Viral properties Flashcards
What is the size of viruses?
20 to 120 nm, smaller than bacteria.
What are the characteristic features of viruses?
Obligate intracellular pathogens which have a DNA or RNA genome. They have no cyptoplasm or nucleus and are not considered cells.
What is the shape of viruses?
Spherical, filamentous or icosahedral.
What is an icosahedral virus?
Geometric shape with 20 sides of equilateral triangles, increasing number of structural units to increase the size of the capsid. Each face has proteins which can self-assemble.
How do viruses replicate?
They are obligate intracellular pathogens which hijack the cell genome to interfere with transcription/translational self-assemble individual viral components such as nucleic acids and a capsid
What is the genome of a virus?
Viral genetic material within a nucleocapsid in the form of DNA or RNA. Capsid is formed of individual subunits of capsomers.
What is a virion?
Extracelullular Infective form of a virus outside the host cell. It has a DNA/RNA core surrounded by capsid. It is only infectious if fully assemlbled.
What is an enveloped virus?
Contains a lipid-protein bilayer for protection of viral genome formed from the host cell membrane. It contains cell surface glycoprotein spikes for virus-cell fusion.
How does the capsid form?
Structural proteins self assemble
What is a naked virus?
Does not have an envelope and more stable in response to environmental stress such as acidity and temperature. It spreads more easily and survives gut and poor water treatment
What are the requirements of enveloped viruses?
More sensitive to environmental stressors. Must stay wet to remain infectious and spreads through large droplets. It does not need to kill cell to spread and membrane is sensitive to detergents.
What is the influenza virus?
Spherical enveloped virus which contains a H (haemogluttin) spike and a N (neuroaminidase) spike. These are susceptible to antigenic shift and recombination
How does the influenza virus initiate infection?
Contains H spike haemogluttin receptor which binds to sialic acid on glycolipid/glycoprotein on respiratory epithelia surface
How does the influenza virus leave the host cell?
The N spike on the influenza virus contains the enzyme neuroaminidase which cleaves sialic acid to cause release of the virus.
What is norovirus?
Icosahedral virus with an RNA genome spread through contaminated surfaces or via people.
What is the viral genome?
Contained inside a capsid of either single/double stranded DNA or RNA which is linear/circular. it codes for all proteins not produced by the target host cell. The genome is delivered to the host cell to produce proteins via transcription + translation.
What does the viral genome encode for?
Proteins to produce the progeny viral particle such as virions
Proteins to attack host defences
Enzymes for genome replication
How are viral genomes classified?
Strategy of nucleic acid and identity/polarity of nucleic acid
How do viruses produce new viral proteins?
THey produce viral mRNA which undergoes translation. Negative strand RNA require viral proteins for this process but positive strand immediately enters translation. This occurs in the cyptoplasm.
How do viruses produce nucleic acids?
Make copies of the genome
Stages of viral infection cycle
Attachment, penetration, uncoating, new genome/protein synthesis, assembly of new particles and release
What is a DNA virus?
Genome with either single stranded DNA or double stranded DNA.
What is an RNA virus?
Genome with single stranded RNA
Double stranded RNA
What do RNA viruses synthesise in host cell?
RNA genome, copy of genome and mRNA.
What is RNA +/ positive strand RNA?
RNA which has a genome that also exists as a single stranded mRNA and upon entry into host cell cyptoplasm, immediately undergoes translation to produce capsid proteins, replicate mRNA or progeny viruses release for budding
Which viruses are + positive strand RNA?
Hepatitis A, Hep C, Hep E
Rubella
Poliovirus, COVID-19
What is RNA -/negative strand RNA?
All are enveloped viruses which requires enzymes on the capsid for mRNA synthesis. It must be converted into positive sense RNA by RNA polymerase in viral transcription + translation to produce viral proteins for capsid or progeny viral proteins to be released by budding.
Which viruses are - negative strand RNA?
Rabies, measles, mumps, influenza, ebola
What is reverse transcriptase?
DNA polymerase enzyme used by viruses to transcribe single stranded RNA into DNA.
Which viruses use reverse transcriptase?
Retrovirus such as HIV
Double stranded DNA Hepadna (Hep B) virus which uses an RNA intermediate
How does an enveloped virus enter host cell
Enveloped viruses attach to host cell membrane and form endosome containing nucleocapsid or via fusion.
Cell membrane participates in viral attachment via receptors
What is the alternative route to endocytosis?
Fusion- Virus attaches to receptor and uses with host cell membrane via envelope and releases nucleocapsid which travels along cytoskeletal filaments
How does the endosome travel to cyptoplasm?
Adaptor proteins attach at the cell membrane to the endosome, allowing association by clarithrin to form a clarithrin coat for uptake
What is the rabies virus?
Negative strand RNA virus which enters host cell via endocytosis and uses polymerases and capsid proteins for transcription and translation
What is hepatitis B?
Double stranded DNA virus which is blood borne and causes liver disease. It is transmitted via exposure to contaminated blood.
Uses reverse transcription to replicate and causes chronic infection which can cause liver cancer.
What is burst size?
Yield of infectious virus produced from an infected cell
HIV
Retrovirus which replicates using the virion enzyme reverse transcriptase
How do DNA viruses replicate?
Immediate early genes are transcribed and translated to form polymerase enzymes for replication of viral DNA. Late genes are then transcribed into mRNA and translated to produce the capsids and progenors.
Why are viral infections so prominent?
Drug resistance, few drugs are available. Viral resistance can occur. There is a high mutagenic rate of the viral polymerase and recombination
What is the effect of viral infection on the host cell?
Cell lysis, Latency before lysis, persistent infection with slow release of virus without lysis, cause transformation of host cell into tumour cell
Why does host cell transformation to tumour cell occur?
Due to insertion of viral DNA into genome
