Lecture 4 Flashcards
How is the one step growth generated?
The one-step growth curve is generated by infecting all cells in a culture at the same time, then following the virions’ growth over time. The Y axis shows infectious units (virions) per cell, and the X axis shows hours post-exposure to the virus. The curve is represented by the viral particles per cell – over time, as the virus takes over the cell’s resources, the cell breaks open and falls apart. After this point, we can observe virus particles outside of the cell (dotted line = “cell-free virus”).
How is the the one step growth curve used?
The one-step growth curve is used to study viruses because in this model, everything is synchronized and occurring at the same time. If we infected different cells in the culture at different times, then we would not be able to tell when different steps in the viral replication cycle were occurring. As different cells became infected and produced progeny virions at different times, then the steps would overlap and we could not distinguish between them
What is MOI?
multiplicity of infection
ie the ratio of infective virions/cell
2) Describe the individual steps/phases of the virus replication cycle using the poliovirus as a model
a) Virus attaches to cell
b) Virus penetrates into the cytoplasm of the cell via endocytosis-like mechanism (this mechanism differs depending on the family of virus).
c) Virus uncoats and exposes its genome (ssRNA in the case of poliovirus).
d) Since poliovirus has a positive sense RNA genome, it is translated immediately into one big polypeptide.
e) Posttranslational modification: regions on this 3D polypeptide have protease activity – the polypeptide undergoes self-cleavage into smaller polypeptides.
a. Some of these smaller polypeptides are regulatory and shut down the cell machinery so that the resources that the cell was using are now available to the virus.
b. Poliovirus codes for its own RNA-dependent RNA polymerase (RdRpol).
c. Structural proteins are also created.
f) The positive sense genome is used as a template by RdRpol to generate a replicative intermediate (RI) – this is a negative strand that can serve as a template to generate more positive strands.
a. Some of the resulting positive strands will be translated further to produce more proteins
b. Other positive strands serve as genomic material for progeny virions
g) The genomic material and proteins come together during assembly and then undergo egression from the cell.
a. In the case of polioviruses, egression occurs via cell lysis – the cell can no longer maintain the integrity of its membranes because the machinery has been taken over by the virus.
Describe the differences in replication time and virus load within/between the DNA and RNA virus families.
The virus replication time represents the amount of time between infection and production of maximum progeny virus yield.
DNA viruses: on this chart, the max amount of DNA viruses in a cell approaches ~1000. Some viruses (ex. poxviruses) only approach a lower amount ( perhaps because they are so large?). Some families take longer amounts of time to reach max virus production.
RNA viruses: compared to the DNA viruses, the number of virus particles that are possible to be produced per cell is much higher – the max amount of viruses approaches ~100,000. There is a range in time required to reach max yield of virus progeny.
Picornaviruses have ability for huge yields.
Describe the role of antireceptors in the replication process and their potential to influence pathogenesis
The attachment phase of the virus replication cycle is highly dependent on the specific interactions between antireceptors and receptors. Receptors on the host cells have evolved for the purposes of the host cells, but viruses have evolved to take advantage of these host cell receptors.
An example of how the aR-R interaction can influence pathogenesis is the HIV model. HIV requires multiple receptors to attach and infect a cell. Different strains of HIV utilize different co-receptors, leading to infection of different cell types, leading to different disease manifestations
