Cycle Of Virus In Cell.Viruses are unique: they are exceedingly small, often made up of nothing more than a nucleic acid molecule within a protein shell, yet when they enter cells, they parasitize the cellular machinery to produce thousands of progeny. This simplicity is misleading: viruses can infect all known life forms, and they comprise a variety of structures and genomes. Despite such variety, viruses are amenable to study because all viral propagation can be described in the context of three fundamental properties, as described in Chapter 1: all viral genomes are packaged inside particles that mediate their transmission from cell to cell; the viral genome contains the information for initiating and completing an infectious cycle; and all viruses can establish themselves in a host population to ensure virus survival.
The objective of research in virology is to understand how viruses enter individual cells, replicate, and assemble new infectious particles. These studies are usually carried out with cell cultures, because they are a much simpler and more homogeneous experimental system than animals. Cells can be infected in such a way as to ensure that a single replication cycle occurs synchronously in every infected cell, the one-step growth curve. Because all viral infections take place within a cell, a full understanding of viral life cycles also requires knowledge of cell biology and cellular architecture. Tese are the topics of this chapter: the cell surface (the site at which viruses enter and exit cells), methods for detecting viruses and viral growth, and one-step growth analysis.
Infectious Cycle Of Virus In Cell
The production of new infectious viruses can take place only within a cell (Fig. 2.1). Virologists divide the viral infectious cycle into discrete steps to facilitate their study, although in virus-infected cells no such artificial boundaries occur. The infectious cycle comprises attachment and entry of the particle, production of viral mRNA and its translation by host ribosomes, genome replication, and assembly and release of particles containing the genome. New virus particles produced during the infectious cycle may then infect other cells. The term virus reproduction is another name for the sum total of all events that occur during the infectious cycle.
There are events common to virus replication in animals and in cells in culture, but there are also many important differences. While viruses readily attach to cells in culture, in nature a virus particle must encounter a host, no mean feat for nanoparticles without any means of locomotion. Afer encountering a host, the virus particle must pass through physical host defenses, such as dead skin, mucous layers, and the extracellular matrix. Host defenses such as antibodies and immune cells, which exist to combat virus infections, are not found in cell cultures. Virus infection of cells in culture has been a valuable tool for understanding viral infectious cycles, but the differences compared with infection of a living animal must always be considered.
The infectious cycle
Many distinct functions of the host cell are required to complete a viral life cycle. A productive infection requires target cells that are both susceptible (i.e., allow virus entry) and permissive (i.e., support virus reproduction).
Viral nucleic acids must be shielded from harsh environmental conditions as extracellular particles, but be readily accessible for replication once inside the cell.
To advance their study, viruses may be propagated in cells within a laboratory animal or in cell cultures, which include immortalized cells or primary cultures derived from the natural host or other animals. Plaque assays are the major way to determine the concentration of infectious virus particles in a sample, though alternative strategies exist for viruses that do not form plaques.
While the goals of quantifying and characterizing virus particles remain fundamental for research in virology, the specific techniques used evolve rapidly, based on developments in detection, ease, cost, safety, utility in the field, and amenability to large-scale implementation.
Viral nucleic acids can be detected and characterized by multiple methods, including direct sequencing of genomes and mRNAs, PCR, and microarrays. Relationships among viruses can be deduced from phylogenetic trees generated from protein or nucleic acid sequences. Viral reproduction is distinct from cellular or bacterial replication: rather than doubling with each cycle, each single cell cycle of viral reproduction is typically characterized by the release of many (often thousands) of progeny virions. The multiplicity of infection (MOI) is the number of infectious units added per cell in an experimental setting; the probability that any one target cell will become infected based on the MOI can be calculated from the Poisson distribution. Application of systems biology approaches to virology can implicate particular cellular pathways in viral reproduction and can reveal signatures of virus-induced lethality or immune protection.
Entering Cells Viral infection is initiated by a collision between the virus particle and the cell, a process that is governed by chance. Consequently, a higher concentration of virus particles increases the probability of infection. However, a virion may not infect every cell it encounters; it must first come in contact with the cells and tissues to which it can bind. Such cells are normally recognized by means of a specific interaction of a virus particle with a cell surface receptor. These molecules do not exist for the benefit of viruses: they all have cellular functions, and viruses have evolved to bind them for cell entry. Virus-receptor interactions can be either promiscuous or highly selective, depending on the virus and the distribution of the cell receptor. The presence of such receptors determines whether the cell will be susceptible to the virus. However, whether a cell is permissive for the reproduction of a particular virus depends on other, intracellular components found only in certain cell types.
Cells must be both susceptible and permissive if an infection is to be successful. Viruses have no intrinsic means of locomotion, but their small size facilitates diffusion driven by Brownian motion. Propagation of viruses is dependent on essentially random encounters with potential hosts and host cells. Features that increase the probability of favorable encounters are very important. In particular, viral propagation is critically dependent on the production of large numbers of progeny virus particles with surfaces composed of many copies of structures that enable the attachment of virus particles to susceptible cells. Successful entry of a virus into a host cell requires traversal of the plasma membrane and in some cases the nuclear membrane.
Te virus particle must be partially or completely disassembled, and the nucleic acid must be targeted to the correct cellular compartment. Tese are not simple processes. Furthermore, virus particles or critical subassemblies are brought across such barriers by specific transport pathways. To survive in the extracellular environment, the viral genome must be encapsidated in a protective coat that shields viral nucleic acid from the variety of potentially harsh conditions that may be met during transit from one host cell or organism to another. For example, UV irradiation (from sunlight), extremes of pH (in the gastrointestinal tract), dehydration (in the air), and enzymatic attack (in body fluids) are all capable of damaging viral nucleic acids.
However, once in the host cell protective structures must become sufficiently unstable to release the genome. Virus particles cannot be viewed only as passive vehicles: they must be able to undergo structural transformations that are important for attachment and entry into a new host cell and for the subsequent disassembly required for viral replication. Making Viral RNA Although the genomes of viruses come in a number of configurations, they share a common requirement: they must be efficiently copied into mRNAs for the synthesis of viral proteins and progeny genomes for assembly. The synthesis of RNA molecules in cells infected with RNA viruses is a unique process that has no counterpart in the cell. With the exception of retroviruses, all RNA viruses encode an RNA-dependent RNA polymerase to catalyze the synthesis of mRNAs and genomes. For the majority of DNA viruses and retroviruses, synthesis of viral mRNA is accomplished by RNA polymerase II, the enzyme that produces cellular mRNA. Much of our current understanding of the mechanisms of cellular transcription comes from study of the transcription of viral templates