Lecture 4- Viral properties Flashcards
What are the four steps to prove that a virus causes a disease?
1) Microorganism must be found in large numbers in all diseased animals but not healthy ones
2) Organism must be isolated and grown in a pure culture
3) The isolated organism must produce the same disease once injected into a healthy animal
4) This microorganism must then be recovered from the host, isolated and be identical to the first microorganism
What is the definition of a virus?
They are infectious OBLIGATE intracellular PARASITES
- Obligate means they have to complete their life cycle after invading a host
What is the average size of a virus?
100nm
What are the two morphological varieties of viruses? What do they look like and what is the exception to these two groups
1) Non-enveloped
- have a protein capsid
- symmetrical
- e.g. Adenovirus
2) Enveloped
- protein around the genome
- lipid envelope around this derived from host membrane
- pleomorphic= different forms
- e.g. Ebola virus
**Combo of capsid and envelope= Herpes virus
What might viruses be named after and give examples
- The disease (poliovirus, rabies)
- The person who discovered it (EBV)
- The place it was discovered (Coxsackievirus)
- The part f he body affected (Rhinovirus, Hepatitis)
- The way it was spread (Dengue, Influenza)
Give the name of the classification system used to classify viruses
Baltimore Classification system
- based on genome
What are the different categories for the classification of viruses?
- dsRNA-> mRNA->protein-> assembly
- ssDNA-> dsDNA
- dsRNA->mRNA
- sense ssRNA-> protein (translated directly)
- -ve antisense ssRNA->+ RNA -> protein (transcribed first)
- ssRNA -> dsRNA (retrovirus)
What is the difference between positive and negative sense RNA?
+ve: translated immediately
-ve: virus has to carry enzymes/ machinery to convert genomes back into sense RNA
What are the largest RNA viruses and how big are they?
Coronaviruses
- around 30kb
What sort of genome do influenza and rotavirus contain?
*how many
Segmented -ve sense RNA genomes
Influenza= 8 segments
Rotavirus= 11 segments
What are the pros and cons of a segmented genome?
PRO: easy form of recombination (reassortment), can pick up new genes–> evolve
CON: difficult to pack
What is the approximate size of DNA viruses?
100s kb
What are the consequences of DNA viruses?
- can be big because DNA is more stable
- lots of accessory genes to modify host immune response
- accessory genes often lost in culture (no immune system in culture)
What are the consequences of RNA viruses
- use their own polymerase to replicate –> lack proofreading capacity –> high mutation rate
- high mutation= fast evolution
- RNA virus genome limited in size becuase of INSTABILITY
- Have to use complex coding strategies to make more protein varieties (use all three reading frames)
Describe the general replication cycle of a virus
1) ATTACHMENT to host cell by interaction between Virus Attachment Proteins (VAPs) and the host cell receptor
2) ENTRY of virus genome into host cell
3) SYNTHESIS of viral mRNA
4) TRANSLATION of viral mRNA to viral proteins using host cell ribosomes
5) REPLICATION of viral genome
6) ASSEMBLY of new virus proteins into virions. ENCAPSIDATION of new genomes
7) EXIT from infected cells and dissemination into new host
How does HIV enter its host cell?
Host cell= Th cell
- gp120 on HIV capsule interacts with CD4 receptor
- it then interacts with co- receptors CCR5/ CXCR4 bringing the virus close to the cell
- membranes fuse
Which two enzymes enter the host cell along with the HIV genome and what is their purpose?
1) Reverse transcriptase- produces viral DNA from ssRNA
2) Integrase- integrates viral DNA with host DNA
Describe the replication cycle of influenza
1) Influenza enters cell by spike protein
2) Engulfed and placed in endosome
3) Genome released and enters nucleus –> -ve sense RNA converted to mRNA
a. mRNA is replicated to form more -ve sense RNA
b. mRNA used to synthesis viral proteins
4) Assembly of new viruses
Describe the replication cycle of Ebola
Worm like!!!
1) enters by macropinocytosis
2) In early endosome undergoes cleavages so that it can bind to different receptors
3) Genome released (-ve sense RNA)
a. replicated to form more genomic material
b. transcribed to mRNA for protein synthesis
4) Assembly of new viruses and budding by host ESCRT
What could cause the cytopathic effect?
- virus lysing the cell
- shut down of protein synthesis
- accumulation of viral proteins
What are plaques?
- viruses form plaques on cell monolayers
- they are a result of an individual virus infecting one cell and then infecting other cells
How would you find out how much virus is in a patient?
Plaque assay
- you make serial dilutions of the patients fluids
- culture the dilution and count the number of plaques and multiply back by the right power
What are syncytia?
Viruses with surface proteins that fuse at neutral pH and fuse cells together
How can you find infected cells using immunostaining?
1) Innoculate a sample onto a cell
2) Wait for cell to produce viral proteins
3) Produce antibodies against viral proteins
4) Visualise the antibodies
What are the stages of viral replication?
1) Eclipse phase- no virus because they have just gone into the cell
2) Logarithmic phase- rapid increase in number of visions synthesised
3) Cell death- number of viruses decreases as cell dies
How can you diagnose viral infections?
- detect viral genome PCR
- detect viral antigen IFA, ELISA
- Detect viral particles EM, HA
- detects viral cytopathic effect in cultured cells
- detect antibodies to virus
How do you generate attenuated vaccines?
Propagate and PASSAGE viruses
- Provide permissive cells which are continuous lines pf transformed cell cultures
- Virus accumulates mutation to survive in the new host
- leads to attenuation
Which viruses do not have permissive cell lines?
Norovirus
Hepatitis C
What new scientific breakthrough was made about passaging Norovirus?
- enteric bacteria promote norovirus infection of B cells
- maybe bacteria transport virus across intestinal epithelium
Why is it possible to manipulate viruses?
Virus genome is incredibly small
