4 Viral Properties Flashcards
Q: What is a virus? meaning?
A:infectious OBLIGATE intracellular PARASITES.
Obligate = cannot complete its life-cycle without being inside a host cell (uses the host cell’s genetic machinery)
Q: What’s the average size of a virus? What’s its size relative to a ribosome and a bacterium?
A: 100nm
1/10 or less than bacterium size
just a little bigger than ribosomes
Q: Describe virus morphology. 2 types.
A: talks about shape
Non-enveloped: Have a protein capsid + Tend to be more symmetrical (some are made of just one type of protein with lots of copies -> assemble to make geometric shapes and encapsulate genome within it)
Enveloped: Have proteins around their genome + have a lipid envelope around that which is derived from the host membrane (it is not encoded in the genome)
Q: How are viruses classified? (2) Name?
A: The classification based on genome is known as the BALTIMORE CLASSIFICATION
based on replication strategy and coding
placed in 1 of 7 groups
Q: How can viral genomes differ? (6)
A: They can be made from DNA or RNA in different forms
Double Stranded DNA or Single Stranded DNA
RNA viruses carry their RNA as:
Positive Strand - sense strand which can be translated directly
Negative Strand - antisense strand
Double Stranded RNA
Q: Describe the central dogma. How does this relate to the genomes viruses carry?
A: describes the two-step process, transcription and translation = DNA –> RNA –> Protein
Most of the time, living things have a DNA genome which is transcribed into RNA and then translated into protein
Some viruses carry their RNA as a positive sense strand - this means that as soon as it enters the cell, ribosomes can translate that into a protein (-> no DNA is involved in life cycle at all)
Some viruses carry their RNA as a negative sense strand - this cannot be translated straight away. It needs to be transcribed back into a positive sense complimentary copy (said viruses with must carry proteins/enzymes/machinery that will convert the genomes back into sense RNA Otherwise, they are inert)
Q: What do RNA viruses and retroviruses use to replicate? Result?
A: Use their own polymerase to replicate = error prone and these lack proofreading capacity (simple) = HIGH MUTATION RATE - hence fast evolution
Q: Why can DNA viruses have a large genome? Up to what size? Have space for? Often lost when?
A: Can be big because DNA is more stable
100s kb
There is space for accessory genes - things that viruses don’t inherently need to survive but may give some advantage (may modify host immune response)
in passage in culture
Q: What are the benefits and downsides of having a segmented genome? What is it?
A: (Some viruses have their genome in one long strand, others have it in several little pieces - SEGMENTED GENOME)
BAD - pieces of genomic material must be gathered together when the virus tries to leave the cell and move to another cell (imposes more difficult packing strategies)
GOOD - allows opportunity to pick up new genes and evolve (allow an additional easy form of recombination known as re-assortment)
Q: Describe the generic replication cycle of a virus. (5)
A: start with the virus on the outside where it is inert ->
virus will have a protein on its coat (viral attachment protein) = binds to a receptor on the host cell surface -> virus docks down onto the cell surface ATTACHMENT
Some viruses enter the cell by fusing with the host cell membrane and injecting their DNA or RNA into the cytoplasm // Other viruses enter via a series of vesicles such as endosomes ENTRY
viral genome is in cell -> converted into messenger RNA if it isn’t already -> viruses uses the host ribosomes to translate its own mRNA and produce proteins TRANSLATION + at the same time the virus begins to replicate its own genome (using its own polymerase or host cell polymerase) REPLICATION
The copies of the genome and the viral proteins assemble to form new virus particles ASSEMBLY which then leave the cell EXIT
Q: How are viruses grown in a laboratory?
A: have to be grown inside host cells
Cells need to be grown in the laboratory before being infected by a virus
If you put a virus on healthy cells you get a change in cell shape and you get cell death
Q: What is the cytopathic effect? What can cause it? (2)
A: the death of the cell as a result of being infected by a virus (usually result of virus lysing the cell)
due to the virus taking over genetic machinery so that the cell is no longer able to produce the proteins that it needs to survive - the cell is destined to die (Death is probably due to apoptosis)
or as a defence mechanism - when viral proteins have accumulated (stops spread/ infection of other cells)
Q: What are plaques in relation to viruses? Allow us to see? Do all viruses make them?
A: the result of an individual virus infecting one cell and then infecting other cells (one single virus that was originally put onto the cell monolayer)
As a result of this death - viruses form plaques on cell monolayers
allow us to see the consequence of cytopathic effect
Not all viruses make plaques
Q: How can plaques be useful?
A: If you count the plaques you can measure how many viruses there were in the sample
number of plaques represent the original number of viruses in the sample
Q: What is a plaque assay? How do you conduct one?
