RNA virus with crown-like shape that uses an animal cell as its host
• Is covid-19 enveloped or non-enveloped?

  enveloped
• genetic material in COVID-19

  one single-stranded positive sense RNA molecule with over 29000 bases
• distinctive features of COVID-19

  caused a pandemic in 2020 - example of zoonosis because it was passed to humans from another species
• Host of COVID-19

  human cells and possible other cells in other mammals
• What type of virus is HIV?

  retrovirus which converts RNA genome to DNA after infecting a host
• Is HIV enveloped or non-enveloped?

  enveloped
• genetic material in HIV

  2 copies of single stranded positive-sense RNA molecule of over 9000 bases
• distinctive features of HIV

  contains enzyme reverse transcriptase which makes double stranded DNA copy of viral RNA genome; then integrated into host cell chromosome
• host of HIV

  T-helper cells in the human immune system