Viral Replication Process
viruses are transferred via virions. when the virion invade the host cell, it disassembles and the genetic material ( DNA or RNA ) of the virus start to interfere with the host cell process.
The duration of replication varies for different viruses; some have slow proliferation rates whereas others have fast proliferation rates.
The simplest definition for viral replication is disassembly followed by reassembly.
Intracellular viral replication cycle phases
A- Eclipse phase:
this phase end up with uncoating of the virus, it takes about 10 to 12 hours after the entry of the virus to the host cell.
uncoating is removing of capsid of the virus, this step will allow the access of the genetic mateial of the virus to the cytoplasm of the host cell.
This phase start by virus attachment to the host cell to enter the cell cytoplasm and disassemble to enable the viral genome expression which is important for replication process.
Adsorption or attachment means entering of the virus to the host or target cell, viruses are host specific and use receptors and anti-receptors for attachment and entry into host cell.
Cellular receptors and anti-receptors are mostly protein but sometimes they may be glycoprotein, carbohydrates or lipids .
❎ Penetration :
penetration or entry is energy dependent process, Virus may enter the host cell by Endocytosis or Translocation or Fusion .
Invagination of the cell wall to enclose the virus in a pinocytic vacule a process called (pinocytosis).
fusion of virus syncytia with cell wall (syncytia producing viruses) and
then releases the virion into the host cytoplasm.
The removal or degradation of capsid, this step lead to the release of the genetic material of the virus into the host cell cytoplasm. the genetic material is then transported to the site where transcription/replication can begin.
Some viruses don’t undergo uncoating/ degradation step because there capsid proteins play a role in viral transcription and replication.
B- synthetic phase
New virus particles are assembled during this period , this phase include two stages and take about 4-6 hours
Early Protein synthesis stage:
This early stage involves the synthesis of non-structural proteins, such viral enzymes and proteins needed in the viral replication process.
Late protein synthesis stage:
This late stage involves the synthesis of structural proteins, such capsid protein and envelope glycoprotein and spike proteins needed in reassembly of the virus.
❎ Genome replication :
Viral genetic material or genome is multiplied within the host , simultaneously viral structural proteins like capsids are synthesised. Type of genetic material varies from virus to virus , some are RNA and some are DNA.
Transcription and Translation
Transcription is production of virus mRNA from viral genome, Carried by host cell (-ve) sense or host enzymes – (+ve) sense. Transcrption can be early and late (before and after nucleic acid replication).
Involves the collection of all components necessary for formation of viron , It takes place at a particular site in the cell For example in pox viruses assembly occurs in the cytoplasm; in adenovirus it occurs in nucleus.
❎ Maturation :
Maturation is the stage of life cycle at which the virus become infectious , It involves structural change in virus particles , For some viruses maturation occurs only after release of viurs particle from the cell .
❎ Release :
Newly formed viruses are released to outside of the cell either by lysis or by budding . Generally non enveloped viruses release by cell lysis which results in the death of host cell .
In general terms, virus replication involves three broad stages carried out by all types of virus; the initiation of infection, replication and expression of the genome, and, finally, release of mature virions from the infected cell. At a detailed level, there are many differences in the replication processes of different viruses which are imposed by the biology of the host cell and the nature of the virus genome. It is possible to derive an overview of virus replication and the common stages which, in one form or another, are followed by all viruses.