Virology Flashcards
what was the first indication of the presence of viruses?
foot and mouth disease would be transmitted even after being passed through bacteria free filtrate (which would kill bacteria)
how were mammalian viruses shown to be corpuscular?
dilution experiments - severity of symptoms did not reduce with dilution but the number of cases did
where is the only place that viruses can grow?
in living tissue
what did the use of embryonated eggs to grow viruses lead to?
identification of human viruses and lead to production of vaccines
what are the 7 key impacts of molecular biology on our understanding of viruses and viral disease?
scientific basis for classification understanding replication and pathogenesis (infection) development of antiviral drugs production of vaccines understanding viral evolution new diagnostic tests molecular epidemiology of viruses
how can the relationship of viruses with host cells be described?
viruses are obligate, intracellular parasites
what are viruses dependent on the host cell for?
source of energy
provision of raw materials
what do viruses use host cell machinery for?
to replicate genome
to synthesise new protein
describe the formation of protein from DNA
DNA is transcribed to RNA. Introns are removed through splicing and a cap and tail are added - mRNA is formed. mRNA is translated to a polypeptide by ribosomes. The polypeptide is then folded to form the protein
what is the virus genome formed of?
nucleic acids
what is the size of a viral genome?
small - only a few genome protein components
what is not included in a viral genome?
no protein synthesis enzymes or ribosomes
no genes for lipid or amino acid synthesis
few or no introns (non - coding DNA)
why is there little/none non-coding DNA (introns) in a virus genome?
less need for control sequences as DNA is less complex than in mammals
how does the virus perform functions that it does not possess DNA code for?
they are supplied by host cells
what DNA genome can be found in viruses?
double strand DNA (dsDNA)
single strand DNA (ssDNA)
what RNA genome do many viruses have?
positive sense single stranded RNA
negative sense single stranded RNA
double stranded RNA
what happens to positive sense single stranded RNA once in the host cell?
directly translated to protein
what happens to negative sense single stranded RNA once in the host cell?
only translated to protein once it has been copied to positive sense RNA
what must be supplied by the virus if it has negative sense single stranded RNA?
RNA polymerase
what is virus nucleic acid enclosed within?
a capsid
what is a capsid?
protein coat which encloses virus genome
what is a capsid formed of?
multiple protein subunits known as capsomers
what are the 4 key roles of the capsid?
packaging of genome and essential enzymes
protection of nucleic acid from UV light
protection from nucleases
provides specificity for attachment of viruses
what do nucleic acid and capsid form together?
nucleocapsid
how can capsids form a flat sheet?
pack together in a hexagonal array
how can the hexagonal array be modified to enclose 3D space?
rolled into a cylinder where units form a new spiral pattern (helical symmetry)
pentagons placed within the sheet to produce gaps - when closing these gaps a spherical space is enclosed
where is the nucleic acid genome located within helical symmetry viruses?
wound around the inside of the cylinder
how many pentagons are required to enclose a spherical space?
12
why are simple shapes used in virus capsids?
reduced number of proteins required which frees up space on the genome
what is a icosahedron?
no hexagons present in structure - may be formed of triangles
what do capsins provide as well as protection of nucleic acids?
attachment sites for cells
direction of virus to replication area
attack point for anti-viral immune response
what extra external structure do some viruses have?
lipid envelope
what does the lipid envelope surround?
capsule
how is a lipid envelope derived?
budding from different lipid bi-layers in the host cell
what type of viruses in animals all have an envelope?
all helical viruses
what type of viruses in animals mostly have an envelope?
many icosahedral viruses
what is a benefit of an envelope to the virus?
hides virus from the host
what is a disadvantage of an enveloped virus?
susceptible to damage by soap and detergent
what is a benefit of unenveloped viruses?
survive better in the environment
what are peplomers?
membrane anchored glycoproteins
in formation of peplomers what are sugars added to glycoproteins by?
host cell enzymes in the golgi apperatus
what is the role of peplomers?
adhere to host cell
influence pathology
target for immune system
what is the usual size range of viruses?
20-300 nM
what are the 5 genome types of viruses?
dsDNA ssDNA dsRNA \+ssRNA -ssRNA
what do viruses utilise in order to replicate?
host organelles and nucleotides
where are peplomer anchored?
envelope
what are the 2 types of protein capsid symmetry?
helical or icosahedral
what are the 7 steps of viral replication?
attachment to plasma membrane entry into cytoplasm/nucleus un-coating of virus particle synthesis of mRNA and protein replication of nucleic acid virus assembly exit of virus from cell
where does viral replication take place?
in host cells
what does the virus do on entry to host cell?
shuts down normal cell working and new virus protiens are syntheised
what happens during the attachment phase?
binding to cellular receptor of suitable host cell
what is the ligand (attachment protein) of unenveloped viruses?
capsid proteins
what is the ligand (attachment protein) of enveloped viruses?
envelope glycoproteins
what does the attachment define?
tropism of the virus
what is the tropism of a virus?
specificity of a virus for particular host tissue
what is the tropism of a virus determined by?
interaction of viral surface structures with host cell receptors
what happens during the entry of a virus?
the bound virus is endocytosed
what happens during uncoating of the virus particle?
genome exits capsule and spreads to the site of replication
what does acidification of the host cell environment cause?
conformational change in virus capsid which releases virus genome
what does acidification do to the capsid?
creates a pore in the membrane through which the genome escapes into the cytoplasm
where does virus assembly take place?
in cytoplasm or ER
what 2 ways may the virus exit the host cell?
lysis
budding
how long does viral replication in a host cell take?
