15-22 virology Flashcards
what is the current definition of a virus?
a genetic element that is protected by a protein coat and cannot replicate independently of a living cell
what are the general features of viruses?
- contain DNA or RNA
- do not divide by binary fission
- they are obligate intracellular parasites
- small
- simple structure
what replication phase are viral components produced in?
the viral eclipse phase
what technology can be used to target viruses?
- siRNA
- CRISPr
- vaccines
- novel therapies
what is the basic structure of a virus?
- contain DNA or RNA surrounded by a protein capsid
- some viruses possess a viral envelope that surrounds the capsid - it is derived from the host cell
- all viruses have viral attachment proteins which help the virus recognise, bind, and enter the host cell
define what a virion is
the complete virus particle
how do viruses recognise their target cells
by their cellular receptors
what does the viral genome encode?
- structural proteins including capsids and VAPS
- non structural proteins including proteins for: viral replication, pathogenesis, transformation, modulation of host defences
what does the viral genome not encode?
- the complete protein synthesis machinery
- proteins involved in cell wall production or membrane biosynthesis
- centromeres or telomeres found in standard host chromosomes
what is a capsomere?
the viral subunits of the capsid
what is the use of viral particles being stable?
it protects the viral genome
what is the use of viral particles being unstable?
it facilitates the delivery of the viral genome into the host cell to dissociate on infection
how is the stable structure of viral particles achieved?
- symmetrical arrangement of identical virus protein subunits provide maximal contact
- each subunit has identical bonding contacts with its neighbours
how is the unstable structure of viral particles achieved?
- the contact is not covalent
- the structure can be dissociated or taken apart once the virus attaches to the host cell to release the genome
what shapes can the capsid structure be?
- helical - the capsid look like a coil and the virus looks like a rod
- icosahedral - the capsid has an icosahedral shape and the viruses look like spheres or cocci
- complex - a capsid with a combination of helical and icosahedral arrangements or no specific arrangement
how is the viral envelope acquired?
- when the virus exits the infected cell, it pushes out of the cellular membrane, acquiring the envelope, endoplasmic reticulum membrane, and nuclear membrane
- the VAPs are located within this envelope
is the viral envelope sensitive to environmental conditions?
yes, including heat, soap, detergents, and stomach acidity
what are the disadvantages of classing viruses based on the diseases they cause?
- focuses on some viruses and ignores others
- a single virus may cause more than one disease
- viruses infect more than one host
how can viruses be classified based on virus particle morphology and nucleic acid?
-> nucleic acid
- type of nucleic acid
- single or double stranded
- linear, circular, single molecule, or segmented
- negative or positive polarity
-> capsid symmetry
- icosahedral, helical, or complex
-> presence of absence of lipid envelope
what is the systematic taxonomy scheme used for viruses?
order -> family -> subfamily -> genus -> species -> member
what are the 7 viral genome types in the baltimore classification?
1 - dsDNA
2 - ssDNA +ve strand
3 - dsRNA
4 - ssRNA +ve
5 - ssRNA -ve
6 - ssRNA -> ssDNA -ve
7 - gapped dsDNA
describe the basic structure of viroids
- novel agents of disease in plants
- contains a single circular ssRNA molecule as infectious material
- no protein components
- smallest self replicating pathogen known
- up to 70% of the nucleotides in the genome RNAs are based paired
- rod shaped or dumb-bell shaped
what are prions?
- agents of infection in diseases which are characterised by slow, progressive neurological degeneration that are fatal
- no nucleic acid
- they replicate slowly in hosts
- they are abnormal forms of normal cellular proteins
- they induce changes in the shape of their normal homologues
what is viral tropism?
- the specificity of a virus to a specific host
there is: - cellular tropism
- tissue tropism
- host tropism
how can enveloped viruses enter the host cell?
they can either:
-> fuse with the plasma membrane
- they fuse directly with the plasma membrane via fusion proteins
- the genetic material and capsid are immediately delivered to the cytoplasm of the cell
-> fuse via endosomes
- occurs at a low pH
- the virus is taken up by invagination of the membrane into endosomes
- the pH inside the endosome decreases, so fusion of the viral envelope with the cell membrane can now occur, resulting in delivery of virion components into the cell cytoplasm
how do non enveloped viruses enter the host cell?
- endocytosis via the endosomal membrane
- direct entry
how does the virus uncoat and where is the contents delivered?
- the capsid destabilizes to release the contents into the cytoplasm
- DNA viruses must deliver DNA into the nucleus
- RNA viruses must deliver RNA into the cytoplasm?
which virus does not deliver DNA into the nucleus?
poxiviruses
which virus does not deliver RNA into the cytoplasm?
influenza
how are the new viral protein and nucleic acids packaged?
