2 - Virus Replication Flashcards
Steps common to all viral life cycles
- Attachment (usually by receptor)
- Entry
- Translation
- Genome replication
- Assembly
- Release
Requirements for successful infection
- Dose (enough virus)
- Access to target cells (susceptible, permissive)
- Absent or insufficient host immunity
Susceptible cell
Has functional receptor for a virus (cell may or may not support viral replication)
Resistant cell
Has no receptor
Permissive cell
Has the capacity to replicate virus
What is the only cell that is able to take up a virus and replicate it
A susceptible and permissive cell
Host range of a virus
Range of species it can effect
Tissue tropism of a virus
The specific tissues that support virus replication inside an infected host animal (e.g. rabies infects nerves not kidneys)
Lytic infections
Host cell releases hundreds of new viruses and then dies as a consequence of viral infection (bursts open or lyses)
Use of the one step growth cycle method
Can track plaque-forming units per milliliter in the broth surrounding the host cells and the events inside the host cell, such as synthesis of RNA, proteins, and DNA.
Molecular events during each stage of the virus replication cycle
- Attachment (virion attaches to host cell via its receptor)
- Penetration and uncoating (virions genome enters host cell)
- Synthesis of early proteins
- synthesis of new genomes and late proteins (may overlap with stage 3)
- Assembly (component parts of the virion assemble into completed virions)
- Release (Can be gradual and may not lyse host immediately)
Three functions of early proteins
- To shut down the synthesis of host proteins
- To regulate the expression of viral genes
- Synthesise viral nucleic acids
Synthesis of new viral genomes and late proteins
- Late proteins are those expressed after genome replication has begun.
- The proteins made at this phase are usually structural proteins, meaning that they will become components of the progeny virions.
The first cell surface molecule that is found to be essential for virus binding
Receptor
Coreceptor
When binding of receptor is not sufficient for entry into the cell and binding to another cell surface molecule is needed
Example of receptor and coreceptor
HIV binds to cells via CD4 and requires interaction with a second cell surface protein CXCR4 or CCR5
Influenza virus (IVA)
- Has 8 genome segments consisting of single-stranded negative sense RNA closely associated with nucleocapsid proteins and enzymes (ribonucleoprotein/vRNP)
- vRNPs travel to nucleus using the cell’s cytoskeleton and motor proteins
Nucleocapsid proteins and enzymes
PB1, PA, and PB2
Influenza virus replication cycle steps 1-6
- Virus attaches to its receptor (sialic acids (SA, N-acetylneuraminic acid) on the surface of the host cell.
- The virus is internalised by endocytosis during the penetration step.
- During uncoating, the (ss -RNA) genome segments and vRNPs are released into the cytoplasm
- And are transported to the nucleus.
- The influenza genome is copied to make a double stranded template,
- ds template can be used for production of mRNA and new genomes.
Influenza virus replication cycle steps 7-11
- The mRNAs are exported to the cytoplasm
- mRNAs are translated to make IV proteins.
- Some influenza proteins reenter the nucleus to assemble with new genome
segments, whereas others assemble at sites of future budding. - The vRNPs assemble with other component parts of the virus.
- The virus exits the host cell through budding, assisted by a viral enzyme (the neuraminidase, NA) that degrades the host surface sugars (sialic
acids (SA, N acetylneuraminic acid) that would otherwise tether the virus to the cell surface.
What does expression of viral regulatory proteins and enzymes results from
Interaction between viral nucleic acids, proteins, and the host transcription and translation systems
Difference in bacterial and eukaryotic mRNA molecules structural features which are required for proper translocation, translation, and stability
- Transcription from a DNA template occurs in the cytoplasm in the Bacteria, but in the nucleus in the Eukarya.
- mRNA of bacteria has internal ribosome-binding sites, and is commonly polycistronic (encodes more than one protein)
- mRNA of eukaryotes has a 5ʹ cap, necessary for ribosome binding and for transport out of the nucleus, a poly(A) tail, also necessary for export from the nucleus and for stability, and usually encodes just one protein.
Viral genome, antigenome and mRNA
- The host transcription machinery in all cells exclusively uses a double-stranded DNA template to polymerize RNA.
- Viruses with RNA genomes must have a viral enzyme that can use that RNA as a template, whether to produce antigenomes, mRNA, or new genomes
Antigenome
- Complementary copies of the genome, used as a template to synthesize new genomes
- Lack 5’ cap and polyadenylation found in mRNA
Retroviruses
- Reverse transcribing viruses
- A viral enzyme is needed to make a DNA copy of an RNA molecule
Virus replication complexes (VRC)
Assembly factories in eukaryotic viruses that contain viral genomes, viral RNA polymerases, and other non structural proteins, and may be fashioned from the membranes of the ER, golgi, or other cytoplasmic organelles
Acquisition of envelope by IVA and virus budding
- Viral proteins are synthesized in the cytoplasm and on the endoplasmic reticulum.
- Viral proteins that will become part of new vRNPs must be trafficked to the nucleus while the viral envelope proteins traffic to the plasma membrane
- New vRNPs are synthesized in the nucleus and are then exported to the
cytoplasm. They travel along the microtubule network at the periphery of the cell, bundling along the way. - Eight different vRNPs gather under a patch of plasma membrane containing matrix and spike proteins
- The new virion buds away from the surface of the host cell.
Extracellular and cell to cell spread
- Many viruses spread from one host cell to another as extracellular virus particles released from an infected cell (Such extracellular dissemination is necessary to infect another naive host)
- Some viruses can also spread from cell to cell without passage through the extracellular environment
- Some viruses can be disseminated by both mechanisms
Multiplicity of Infection (MOI)
- Number of infectious virus particles added per cell
- Amount of virus (PFU) / number of cells
- Infection depends on random collision of virus particles and cells
PFU
Plaque forming units
Poisson distribution
When susceptible cells are mixed with virus, some cells are not infected, other cells receive 1 or more virus particles
Method of detection used for viruses that do not form plaques
Endpoint dilution assay
Are persistent long term infection common or uncommon
Common, most adult people have at least one longterm infection caused by a member of the Herpesviridae family
Chronic infection
Viral replication is continual, though the immune system holds clinical disease in check
Latent infections
Those in which there is little or no production of virions, yet the virus persists, typically as viral DNA in the nucleus of a differentiated cell
Example of a chronic viral infection
HIV
HIV Infection
- Over the course of a typical HIV infection, virions are found in very low levels in blood
- HIV is not a latent infection in which viral replication is not ongoing but instead persists as a chronic infection during which viral replication is ongoing
- Within weeks of infection, the number of host cells (CD4+ lymphocytes) in the blood declines (susceptible to infections
How are viruses quantified
- Plaque assay for cytopathic viruses
- Endpoint diultion assay for non cytopathic viruses