3 hours
what are the 5 key properties of viruses?
small intracellular parasites grow only in living cells genomes of RNA or DNA no genes for energy metabolism genomes are replicated and new virus proteins are made using host cell machinery
define pathogenesis
the process by which viruses causes disease
what is the route of infection of a virus?
route of entry into the host
what are the main routes of viral infection?
respiratory oral cutaneous percutaneous sexually transmitted
what does the respiratory route of viral infection lead to?
localised respiratory disease
what cells are targeted by respiratory route of viral infection?
ciliated epithelial cells of the upper respiratory tract
what is the result of respiratory route of viral infection?
inflammation and reduced removal of microorganisms
how is inflammation caused during respiratory viral infection?
damaged cells release inflammatory mediators which lead to altered blood flow and increased sensitivity
how is reduced removal of microorganisms caused during respiratory viral infection?
due to damage to cilia that are unable to remove mucus from the lungs - can lead to secondary infection
what type of infections are the most common cause of diarrhoea?
viral infections
what cells are affected in diarrhoea?
enterocytes - causes diminished absorption of salts and water in small intestine
why can there be rapid recovery from viral diarrhoea if dehydration and acidosis corrected?
crypts of small intestine are intact
what are 3 types of percutaneous injection viral infection?
bites from infected animals
bites from insect vectors
aeatrogenic
what is viraemia?
virus within the blood
what can aeatrogenic viral infection be caused by?
dirty needles
what is the first step of viral infection?
receptor/ligand binding between virus and host cell
how can receptor/ligand binding affect the specificity of the virus to host and cells?
depending on specificity the virus may only be able to bind to one type of cell in a single host. Others can bind to a much wider range
what are the 2 types of receptor/ligand binding?
specific interactions
non-specific ligand
describe specific interaction receptor/ligand binding
receptor/ligand interaction depends on exact complementary fit. The virus exactly matches it’s host cell
what happens during HIV/FIV?
specific binding to CD4 T-cells, leads to immune deficiency. HIV cannot infect cats and FIV cannot infect humans even though the disease is the same due to specific interactions
what is the host potential of non-specific ligands?
wide range of host potentials
what does influenza haemagglutinin bind to on body cells?
sialic acid
what is sialic acid?
glycoprotein found on many cell surfaces
how is the route of infection in influenza limited to the respiratory tract?
infection of cells depends on protease cutting the virus peplomer. Not all cells possess this enzyme so the virus is unable to infect them
what can alter host specificity for infection and severity of disease?
a single amino acid change to the cleavage site
what is the outcome of the disease determined by?
function of the infected cell
what is lytic infection?
acute virus infection resulting in cell death accompanied by release of new virus particles
what is a cytopathic virus?
one which results in cell death
what does the release of virus from one cell cause?
infection of many thousands of new cells leading to extensive loss of cell function
what are latent infections?
virus infections which persist within a cell and do not cause cell death (herpes)
how long can latent virus infections last for?
the lifetime of the host
what is the effect of latent viral infections on the cell?
alter the rate of cell division and growth
how are virus particles released from cells in latent infection?
periodically without killing the cell, usually through budding
what occurs during persistent viral infection?
the immune system fails to clear infection leading to continual shedding of the virus by cells and production of viral proteins
how do viruses modify the function of the cell?
increase cell division leading to cell growth and tumor formation
what is the typical host response to viral infection in epithelial cells?
inflammatory response in cells an adjacent tissues
what factors are produced by epithelium following viral infection?
interferon
pro inflammatory cytokines
chemokines
what pro inflammatory cytokines are released in response to viral infection?
IL1, IL6 and IL8
what is the role of chemokines?
attract leukocytes to enhance immune response
how is a fever caused during viral infection?
production of endogenous pyrogens
binding to opioid receptors on the hypothalamus
activation of COX-2 in the hypothalamus leading to increased prostaglandin secretion
altered firing of temperature sensitive neurons in the anterior hypothalamus
increase in whole body temperature
what endogenous pyrogens are used to fight viral infection?
interferon
TNF-alpha
what is sickness behaviour?
motivational state responsible for re-organising perceptions and actions to enable ill individuals to cope better with infection
what behaviours are involved in sickness behaviour?
decreased motor activity social withdrawal reduced responsiveness reduced food and water intake increased slow wave sleep altered cognition increased pain sensitivity
what factors are responsible for sickness behaviour?
IL1 and IFN
what is the role of interferon alpha?
bind to specific receptors on neighboring cells to prevent them becoming infected
how does interferon alpha prevent cells becoming virally infected?
binding induces synthesis of antiviral proteins
what produces interferon alpha?
virally infected cells
