M2:T1 viruses

• define viruses

  • non-cellular, sub-microscopic infectious agents (10-330nm in size)
  • they are obligate parasites <—outside host cell = dormant
• what cells do viruses infect

  • all cells
  • bacteria
  • archeal
  • eukaryotoes
• can viruses reproduce on their own

  no because they are metabolically inert and lack independent metabolism
• what are the 2 types of virion 


  1.naked

  2. envelope

  • both have a capsid “protein coat“ with lipid layer around it
• capsid 


  -protein coat

  • made up of capsomeres
• nucelocapsid

  • genome and capsid surrounding it
• list what some viruses have

  1. lipid envelopes
  2. serval parts
  3. enzymes ie. NA polymerase and lysosome
  4. genome: DNA or RNA
• 2 types of capsid

  1. isoherdal: symmetrical spherical shape ie. HPV
  2. helical: protein caposomer wrap around genome
• define envelope virus

  • they are enclosed by lipid envelope bilayer
  • mainly infect animal cells
  • have specific viral proteins
• what is the classification viruses based on

  1. NA characteristics
  2. characteristics of host
  3. size
  4. architecture of capsid
  5. possession of lipid envelope
• what is the Baltimore classification based on

  1. mRNA
  2. preduction of RNA
  3. genome replication - transcprit or translate
• class 1

  • double stranded DNA
  • transcibe —> translate +ve sense mRNA
• class 2

  • single stranded DNA (+ve sense)
  • synthess of -ve —> +ve
  • 5’ —> 3’
• class 3

  • double stranded RNA
  • transcipt —> mRNA
• Class 4

  • single stranded RNA
  • mRNA is directly used
• class 5

  • single stranded virus
  • 3’ —> 5’
• class 6

  • single stranded RNA retrovirus
  • rerevse transcription
• class 7

  • double stranded DNA
  • different replication method to class 1
  • transcribes mRNA but has reverse transciption
• semiconservative replication 


  1 strand acts as template = complmentary strand
• +ve sense 


  5’ —> 3’
• -ve sense 


  3’ —> 5’
• when do virus appear

  1. viruses arisen after cell remnant of a cell
  2. RNA word - play a role in transition
  3. 1st from of life
• why did viruses appear

  Rapid movement of genes
• different types of bacteriophages

  • target proteins

  1. SS RNA/DNA
  2. DS RNA/DNA
• Define infectious process

  • viral growth
  • very specific
  • differs b/w pro and eukaryotes
  • only genome enters the viruse
• 2 types of viruses

  1. lytic virus: kills host
  2. temperature virus “lysogenic“: doesn‘t kill host
• stages of lytic virus

  1. attach to host
  2. penerate
  3. synthesis
  4. assembly
  5. lysis
• stagets of temperate virus

  1. attaches and inject genome
  2. genome integrates with chromosomes = phage
  3. lysogenic reproduce
  4. prophase excise and initates lytic
• prophage 


  when genome intergrates with chromosome by recombination
• explain tails on receptors


  tails attached to receptor which has ly some to breakdown cell wall
• stages of viruses

  1. Eclipse: DNA replication and translation
  2. maturation: packaging
• a virus 


  can be seen under an electron microsopce
• mature virus is called virion
• bacteriophage is 


  virus that infects bacteria
• lysogeny 


  phage DNA is incorporated into host cell DNA
• why antibiotics are ineffective against viruses

  • cell wall, ribosomes, DNA replication and envelope for all protection
• why are virises not included in domains

  • non-cellular infectious particles
  • do not have SSU rRNA which is what domains catergories