Viruses Flashcards
Important features of viruses?
Only able to function within the living cells of a host animal, plant or microorganism.
Consists of a nucleic acid molecule (DNA/RNA) surrounded by a protein coat, often with an outer lipid membrane.
Originally distinguished by their size.
Methods of studying viruses?
Pathology/transmission studies
Electron microscopy
Culture
Detection of virions and immune responses
X Ray crystallography
Nucleic acid detection, sequencing and analysis
Structure of a virus?
The virion is the infectious form of the virus outside of the host cell - fully developed.
Capsid is formed of one or more protein subunits and is tightly packed with viral DNA giving a large pressure. Once the tail tube is open it will unpack due to high pressure.
Virions have molecular receptors, of varying degrees of complexity, these enable recognition of and attachment to host cells.
Some bacterial viruses have mechanisms to inject their DNA into the host.
Some eukaryotic viruses are enveloped with a host-derived envelope.
Define bacteriophage?
Bacteriophage, also called phage or bacterial virus, any of a group of viruses that infect bacteria.
Differences between DNA and RNA viral genomes?
DNA:
- deoxyribose is less reactive due to C-H bonds
- stable in alkaline conditions
- smaller grooves make it more resistant to enzymatic attack
- mostly double stranded
- lower mutation rate
RNA:
- ribose more reactive due to C-OH bonds
- not stable in alkaline conditions
- larger grooves so more sensitive to enzymatic attack
- mostly single stranded
- higher mutation rate
Advantages of a segregated genome in RNA viruses?
Enables re-assortment and recombination of their genomes.
Important consequences for evolution and immune escape.
Influenza virus has 8 genomes.
Bacterial virus life cycle?
Virion gains entry Replication Transcription Translation Host-virus interactions Exit
Some phage can be latent in bacterial genomes. Insert to form a prophage (the genetic material to form a phage) in the host genome.
The lytic/lysogenic state is controlled by a genetic switch. Enables long term survival and introduces complex interactions of host and phage genome.
Pseudolysogeny normally occurs under nutrient deprivation when the host cannot support DNA replication.
Define pseudolysogeny?
Pseudolysogeny is a form of phage-host cell interaction in which the nucleic acid of the phage resides within its starved host in an unstable, inactive state.
Define lysogeny?
Characterised by integration of phage nucleic acid into the host’s (bacterial) genome.
Define lytic?
Results in destruction of the injected cell and its membrane.
Eukaryotic virus life cycle?
Virion enters the cell Transcription Replication Assembly Exit
How do viruses enter cells?
Receptor mediated endocytes.
Receptor mediated signalling causing fusion.
Both depend on displaying proteins which interact with cell receptors.
How does viral replication depend on genome structure?
Double stranded virus synthesises positive RNA strands from the negative strand.
Translated.
Synthesises negative RNA strands which are packaged into virions.
Positive RNA virus.
Translation
Creates replication proteins to create negative RNA which is then used to create positive RNA.
Retrovirus.
Uses reverse transcriptase to create DNA for integration into the genome.
Transcription creates +RNA.
Translated and packaged into the virions.
What happens in the Epstein-Barr virus?
Persistently infects almost all humans worldwide. Intrahost reservoir is B cells.
Normally a harmless passenger but can promote development of B cell lymphomas.
EBV can enable survival of B cell receptor deficient B cells.
EBV proteins also mimic cell signalling pathways.
Major EBV associated B cell lymphomas:
Burkitt lymphomas
Hodgkin lymphoma
Post transplant lymphoma
Retrovirus lifecycle?
Adsorption and entry into the cell Synthesis of reverse transcribed DNA Integration Transcription, splicing and nuclear export Translation and encapsidation Viral assembly and budding