A: used to find out how much virus is in the patient
Take a sample from the patient that contains the virus ->
Make serial 10 fold dilutions -> Take a known volume of the dilutions and put it onto the susceptible cells -> there will be plaques on the susceptible cells showing how many viruses there are in the dilution
Q: What are syncytia? How do they form? (2)
A: multinucleate enlarged cells (as a result of host cells merging- defence mechanism)
some viruses cause their formation
virus has envelope and surface proteins on it -> capable of fusing directly with plasma membrane which contains receptor
when virus fuses with cell, cells in culture fuse with their own plasma membranes (provided there are a certain number of viruses to stimulate)
Q: How can you visualise viruses? (3)
A: -immunostaining the infected cells (immune fluorescence)
- can inoculate sample onto a cell and wait for the cell to start producing a protein that is unique to the virus
- You then use an antibody which can detect that protein
- Stain will show the protein in a specific colour
- plaques
- take a sample of blood, drop it on some susceptible cells and if you see syncytian formation, it suggests that there is virus present
Q: Draw a single step growth kinetics graph of a virus. Explain it.
A: time by log[infected virus] (amount of virus)
down then up, then plateaus after rising steadily
At the start of the infection there is a known amount of virus
If you take a sample quite early on, there is NO virus at all - because virus has gone inside the cell
This is the ECLIPSE phase because we can’t see it.
After a while the virus begins making new copies and exiting the cell and becomes detectable
You get logarithmic increase in the number of viruses in the sample
It then tails off because cells start dying (all cells in system are infected)
Q: How can you detect a viral genome? Used when?
A: PCR : Have primers which are complementary to a specific part of the viral genome which identifies the virus
have an idea of what it is
Q: How can you detect a viral antigen?
A: Indirect Fluorescence Antibody (IFA) or ELISA
Q: How can you detect viral particles? Used when?
A: Electron Microscopy or Haemagglutination Assay
new viruses
Q: How can you detect antibodies to virus? Good for?
A: Serology
confirming the virus they had
Q: How can viruses be propagated? Can lead to?
A: can passage viruses in the lab by providing permissive cells, often continuous lines of transformed cell cultures
can lead to attenuation
Q: What is attenuation and what can lead to it? Basis of?
A: Viruses may accumulate mutations when passaged that adapt them to the new host (e.g. losing some accessory genes)
This can lead to ATTENUATION and was the basis of generation of vaccines in the past (evolves so v good at infecting certain types of cells but not healthy human ones)
Q: What are 3 examples of viruses with a non-enveloped virus morphology?
A: adenovirus
picornavirus
callicivirus
Q: What are the 2 types of enveloped viruses? Provide an example for each.
A: pleiomorphic - have lots of different shapes eg. measles virus
typical shape eg. ebola virus
Q: How does a particular replication strategy determine morphology?
A: doesn’t
Q: Why are RNA virus genomes limited in size? What do they do to compensate this?
A: due to the instability of RNA
They often use complicated coding strategies to encode more proteins than expected from a small RNA genome (eg coding for proteins in all 3 reading frames)
Q: Describe the details of the HIV-1 replication cycle. (6)
A: protein called GP120 in its coat (spike protein) that attaches to host cell via CD4 cell surface receptor (on immune cells) -> also uses co receptor on host ATTACHMENT
membranes fuse -> internal capsid containing genome is released ENTRY
goes to nucleus of cell and on the way the RNA is reverse transcribed into DNA
DNA is integrated into host chromosome
REPLICATION and TRANSCRIPTION of virus
ASSEMBLE as plasma membrane of host cell bulges out -> mature virus EXITs
Q: What’s the benefit of understanding the HIV-1 life cycle well? (2)
A: we can treat people
attack virus at many stages using specific E and proteins used
Q: Describe the details of the influenza replication cycle. (5)
A: spike proteins are haemagglutinin (and neuraminidase) -> bind to different receptors - for H= sialic acid
(doesn’t fuse with membrane to enter) convinces host cell to undergo endocytosis -> virus is then triggered when in the host cell to fuse membrane with endosomal membrane -> allows nucleic acid release -> enter nucleus
mRNA synthesis and RNA replication occurs and proteins are made
proteins and genome meet at plasma membrane -> cission event releases virus EXIT
Q: Where is CD4 (and co receptor) uniquely found?
A: on immune response cells
Q: Describe the details of the ebola replication cycle. (5)
A: latches onto cell surface receptor
fold in plasma membrane comes over top of the attached virus -> forming compartment -> enter -> early endosome (convinces host cell to undergo endocytosis)
fusion event occurs and nucleic acid takes over cell in cytoplasm
viral mRNA is made and negative strand of viral genome made
at end all come together and pinched off EXIT
Q: Why and how can viruses be manipulated? Allows?
A: their genomes are so small, that they can be synthesised
when introduced into permissive cells they direct synthesis of all their components and new viruses are made de novo
allows reverse genetics, the creation of viruses at will with engineered mutations in their genomes (can make vaccines better than before)
