Introduction to Virology

Virus Tropism

preference to infect specific cell types or organisms

respiratory viruses eg influenza, rhinovirus

usually acquired by droplet inhalation and replicate in the respiratory tract

enteric viruses eg polio, rotaviruses and reoviruses

replicate in the gut and cause gastric infection. Acquired by ingestion of faecal-contaminated material

arboviruses eg flaviviruses, bunyaviruses and some rhabdoviruses

infect insects which go on to ingest vertebrate blood. Replicate in insect tissue and are transmitted to the vertebrate host.

Why are plant viruses difficult to grow?

Study often requires growth of a whole plant

titer

number of infectious units per volume of fluid

Plaque Assay

Viral infectivity measured by counting plaques on host cell lawns, each indicating infection by a single virus particle.

Capsid

Protein coat of a virus formed by structural subunits. Large assemblies known as capsomers.

bacteriophage structure

icosahedral head and helical tail

What symmetry do most mammalian viruses exhibit?

Icosahedral, with 20 sides and 12 vertices

Virion

complete virus particle consisting of nucleic acid packaged into a protein coat (capsid)

diversity of virion structures

many different shapes, chemical compositions and sizes ranging from 0.018 to 0.3µm

nucleocapsid

nucleic acid and protein packaged into a virus

lysozymes

make hole in cell wall and lyse bacterial cells

neuraminidases

cleave glycosidic bonds, liberating viruses from the cell

Baltimore Classification

Viral classification system based on genetic makeup

class I (most bacteriophages, herpesviruses and poxviruses)

dsDNA

class II (eg parvoviruses)

ssDNA (+) sense

class III eg reoviruses

dsRNA

class IV eg picomaviruses and coronaviruses

ssRNA (+) sense

class V eg rhabdaviruses and influenza

ssRNA (-) sense

class VI eg retroviruses

ssRNA(+) sense with dsDNA intermediate

class VII eg hapadnaviruses

dsDNA with RNA intermediate

initiation phase

recognition, attachment, penetration and uncoating

recognition and attachment

Involves target cell receptors and viral attachment proteins. Virions often change structure when they bind receptor to expose hydrophobic amino acids so it can cross membrane

viropexis aka pore-mediated penetration (naked viruses only)

Virus attaches to cell and ejects its genome into the host cytoplasm

Endocytosis

viruses enter host cells by being engulfed in vesicles.

Membrane Fusion (enveloped viruses only)

virus fuses envelope with host cell membrane for penetration

Eclipse period

phase where virus penetrates and enters the cell, and capsid is no longer present

replication stage

transcription, protein synthesis, genome replication and assembly

nucleic acid polymerases present in all organisms

DNA dependent RNA polymerase and DNA dependent DNA polymerase

Why do many viruses promote cell growth?

smaller DNA viruses are more dependent on the host cell for replication

What regulates viral transcription?

interaction of DNA binding proteins with regions of the viral genome

What is transcribed first?

mRNA for non-structural proteins. Genome replication initiates transcription of ‘late’ genes

What is always formed in genome replication of RNA viruses?

dsRNA replicative intermediate

Why are RNA viruses more prone to mutation?

reduced genome stability

virus specific nucleic acid polymerases

RNA dependent RNA polymerase and RNA dependent DNA polymerase

Viral Release methods

lysis for naked viruses, budding for enveloped viruses and exocytosis for both types