Virus Replication: Introduction, and Steps of Replication with Short Description

Introduction of Virus Replication

Virus replication is s a complex process and totally depends on the host cell due to the lack of an enzyme system. Single replication of virus completes in 15-30 hours.

Steps in Virus Replications

Virus replication steps are as follows-

• Adsorption (attachment)
• Penetration
• Uncoating
• Biosynthesis-It further completes in following steps-

1. Replication of the viral genome.
2. Transcription of the viral genome into mRNA.
3. Translation of mRNA into viral proteins.
4. Protein synthesis,

• Viral assembly
• Release

Adsorption (Attachment)

Viruses must recognize and bind to specific cellular receptors on the surface of the infected cell via particular glycoproteins. e.g. acetylcholine of neural cell bind to rabies virus, HIV-1 for CD4 cell and Influenza virus for the sialic acid residue of respiratory epithelium

Penetration

Enveloped viruses that have the ability to form syncytia (multi-nucleated giant cells) enter the cell through the fusion of the viral envelope with the cell plasma membrane e.g. Paramyxovirus and herpes viruses. The remaining enveloped viruses enter the cell through endocytosis. Unenveloped viruses enter the cell either by endocytosis (endosome lyses as with adenoviruses) or by forming a pore in the membrane of the cell. The viral RNA is then released inside the cell (picornaviruses).

Uncoating

Release of the viral genome from its protective capsid to enable the viral nucleic acid to replicate.

Biosynthesis

It has the following steps:

1. Replication of viral nucleic acid
2. Translation of messenger RNA from viral nucleic acid
3. Translation of mRNA into early protein or nonstructural protein is are enzymes that initiate the maintained synthesis of the viral components.
4. Synthesis of late or structural proteins which are components of daughter virion capsids.
5. Critical step in viral biosynthesis is the transcription of mRNA from viral nucleic acid.

Baltimore Classification

• dsDNA viruses
• Depending on the structure of their genome, viruses use different strategies for the transcription of mRNA
• ssDNA viruses
• dsRNA viruses
• ssRNA viruses with positive strands( positive polarity)
• ssRNA viruses with negative strands(negative polarity)
• ssRNA viruses associated with the enzyme reverse transcriptase
• dsDNA virus with Reverse transcriptase (partially Ds DNA)

1. Double stranded DNA families of medical importance

dsDNA  → mRNA →  protein

e.g. Poxviridae  (small Pox virus, variola virus),Herpesviridae, Hepadnaviridae, Adenoviridae, Papovaviridae (papiloma)

2. Single stranded DNA family

e.g. Parvoviridae

ssDNA  → dsDNA → mRNA → protein

3. Double-stranded RNA family

e.g. Reoviridae

dsRNA →  mRNA → protein

4. Single-stranded RNA families with positive strands

The viral genome acts directly as mRNA.

ssRNA+ sense   → protein

Picornaviridae (hepatitis A, Polio), Coronaviridae, Flaviviridae (dengue, yellow fever), Togaviridae (Chikungunya, WEEV, EEEV)

5. Single stranded RNA families with negative strands

The viral genome does not act as m-RNA. It must be transcribed by the viral enzyme transcriptase into mRNA. Virions contain the  enzyme  transcriptase. e.g. Orthomyxoviridae (Influenza), Paramyxoviridae (measles, RSV), Rhabdoviridae (lyssavirus), Filoviridae (Ebola, Marburg)

6. Single-stranded RNA viruses associated with the enzyme reverse transcriptase

The viral genome is reverse transcribed into a complementary DNA strand using the enzyme reverse transcriptase. e.g. Retroviruses

7. Ds DNA virus with Reverse transcriptase

e.g. Hepadna virus 

Assembly

New virus genomes and proteins are assembled to form new virus particles.

Release

Enveloped viruses are released by budding from the infected cells. Unenveloped viruses are released by the rupture of the infected cells.

Keynotes on Viral Replication

• ds: Double Stranded
• ss: Single Stranded
• mRNA: Messenger Ribonucleic Acid
• WEEV: Western Equine Encephalitis Virus
• EEEV: Eastern Equine Encephalitis Virus
• RSV: Respiratory Syncytial Virus