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
