Viral Replication Flashcards
permissive cell
a cell which a virus is able to replicate within
non-permissive cell
cells in which a factor or factors necessary to viral reproduction is not present or one detrimental to viral reproduction is present
MOI
number of virions that are added per cell during infection
adsorption
period in which the virus attaches to and enters the cell, and the titer of free virus in the medium may actually decline
eclipse period
time interval between uncoating (disappearance) and appearance, intracellularly, of first infectious progeny viruses. No infectious virus is detected during this time
latent period
time before new infectious virus appears in the medium, time from uncoating to just prior to the release of the first extracellular virions
burst size
number of infectious virions released per average cell
steps of viral replication (6 steps)
attachement, penetration, uncoating, synthesis of viral components, assembly and maturation, release in large numbers
attachment
binding of the virus to receptors on the host cell
endocytosis
a process in which a substance gains entry into a cell without passing through the cell membrane
most common form of endocytosis
receptor mediated endocytosis
types of receptor mediated endocytosis
clathrin-mediated endocytosis of virus by host
clathrin- and caveolin-independent endocytosis
caveolin-mediated endocytosis
how do enveloped viruses get out of the cell
fusion of virus membrane with host endosomal membrane releases viral genome. virus with glycoproteins bind to host membrane and fuse
how do non-enveloped viruses get out of the cell
lysis
ADCC
suface membrane fusion of enveloped viruses. viral glycoproteins are retained on the cell surface, and since these are antigenic, the cell can become a target of the host immune system
pH independent fusion proteins and examples
receptor binding, virus with host membrane fusion, release of viron contents into host cytoplasm
ex. HIV & Measles
pH dependent fusion proteins and examples
endocytosis, acidification(low pH) of endosome, HA configuration changes, membrane fusion, RNA genome enters cytosol
ex. HA in Influenza
pore-mediated penetration
some non-enveloped viruses inject their genome into the host cytoplasm through creation of a pore in the host membrane
antibody mediated attachment and penetration
antibodies against spike proteins of FIP cannot clear the virus from host. Rather these antibodies bind to FIP virus spike proteins and facilitate entry of viruses into host cells via antibody Fc receptor
ex. FIP- in macrophages
uncoating within endosome (enveloped viruses)
low pH in endosome promotes fusion of envelope with endosomal membrane; lysis of nucleocapsid by lysosomal proteases
uncoating at the nuclear membrane (4 ways)
1-pass directly through nuclear pores
2-attach to nuclear pore complex and release genome inside nucleus
3-wait for mitosis
4-genome released in nucleus and pass through pores
poliovirus uncoating
not completely uncoated until inside the cell
poxvirus uncoating
complex series of steps involving both host and viral gene products
retroviruses and reoviruses replication
first stages of viral replication cycle occurs inside the capsid
replication of double-stranded DNA viral genome and production of viral mRNA
dsDNA–> (+)RNA –> viral protein
*dsDNA for progeny viruses
replication of single-stranded DNA viral genome and production of viral mRNA
(+)ssDNA –> dsDNA intermediate –> (+)RNA –> viral protein
*(+)ssDNA for progeny
replication of double-stranded RNA viral genome and production of viral mRNA
dsRNA –> (+)RNA –> viral proteins
*dsRNA for progeny
replication of single-stranded (-)RNA viral genome and production of viral mRNA
(-)ssRNA –> (+)RNA –> viral proteins
*(+)ssRNA intermediate –> (-)ssRNA for progeny
replication of a single-stranded (+)RNA viral genome and production of viral mRNA
(+)ssRNA –> (+)RNA –> viral protein
or
(+)ssRNA –> (-)ssRNA –> (+)ssRNA for progeny
replication of single-stranded (+)RNA viral genome and production of viral mRNA by way of REVERSE TRANSCRIPTASE
(+)ssRNA –> (-)ssDNA –> dsDNA intermediate –> (+)RNA –> viral protein
or
dsDNA intermediate –> (+)ssRNA for progeny
replication of double-stranded DNA viral genome and production of viral mRNA by way of REVERSE TRANSCRIPTASE
dsDNA –> (+)ssRNA –> (-)ssDNA –> dsDNA –> (+)RNA –> viral protein
mRNA cannot be recognized until it has __________ & a ____________
cap & a tail
capping
addition of 7-methylgluanosine to the 5’ end of RNA, binds to the mRNA ribosomes, mark mRNA as self
how are caps synthesized (3ways)
1- synthesized by host cell enzymes (eg. Retrovirus, Adenovirus)
2- by viral enzymes (eg. Poxvirus, Reovirus)
3- cap snatching from host mRNA (eg. Influenza)
addition of tail
3’ poly- adenylated tail, tail interacts with poly-A binding proteins- important for translation
3’ polyadenylation
a stretch of adenylate residues are added to the 3’ end. The poly-A tail contains ~250 A residues
major signal for the 3’ cleavage for the tail is
“AAUAAA” - cleavage occurs 10-35 nucleotides downstream from the specific signal sequence
exon
portion of a gene that codes for AAs
intron
portion of a gene that does not code for AAs or proteins
constitutive splicing
every intron is spliced out and every exon is spliced in
alternative splicing
all introns are spliced out and only selective exons are spliced in.
monocistronic
mRNA that encodes for only one polypeptide
polycistronic
mRNA that encodes for several polypeptides
two types of polycistronic and what they yield
1- translation: polyprotien (multiple proteins stuck together), proteases chop them up to yield functional proteins
2- endonuclease: chops up mRNA into monocistronic mRNA, each encodes for a specific protein, they are translated into functional proteins
viral proteins
enzymes, structural proteins, viral nonstructural proteins, regulatory proteins, inhibitors
where does assembly and maturation occur
nucleus, cytoplasm, plasma/cell membrane
release of progeny viruses (how in naked viruses? in enveloped?
naked- lysis
enveloped- budding or lysis
layers of enveloped virus once budding occurred
matrix protein, host cytoplasm, virus glycoproteins
viruses that can bud through the golgi or ER
flaviviruses, arteriviruses, coronaviruses and bunyaviruses
extracellular spread of viruses
virus is adsorbed, enters, replicates, assembles, released and infects new host cell
intracellular spread
spread from cell to cell without contact with extracellular milieu.
mechanisms of intracellular spread
- cell-cell plasma-membrane fusion (Herpesvirus, Paramyxovirus and Retrovirus)
- passage of virions across tight junction (Herpesvirus)
- movement of virions across neural synapse (Rabies)
- viral induction of actin (Poxvirus)
- viral subversion of actin
- membrane nanotube subversion (HIV-1)
- virological synapse (Retrovirus)
nuclear spread of virus genome
viral genome is integrated into the host cell genome and passed down to next progeny or generation of host cells (Retroviruses)
