INF2 - E. VIRUSES-COVERED

Question 1

what are viruses

Answer 1

small, sub cellular organisms with an obligate intracellular lifestyle ie - need to live in our cells to replicate

between living and non-living

Question 2

what size are viruses

Answer 2

20-100 nm
smaller than bacteria and animal cells

Question 3

where are viruses found

Answer 3

ubiquitous
each of us is infected with at least 2 different viruses ie - HSV-1 or 2
viral infections are persistent (ie always infected as they incorporate their genome into our DNA)

Question 4

properties of viruses

Answer 4

1. DNA or RNA packaged into particles for transmission between hosts
2. viral genome contains info needed for replication within a permissive host cell
3. viral survival ensured by establishing its genome in a host cell
4. obligate intracellular parasites: need the cellular machinery to replicate themselves

Question 5

what are the 2 ways viruses can be classified by

Answer 5

1. absence/presence of an envelope (envelope or naked)
2. shape of capsid (protein) - helical or icosahedral (20 sided)

Question 6

why icosahedral shape

Answer 6

• nearest geometrical shape to a sphere
• highest volume/surface ratio (ie lots can be packed without using too much protein)
• only one gene needed: multiple repetitions of single protein

Question 7

how do viral capsids self-assemble

Answer 7

• each subunit has identical bonding contacts with its neighbouring proteins (ie easily bonds)
  achieved by symmetrical assemblies of oligomers
• proteins linked by non-covalent bonds so rapid assembly and disassembly

Question 8

what is the function of the capsid

Answer 8

1. protection for nucleic acid outside cell
   - capsid made from multiple subunits, not covalently linked
   - symmetry provides maximal contact points
   - capsid is metastable: unfolds in cells as conditions like pH change
2. provides specific attachment to cell receptors (for naked)

Question 9

differences between naked and enveloped regarding transmission

Answer 9

• enveloped are sensitive to dryness, heat, detergents, acids
• must stay wet to be transmitted (air droplets, warm air breathe in), don’t typically survive in GI tract eg - HIV1
• naked are resistant to dryness, heat, detergents, acids, proteases
• can remain infective upon drying, on fomites, survive in GIT and surfaces eg - Adenovirus

Question 10

key terms

Answer 10

• capsid: protein shell containing viral genome
• nucleocapsid: capsid containing nucleic acid
• capsomer: individual protein molecules which form capsid
• envelope: lipid bilayer derived from host cells around capsid
• tegument: additional protein layer located between capsid and envelope
• virion: complete, infective viral particle outside the cell

Question 11

attachment of viruses

Answer 11

• bind to specific receptors on host cells
  HIV1 to CD4 and CXCR4 (co-receptor) on T-cells
  influenza virus to sialic acid on respiratory epithelium
• specific cell type/tissues = tropism
• specific host range: ability of virus to infect different species

Question 12

uncoating of viruses

Answer 12

• fusion of viral envelope with hosts cells plasma membrane followed by capsid disassembly
• naked viruses: endocytic uptake through a endosome (vesicle), pH drops in endoscope and clathrin disassembles and virus released

Question 13

reproduction of viruses

Answer 13

• make new copies RNA/DNA
• produce viral proteins and enzymes
• assemble and release new viral particles
• replicate their nucleic acid, packaged into new capsids, released
• virus has small number of genes and relies on host cell machinery to replicate
• begins immediately after viruses have entered host cell and release genetic material
• eclipse phase: incubation period

Question 14

assembly and maturation of viruses

Answer 14

• assembled into new virions
• spontaneous self-assembly process
• may still be non-infectious so maturation has to occur
• maturation: proteolytic cleavage by viral or cellular proteases
  ie - HIV gag polyprotein by HIV protease

Question 15

release of viruses

Answer 15

• budding eg HIV
• cell bursts open: lytic viruses (cell lysis then occurs of virus) eg - polio virus

Question 16

how do viruses cause cellular damage

Answer 16

direct damage to infected cells: cytopathic effects

1. cell lysis: cells burst open to release new virions
2. cell fusion: several cells fused as viruses move from one to another - multi-nucleated syncytia
3. transformation: DNA or RNA tumour viruses may medicate multiple changes that convert a normal cell into a malignant one (oncogenic viruses)
4. DNA damage: breakage, fragmentation, rearrangement and changes in no of chromosomes may occur of our own cells (genotoxic effects - mutations - cancer)