virus replication L13 Flashcards
what does entry to host cell by virus depend on
the interaction between viral surface proteins and host cell surface protein AKA virus receptors
why are viruses referred to as parasites
they rely on host cell machinery (ribosomes, enzymes and precursors) for biosynthesis
- biosynthesis is genome replication + mRNA synthesis + translation
what are the 4 stages of attachment
- diffusion - through extracellular fluid to get to receptor
- binds to low affinity receptor
- increase concentration of virus on host so more likely to bind to primary receptor - binds to primary receptor
- binds to co-receptor
- like using multiple passwords or keys to enter cell, before virus structure changes to release the genome. Is critical that virus doesn’t release genome to early or into wrong (non-permissive cell type).
give the example of HIV attaching
Attachment of HIV to a CD4+ cell. The outer domain of gp120 binds to the CD4 antigen. This leads to a conformational change in gp120 and a co-receptor binding site is exposed. This region of gp120 binds to the chemokine receptor. Binding to the chemokine receptor allows another conformational change to occur so that regions of the gp41 HIV protein interact to form a fusion domain that allows the viral and cell membrane to fuse. As a result the viral core enters the cytoplasm
why cant we just target the host receptors to stop viruses from entering
- give examples
the receptors have functions vital to cell process
- if we target receptors, will not get same biological effect
- Acetylcholine receptor is a neuronal signal transducer but is hijacked by rabies
- Angiotensin-converting enzyme 2 (ACE2) important in vasoconstriction and blood pressure but is hijacked by COVID 19
describe the two methods of internalisation
- fusion
- envelope viruses bind to receptors on plasma membrane
- fuses with plasma membrane releasing genome wrapped in capsid into cytoplasm - receptor mediated endocytosis
- enveloped virus bind to receptor
- it is then endocysed forming vesicle
- vesicle fuses with endosome lowering pH in vesicle
- low pH induces viral membrane to fuse with vesicle membrane – releases nucleocapsid
- for non-enveloped viruses, they are taken up the same but the low pH causes conformational change allowing it to form pores in the vesicle where nucleocapsid can escape from
what happens to virus once it has entered
genome must be uncoated, replicated, transcribed to mRNA and translated to proteins
The mechanisms of genome replication and transcription vary depending upon the nature of the genome
what features can viral genome be
- DNA or RNA
- single or double stranded
If ss: +ve sense (coding) or –ve sense (complimentary to coding strand) - linear or circular
how do DNA viruses replicate and transcribe their genome
DNA genome can be replicated by host DNA polymerases
DNA genome can be transcribed by host RNA polymerases
how do RNA viruses replicate and transcribe their genome
RNA virus genomes cannot be replicated and transcribed by host polymerases as host cell DNA and RNA polymerases can only use DNA as a template
RNA viruses MUST encode their own polymerases which can use RNA as a template
what are polymerases that can use RNA as a template called
RNA dependant RNA polymerase
make DNA from RNA not DNA
what enzyme to retroviruses contain to convert RNA to DNA
reverse transcriptase
What is the function of the proteins encoded by the virus genome?
- replication of the genome
- package the genome into virus particles
- alter the metabolism of the infected cell
what’s the difference between structural and non-structural proteins
structural form part of the virus particle whereas non-structural are not e.g enzymes involved in transcription
how do viruses use host machinery to translate mRNA
Eukaryotic initiation factors (eIFs) bind to cap recruiting 40S ribosomal subunit
- translation starts at AUG
how have viruses evolved to to ensure viral mRNA is translated more efficiently than host cell mRNA
Protein synthesis is often inhibited in virally infected cells
- done directly by virus or by the action of interferon
Some viral RNAs can be translated via cap-independent mechanism (internal ribosome entry site: IRES) to get round this issue
- Initiation complex is recruited directly to IRES
what features of post translational modification do viral proteins go through
Viral proteins undergo the same range of post translational modifications as the host cell proteins eg. glycosylation, acylation, lipoylation.
Some virus genomes are translated to produce a single large viral polyprotein which must be post-translationally cleaved to release individual viral proteins
describe the pathway of forming a virus P1
- viral mRNA translated into GLYCOproteins at ER
- viral glycoproteins inserted into ER
- ER is pinched off with viral glycoproteins inside
- vesicle goes to Golgi where is it glycosylated
- vesicle is trafficked to cell surface where is fuses with plasma membrane so viral glycoproteins are displayed on the surface
Describe the pathway of forming a virus P2
- nucleocapsid forms in cytoplasm
- it migrates to surface and interacts with proteins in plasma membrane in cytoplasmic domain
- causes part of membrane with viral glycoproteins to wrap around it
- buds off
what are the 4 ways virus can be released from host cell
- Release by cell lysis, eg poliovirus
- Release by budding:
from plasma membrane (eg influenza)
into lumen of endoplasmic reticulum (eg rotavirus) - Virus can also spread from cell to cell
via pores between cells (eg herpesvirus?)
by inducing fusion of their membranes (eg RSV) - Release may be preceded by or followed by a maturation step
where may viruses shed from
- primary site of replication
- secondary sites of replication
- if virus is carried from primary to secondary via lymph or blood
- secondary sites may be liver, bone marrow, spleen, blood vessel endothelium - target organs
what are bacteriophages
viruses that infect bacteria
There is probably a phage for every type of bacterium
There are more phage on earth than all other organisms put together.
what types of phage’s can you get and give examples
- single stranded DNA
- ΦX174 phage
- M13 phage - ds DNA
- most common
- Caudovirales - SS or DSRNA
Describe M13 filamentous bacteriophage
Circular ssDNA
Cloning and DNA-sequencing
Released without lysing host cell
(Wouldn’t see plaques in an assay)
‘budding’ makes own channel
how do phage’s infect
bind to surface of host using tail fibres
base plate makes contact with membrane
lysosomes create holes in peptoglycan layer
DNA extruded into host
describe the two cycles of bacteriophages
- Lysogenic
viral DNA enter host chromosomes and lies dormant
just replicates along with host chromosome
only becomes active when the cell or extracellular environment changes - lytic
- viral DNA is translated to proteins
- viral DNA and proteins are packaged to form bacteriophages that break out host
why are bacteriophages important
- Can turn bacteria virulent (phenotypic change)
C. diptheriae
S. typhimurium
EPEC - Alternative to antibiotics – phage therapy
- Great tool in the lab – recombinant DNA technology
