Replication Flashcards
Replication of viruses
Viruses do not have genetic capability to multiply by division
Virus hijacks and uses host cell machinery to produce its proteins and nucleic acid for next gen
Process of virus replication in host cell=assembly line
Permissive cell
Cell in which virus is able to replicate
Cell machinery supports replication of virus
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
Absence of appropriate receptors
MOI- multiplicity of infection
Refers to the number of virions that are added per cell during infections
Latent period
After uncoating and til just before 1st appearance of extracellular new virus particle
Time before new infectious virus appears in the medium
During this phase no extracellular virions are detected
Eclipse period
After uncoating and till just before 1st appearance of intracellular new virus particle
Time interval between uncoating (dissapearance of viruses) and appearance, intracellularly of first infectious progeny virions
Burst size
Number of infectious virions released per average cell
Steps of virus replication
Attachment Penetration Uncoating Synthesis of viral components (nucleic acid and protein) Assembly and Maturation Release in large numbers
Attachment
Virus attachement to receptor on host cells is very specific
Lock and key
Each virus has its own specific receptor or a few receptors on specific host cells.
Attachment to host cell surface
Mediated by interactions between the virus and complimentary receptor on host cell surface
Cell that lack appropriate receptor escape from being infected
In some cases, binding to a cellular receptor is not sufficient for infection: an additional cell surface molecule, or co-receptor, is required for entry
Some viruses may use more than one host cell receptor- like HIV
Co-receptor
In some cases, binding to a cell receptor is not sufficient for infection. An additional cell surface molecule or co-receptor is required for entry
Virus penetration and uncoating in host cells- naked viruses
Receptor mediated endocytosis (commonly seen)
Poor mediated penetration (in some naked viruses)
Virus penetration and uncoating in host cells- enveloped viruses
Depends on type of Fusion protein
Surface membrane fusion (have pH independent Fusion protein)
Receptor mediated endocytosis (have pH dependent Fusion protein)
Virus penetration and uncoating in host cells- other methods
Antibody-mediated attachment and penetration
FIPV
Clatherin mediated endocytosis
Or other receptor mediated
Host cell membrane form vesicle wall
pH of endosome falls and virus capsid bursts (rupture) from endosome.
Are uncoated (by enzymes) and nucleic acid released into cytoplasm
Pore mediated penetration of viral genome
Non-enveloped viruses
Some inject genome into host cytoplasm through creation of a poor in the host membrane
Surface membrane fusion
pH independent Fusion protein
Fusion of virus envelope with host cell membrane occurs on surface of host cell.
Facilitated by pH independent fusion protein.
capsid digested by proteolysis enzymes.
released into cytoplasm
Receptor mediated endocytosis (pH dependent)
Fusion of virus membrane with host endosomal membrane release viral genome. Fusion protein requires low pH to get activated, which is achieved in endosome and thus facilitate virus envelope fusion with endosomal membrane
Fusion- not rupture!
Antibody mediated attachment and penetration
Antibodies bind to virus and then can bind to host cell and allow virus inside
Virus uncoating
Release of viral genome in host cell
Virion can no longer be detected
loss of infectivity of virions
Parent virus
Function 1: multiple copies for new viruses
Function 2: viral proteins for capsid and successful replication
Reverse transcriptase
Conversion of viral RNA to cDNA during virus replication
Processing of primary RNA transcript
The viral mRNA must conform to the requirements of host cell translation system– host cell can recognize the mRNA and translate same
A series of modifications occur: capping, addition of polyA tail, splicing
After processing, mRNAs are translated in cytoplasm
Viral mRNAs produced in the nucleus must also be exported to cytoplasm
Capping
Addition of 7-methylguanosine to the 5’ end of RNA
PolyA
Addition of 3’ poly-adenylated tails
Splicing
RNA splicing is a process that removes introns and joins exons in a primary transcript
Types of viral mRNA
Monocistronic: mRNA that encodes one polypeptide
Polycistronic: mRNA that encodes several polypeptides
Polycistronic mRNA
translation to polyprotein and protease to functional proteins
OR
endonuclease to monocistronic mRNAs then translation to functional proteins
Assembly and maturation
Assembly of virus genome and proteins into new virions follow a specific order
All components, including nucleic acids and proteins, are packages to form mature virions
May take place in nucleus, cytoplasm, plasma/cell membrane (enveloped viruses)
Release of progeny virions
Naked virions: lysis of host cell
Enveloped virions: budding
Naked cant do budding bc they dont have an envelope
Exocytosis
Viruses mature by budding through the membranes of golgi apparatus or ER.
Vesicles containing the virus then migrate to the plasma membrane and released by exocytosis
Viruses that acquire the envelop while budding from ER membrane leave infected host cell by exocytosis
Replication of retroviruses
Reverse transcriptase: synthesized RNA into DNA
Integrase: integrated viral DNA into host genome
Cell-to cell spread
Extracellular spread- released to outside travel to new cell
Intercellular spread- cell to cell without contact with extracellular milieu
Nuclear spread- passed down to progeny of host cell
