1.4 THE VIRUS MULTIPLICATION CYCLE

 

We now know a great deal about the processes which occur during the multiplication of viruses within single cells. The precise details vary for individual viruses but have in common a series of events marking specific phases in the multiplication cycle. These phases are summarized in Fig. 1.3 and are considered in detail in section II of this book. The first stage is that of attachment when the virus attaches to the potential host cell. The interaction is specific, with the virus attachment protein(s) binding to target receptor molecules on the surface of the cell. The initial contact between a virus and host cell is dynamic and reversible, and often involves weak electrostatic interactions. However, the contacts quickly become much stronger with more stable interactions which in some cases are essentially irreversible. The attachment phase determines the specificity of the virus for a particular type of cell or host species. Having attached to the surface of the cell, the virus must effect entry to be able to replicate in a process called penetration or entry. Once inside the cell the genome of the virus must become available. This is achieved by the loss of many, or all, of the proteins that make up the particle in a process referred to as uncoating. For some viruses the entry and uncoating phases are combined in a single process. Typically these first three phases do not require the expenditure of energy in the form of ATP hydrolysis. Having made the virus genome available it is now used in the biosynthesis phase when genome replication, transcription of mRNA, and translation of the mRNA into protein occur. The process of translation uses ribosomes provided by the host cell and it is this requirement for the translation machinery, as well as the need for molecules for biosynthesis, that makes viruses obligate intracellular parasites. The newly synthesized genomes may then be used as templates for further rounds of replication and as templates for transcription of more virus mRNA in an amplification process which increases the yield of virus from the infected cells. When the new genomes are produced they come together with the newly synthesized virus proteins to form progeny virus particles in a process called assembly. Finally, the particles must leave the cell in a release phase after which they seek out new potential host cells to begin the process again. The particles produced within the cell may require further processing to become infectious and this maturation phase may occur before or after release. Combining the consideration of the steps which make up a virus multiplication cycle with the information in the graph of the results of a single step growth curve it can be seen that during the eclipse phase the virus is undergoing the processes of attachment, entry, uncoating, and biosynthesis. At this time the cells contain all of the elements necessary to produce viruses but the original infecting virus has been dismantled and no new infectious particles have yet been produced. It is only after the assembly step that we see virus particles inside the cell before they are released and appear in the medium

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