INF2 - E. VIRUSES-COVERED Flashcards
what are viruses
small, sub cellular organisms with an obligate intracellular lifestyle ie - need to live in our cells to replicate
between living and non-living
what size are viruses
20-100 nm
smaller than bacteria and animal cells
where are viruses found
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)
properties of viruses
- DNA or RNA packaged into particles for transmission between hosts
- viral genome contains info needed for replication within a permissive host cell
- viral survival ensured by establishing its genome in a host cell
- obligate intracellular parasites: need the cellular machinery to replicate themselves
what are the 2 ways viruses can be classified by
- absence/presence of an envelope (envelope or naked)
- shape of capsid (protein) - helical or icosahedral (20 sided)
why icosahedral shape
- 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
how do viral capsids self-assemble
- 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
what is the function of the capsid
- 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 - provides specific attachment to cell receptors (for naked)
differences between naked and enveloped regarding transmission
- 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
key terms
- 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
attachment of viruses
- 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
uncoating of viruses
- 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
reproduction of viruses
- 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
assembly and maturation of viruses
- 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
release of viruses
- budding eg HIV
- cell bursts open: lytic viruses (cell lysis then occurs of virus) eg - polio virus
