Intro

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Viruses - nucleic acid contained within a protective protein coat

*viruses are not affected by antibiotics and cannot replicate outwith a cellular environment (if antibiotics are used to treat viral infection, it is used to treat secondary bacteria infection

Viruses are smaller than bacteria - huge variety in shape and structure, eahc virus has evolved to live in its own niche

Bacteriophages - viruses that infect bacteria

replicate via assembly of preformed components within the infected cell *not by binary fission

Genome (copies of virus nucleic acid) + caspid (virus-encoded proteins) = nucleocaspid (some enveloped)

*some viruses are enveloped with envelope glycoproteins

Lipid bilayer of capsid is derived from cellular membranes

Enveloped virus is sensitive to attack in the environment and more easily destroyed by temp, pH and chemicals

Un-enveloped virus (foot and mouth etc) are much more hardy in the environment that enveloped virus (flu)

*Icosahedral Helical - common symmetry

Icosahedral (cubic):

capsid is made up of many identical repeated units that share the same proteins

Helical:

simple viruses look like old telephone cord (all are enveloped)

Why do viruses demonstrate symmetry/subunit construction?

thermodynamics

strength

space

simplicity in making new virions - almost an automatic process

the bigger the genome the more errors will occur

if more errors the more genomes that will not be able to replicate

repeated units means that if something goes wrong with one - other identical units can fulfil that function

*structure can be more complex in some viruses - pox viruses (multiple envelopes and capsid layers)

DNA of virus genomes can be single or double stranded - RNA or DNA

Can be segmented or non-segmented - important for mutation

Viruses that have bigger genome - come with cost but they can generate additional proteins which provides an advantage = balance

Virus Replication:

means by which virus is perpetuated

genetic changes occur during replication

aim of virus is to reproduce its genetic material

viruses are 100% reliant on hosts to replicate themselves

*Viruses must infect cells which support viral replication in order to multiply (entry mechanism - receptors + intracellular physiology)

Viruses replicate using assembly of pre-formed components inside infected cells - different viruses exploit the host cell in different ways to replicate

Entry into cell - virus has to attach to a permissive cell that supports its replication

Stabilisation of binding

Uncoating - processes that allow the genome to be unraveled so it is available for expression within the cell

viral proteins made

once plenty of copies have been made - need structural proteins to make new viral particles

process of assembly of genomes and proteins and/or envelope

maturation steps - protein changes (change particle into form that is infectious)

Release of virus particle

*most DNA viruses replicate in nucleus

*most RNA viruses replicate in cytoplasm

Viruses are released by budding - assembled within the cytoplasm and as they exit the cell (via exocytosis) they require their envelope from the plasma membrane

Tissue tropism - some cells within the body will be more liable to be infected and host the virus

Transport systems can be hijacked by viruses in order to travel to distal cells/organs via blood and lymphatics or infect nerves

Effects of viruses on hosts:

cel/tissue destruction (direct or ini​​​​​​direct) - due to replication and lysis

virus induced immunopathology/immunesuppression

viral oncogenes - lead to tumour genesis/genetically transform cells

increased susceptibility to other pathogens

latent infection (sub clinical) - welfare/production losses

Summary:

•Structural and non structural features of viruses define the cells and hosts they infect [and influence the diagnosis and control of virus disease]

•Viruses rely on cells for replication

•Wide range of replicative strategies used by viruses – common stages

•Viral infection can have many consequences at cellular and host level

•Clinical consequences depend on interplay between virus and host