Flashcards in MCB Lecture 52 Intro to Viruses Deck (21)
What are the features of viruses? (6)
1. Subcellular (not living cells)
2. Obligate parasites
3. Can not self replicate
4. No energy metabolism
6. Can infect all sorts of cells (plant, animal, Protozoa, bacterial, fungal)
What is a virion?
A virion is a virus particle
Describe the structure of viruses
RNA, DNA, ss or ds, + or - sense
Made of capsomeres
3. Optional: envelope
What is the general size of a virus?
How do we visualise viruses?
X ray chrystallography (only those without a membrane)
Describe the different types of capsid
What are two other features that are seen? In which viruses are these seen?
2. Helical capsid
Envelope always present
Complex structure, eg. Poxvirus
Multi shelled capsids, eg. rotavirus
Which types of capsid must have a membrane associated with it? What about the other type?
Helical capsid viruses always have an associated envelope
Icosahedral viruses may or may not have associated envelopes
What is interesting about the rotavirus?
It has a multi shelled capsid, making it very hardy
It can survive the acidic stomach
How do we classify viruses?
Based on :
- Structure of genome
- Mode of replication
We don't classify based on the infection caused by the virus
What is the purpose of the infectivity assay?
Describe the steps involved
This tells us how much virus is present in a source
1. Monolayer of cell culture
2. Add the virus
3. Layer of agar
4. Let develop
5. Remove the agar, wash and stain
6. Dead cells will not stain or be washed away
We can thus visualise the number of viruses present in the original dilution and can calculate the number in the original source
Describe the steps of replication of viruses
1. Adhesion to host
2. Entry into host
4. Genome replication
6. Exit from host cell, shedding
How does bacterial and viral replication differ?
Viral replication uses host machinery
Also, there is a much longer eclipse period (12 hrs) for viruses
How do viruses get into cells? (3)
1. If envelope: membranes fuse
Where are the possible locations for genome replication of viruses? Which viruses do which?
Cytosol: RNA viruses
Nucleus: DNA viruses
Influenza: nucleus, because it needs splicing
Poxvirus: DNA, however it is so big that it does not rely on host
HIV: RNA, however, it is a retrovirus, so it goes to nucleus
Which enzymes are available in the host cell for use in viral replication?
Why does this present a problem?
DNA dependent DNA pol
DNA dependent RNA pol
This presents a problem for RNA viruses, who require RNA dependent RNA pol
They must thus get this from somewhere else
Describe the replication of the genome of RNA viruses using an example
+ sense RNA virus
Once in cell and uncoated:
1. RNA strand is translated (mRNA)
2. One big protein translated
3. Auto cleavage of the big protein yielding structural proteins, proteases and RNA dep RNApol
4. Proteases cleave the proteins further
5. RNA-dep RNA pol transcribes + RNA into - RNA
6. - RNA then much transcribed into + RNA for packaging
What different types of genes are present on the polio genome?
Structural proteins (capsid)
Non-structural: proteases (to further cut up the single protein product) and RNA dependent RNA polymerase
What is the Baltimore classification system?
This describes the ways that the genomes of different viruses can be replicated, based on whether they are ss or ds, and RNA or DNA
What modifications are made to viral proteins?
Glycosylation in the Golgi
How do viral proteins destined for the envelope get inserted into membrane that will go onto envelope the virus?
They are transported through the synthesis pathway and are placed in the plasma membrane of the host cell
Then, when viral shedding is occuring, viral genomes and the membrane proteins are concentrated into these budding vesicles