- each nucleic acid copy is packaged in viral capsid proteins
- usually occurs in the cytoplasm
- helical and icosahedral complexes assemble around the viral nucleic acid
- complex capsids first assemble into empty capsule structures (procapsid) using scaffolding proteins. scaffolding proteins are then removed and replaced by viral NA
how is the new virus released?
- naked viruses usually lyse the cells, causing them to burst and release the new virions
- enveloped viruses bud from the cells, acquiring their envelope from the host cell membrane
how do dsDNA viruses replicate?
to produce viral proteins - dsDNA is transcribed by RNA polymerases to produce viral mRNA that is read and translated by the host ribosome
replication of viral DNA - the genome is replicated by DNA polymerases
how do ssDNA viruses replicate?
- it is first replicated by host DNA polymerase to produce dsDNA - this is transcribed by host RNA polymerase to produce viral mrna to make proteins
- viral DNA is just replicated as normal by host DNA polymerase
how is gapped dsDNA replicated?
- the genome is first repaired by cellular DNA repair polymerases to remove the RNA piece with the protein and to fill the gap in the DNA - this is now recognised by host RNA polymerase to produce viral mRNA to make proteins
- some mRNAs are used as templates to produce incomplete dsDNA using RT
- A DNA strand is produced by RT, which is then used again to product a tDNA strand that is incomplete and contain RNA and RT itself
what does RT stand for?
reverse transcriptase
how are ssRNA+ genomes replicated?
- the genome is translated straightaway by the ribosomes to viral proteins including replicase
- RNA replicase is then used to replicate the viral genome
how are ssRNA- genomes replicated?
- RNA must first be replicated to tRNA using carried RNA replicase
how is dsRNA replicated?
in the same way as ssRNA-
how is ssRNA+ sense with DNA intermediate (retrovirus) replicated?
- RT is carried and converts ssRNA to dsDNA
- the dsDNA copy is delivered to the nucleus and is integrated into the host DNA using integrase
- now, when the cell replicates its own DNA, the viral dna is also transcribed to produce viral mRNA
what are cytopathic effects?
distinct observable cell abnormalities due to viral infection
what is hemadsorption?
- cells infected with certain viruses acquire the ability to bind to and adsorb red blood cells
- many viruses contain haemagglutinin surface glycoproteins
which viruses are capable of hemadsorption?
- influenza
- measles
- mumps
what are the main stages of the viral single step growth curve?
1 - inoculation - inoculum of virus binds to cells
2 - eclipse - virions penetrate the cells
3 - burst - host cells release many viral particles
4 - burst size - the number of virions released per bacterium
define what is meant by susceptible cells
a cell that expresses the specific receptors that are recognised by a specific virus through specific VAPs
define what is meant by a permissive cell
a cell that contains the proteins and molecules within the cell that are necessary for replication to occur
what are common portals of entry in the body?
- the skin
- the respiratory tract
- the GI tract
- the genital tract
what is a localised infection?
when the virus infects and replicates only within cells at the site of infection
what is meant by systemic infection?
when the virus infects the initial site of entry and replicates locally to spread to other sites within the body
define what is meant by pathogenesis
the ability/capacity of a virus to cause disease
define what is meant by virulence
a quantitative or relative measure of the pathogenesis of the infecting virus
how can viruses carry out direct killing?
- poliovirus kills neurons, causing paralysis of muscles innervated by those neurons
- ebola virus damages endothelial cells, causing haemorrhage
what is virus induced transformation?
- cancers associated with some viral infections are due to indirect effects, such as immune suppression
- some viral infection directly promotes tumour development
define what is meant by a vaccine
a biological product that can be used to safely induce an immune response that confers protection against infection and/or disease on subsequent exposure to a pathogen
what is a live attenuated vaccine? what are its advantages and disadvantages?
- it is a vaccine which contains the whole virus that has been weakened or attenuated to produce an immune response similar to that during natural infection
pro - tends to create a strong and lasting immune response
cons - may not be suitable for immunocompromised individuals
what is a killed/inactivated vaccine? what are its advantages and disadvantages?
- it is a vaccine which contains the whole virus which has been killed or altered so it cannot replicate
pros - may be suitable for immunocompromised individuals
cons - doesn’t create a strong and lasting immune response
what is a subunit vaccine? what are its advantages and disadvantages?
- it is a vaccine which does not contain the whole virus but instead contains one or more specific component or antigen usually from the surface of the virus
pro - may be suitable for immunocompromised individuals
cons - doesn’t create a strong and lasting immune response
what is a nucleic acid vaccine?
- vaccines which do not provide the viral protein/antigen and instead provide the genes that encode for the specific viral antigens to host cells
- these genes are then expressed by the host cells to produce the viral antigen, stimulating an immune response against it
why is there a limited number of marketed antiviral drugs?
- many compounds that interfere with virus growth cause adverse effects in the host
- some medically important viruses cant be propagated, have no animal models, or are very dangerous
- antiviral drugs must be potent to completely inhibit viral replication
- many acute viral infections are short lived
