VIRAL REPLICATION Flashcards
What are the steps of viral replication?
I* Attachment of virus to host cell
II* Penetration
III* Uncoating
IV* Transcription of early mRNA
V* Translation of early proteins
VI* Replication of viral genome
VII* Transcription of late mRNA
VIII* Translation of late proteins
IX* Assembly of virions
X* Release.
Attachment of Virus to Host Cell
*This process is also called as binding or adsorption.
*This is a specific process wherein the structures present on the surface of the virus (sometimes called as ligands) bind to the receptors on the surface of the host cell.
The presence of appropriate receptors on the cell surface is a prerequisite for virus attachment and subsequent infection.
Discuss the 4 mechanisms of Penetration
There are four possible mechanisms which the viruses use to enter a host cell. These are:
- Translocation is the process by which the whole nonenveloped virus enters the host cell by moving across the cell membrane.
- Endocytosis is the engulfment of the virus by the invagination of a section of plasma membrane. This is a common mode of penetration of nonenveloped viruses.
- Fusion is the endocytosis of enveloped viruses in which the envelopes of the viruses fuse with the membrane of the endosome.
- Direct fusion of the virion envelope with the surface membrane of the cell also takes place in some of the families of viruses.
- Certain naked viruses (e.g. enteroviruses) are taken into the cells by viropexis, a process that is similar to phagocytosis.
Uncoating
Immediately after entry into the host cell, interaction
of the virion with host components and enzymes leads to a breakdown of the virion and exposure of the genome so that it can express.
Transcription of Early mRNA
•Transcription of early mRNA is required to produce certain enzymes and proteins which are necessary for the replication of viruses and that of late mRNA is for viral structural proteins.
•The DNA viruses which replicate in the host nucleus, use the host DNA dependent RNA polymerase to perform this function.
•All other viruses require virus coded transcriptase to synthesize required mRNA .
Translation of Early Proteins
•The viral mRNAs use the ribosomes of the host cell to synthesize proteins in the same way as the host mRNA.
• A wide variety of proteins are synthesized and their production is regulated by a mechanism at the level of translation.
• Two types of proteins are synthesized: early and late, depending upon their time of appearance.
Replication of Viral Genome
•Viruses reproduce from their nucleic acid, which directs not only its own replication but also the synthesis of virus specific proteins.
• Some of these are structural components of the virion, while others are proteins leading to the selective shutdown of synthesis of cellular
macromolecules or enzymes involved in the synthesis of viral components.
•The viral nucleic acids replicate by Watson-Crick base pairing.
Transcription of Late mRNA
•This is required for the production of structural proteins.
• The cytoplasmic double stranded DNA carries DNA dependent RNA polymerase where ssRNA viruses make use of their own ssRNA dependent RNA polymerase to transcribe late mRNA.
Translation of Late Proteins
•These result after the synthesis of late mRNA and are usually the result of instructions of the newly formed progeny of the virus in the host cell.
Assembly of Virion
*Virion assembly may take place in the host cell in the cytoplasm or the cell nucleus.
* Herpesviruses and adenoviruses are assembled in the nucleus whereas the picornaviruses and the poxviruses are assembled in the cytoplasm of the host cell.
There are three mechanisms by which mature virions can escape from the host cell and start their cycle once again in new host cells. These are:
- Cell Lysis: This occurs with most of the nonenveloped viruses when the cell lysis occurs after the completion of the replication of the viruses. This cell lysis is not the result of natural death of the cell but is because of the large number of viruses in the host.
- Cell Degeneration: Many viruses such as parvoviruses accumulate within the nucleus of the host and are released only after the death of the cell which follows the degeneration of the cell.
- Budding: This is the mechanism by which most of the enveloped viruses exit from the host cell without damaging the cell.
- This process takes a long time and does not kill the host cell.
General features about viral replication.
1• Occurs only in living cells
2• Involves many host cell enzymes and functions
3• Follows a sequential pattern as described above
4• May be incomplete in some cells (abortive infection)
5• May lead to the death of host cells (virulent viruses)
6• May occur without apparent damage to the host cell (moderate
viruses)
7• Is very similiar for all viruses in a specific family
What are Abortive Viral Infections?
•Viral infections of the host cells are not always productive. Infection of a cell with a virus shall not necessarily result into production of large number of viruses.
• Either the viral coded products are not synthesized or these are not properly assembled. Such processes are known as abortive infections.
•These may be due to defect in the host cell which is unable to manufacture products as per the demands of the virus or because of an inherent fault in the virus itself.
DEFECTIVE VIRUSES
•Certain viruses which are unable to replicate themselves in a responsive host cell are called as defective viruses. Some of these can replicate in the presence of a helper virus. Adeno-associated viruses are important examples because these cannot multiply unless adenovirus is present in the same host cell.
•The extreme example of defectiveness is exhibited by papillomaviruses which on infecting a cell either replicate as episomes or get integrated into the chromosome of the host and replicate with it.
