what are the three basic function of a capsid?
protect the genetic material
aid entry into the cell
package viral enzymes
what are the three general capsid types?
helical, icosahedral, or complex(combination of helical and icosahedral)
helical virus
composed of multiple protein subunits packed into a helical arrangement with the genetic material coiled inside
Icosahedral virus
composed of repeating subunits called capsomeres, which are composed of 5-6 individual protein structures called a protomer
what makes up a capsomere?
5-6 protomers
when a virus is encompassed in a host cells’ lipid membrane they are said to be what?
enveloped
what are possible sources of an envelope?
plasma membrane
nuclear membrane
cytoplasmic membranes
what holds the envelope to the capsid?
tegument or matrix proteins
* can also be involved with delivering viral genetic material/enzymes for replication
RNA virus can have orientation of RNA
positive sense, negative sense, double stranded, or ambisense
positive sense
the RNA virus can be directly translated into viral proteins (5’-3’)
-basically is mRNA
negative sense
needs to form a complimentary strand to make a positive sense strand to make proteins (3’-5’)
what are the 7 groups of the baltimore system?
- DS DNA
- SS DNA
- DS RNA
- SS +RNA
- SS-RNA
- SS +RNA with RT has DNA intermediate
- DS DNA with RT
what do viral cells rely on to reach host cells?
collision
how to viruses attach to host cells?
via glycoproteins located on envelope or capsid, and target glycoproteins on host cell surface
is a virion receptor and attachment site a 1:1 ratio?
nope, remember the cell membrane is fluid
what determines the range of host/tissue specificity?
viral attachment proteins, thus a virus species will on infect certain cells or tissues
what is the eclipse phase?
once the virus is inside a cell and the host immune system can no longer detect it
antibodies for viruses prevent what step of a viral life cycle?
attachemnt
what are the two ways that viruses get into the cells?
direct fusion
viropexis
what is direct fusion?
viral attachement, protein mediated fusion of the viral envelope and plasma membrane, and results in the capsid directly entering the cell.
* therefore the viral envelope stays at the plasma membrane, and the nucleocapsid enters the cell
what is viropexis?
receptor mediated endocytosis. once in the endosome the low pH causes fusion and release of nucleocapsid into the cytoplasm.
* viral envelope does enter the cell, but fuses with the host endosomal membrane NOT the plasma membrane
naked capsid penetration
basically they enter through viropexis, but must disrupt the endosomal vesicle because they have no envelope to fuse with.
SS DNA parvovirus replication
- DNA folds that allows replication to produce a series of repeated viral genomes called a concatemer, creating a long continuous mRNA via
- The concatemer is then cleaved into single viral genomic DNA prior to assembly
DS DNA herpesvirus
- complex expression, alpha components, beta components, and gamma components
- latent phase-virus is replicated with host DNA
- lytic phase- when virus is in active replication
- can cause cancers due to DNA disruption
herpes alpha component
alpha are made first and are transcription factors for beta components
herpes beta component
beta proteins are helicases, primases, DNA polymerases that will drive viral replication and transcription of gamma components
herpes gamma components
gamma component is the structural components
SS + RNA virus
eg: picornaviurs, poliovirus, rhinovirus
usually replicate in cytoplasm
- RNA enters cell and makes viral proteins directly called a polyprotein precursor, which is later cleaved into functional parts.
- Some of these proteins make RNA replication enzymes, which make -RNA to make more +RNA
- as more +RNA are made they are transcribed and make more viral enzymes and proteins to allow formation of progeny.
SS-RNA virus
eg: orthomyxovirues aka influenza
need to make comp +RNA to be read by host ribosome
*packs its own RNA dependent polymerase to make this happen
-can replicate in cytoplasm or nucleus
comp +RNA stands are produced and sent to cytoplasm to be translated or retained as a template to make further -RNA
-viral structural proteins made by viral RNA+ which are assembled with -RNA
+ SS RNA with own reverse transcriptase
- bring own reverse transcriptase to make DNA
- from here it can be integrated into host by viral integrase and is said to be a provirus
- then viral mRNA can be made, and thus viral proteins to make more progeny
how does assembly happen?
- in such a way to favor the incorporation of viral genomes.
- site specific on genome for initiation of protein assembly.
- process is energetically favorable
naked viral egression
cause the cell to go through apoptosis by disruptions on protein synthesis/ and normal cell cycles.
-they keep the cell alive long enough to complete replication and then allow the cell to die to release the virus
enveloped virus egression
need to acquire host membrane, therefore tend to proceed down the normal protein secretion pathways.
- they are then expelled either by exocytosis or more commonly via budding from the plasma membrane.
- *they are now accessible to the immune system and have become visable
how to viral surface proteins get onto the host cell membrane?
the proteins are programmed to appear at the site where the envelope will form.
