Viruses replicate inside host cells using the cell's machinery. The replication process involves 1) attachment to and entry into the host cell, 2) uncoating of the viral genome from the protein capsule, 3) replication of the viral genome, and 4) assembly of new virus particles and their release from the cell. Different viruses follow different pathways depending on whether their genome is DNA or RNA and whether the RNA is single-stranded, double-stranded, or requires an intermediate DNA copy. All viruses must package within their particles any enzymes required for their specific replication strategy.

1. virus

2. Viral replication
1. Adsorption
2. Penetration
3. Uncoating
4. Viral genome replication
5. Assembly and release

3. Steps in replication
1. Attachment. This is the first step in viral replication.
Surface proteins of the virus interact with specific
receptors on the target cell surface
2. Penetration. Enveloped viruses (e.g., HIV, influenza
virus) penetrate cells through fusion of the viral envelope
with the host cell membrane. Non-enveloped viruses
penetrate cells by translocation of the virion across the
host cell membrane or receptor mediated endocytosis of
the virion with accumulation of viruses in cytoplasmic
vesicles.
3. Uncoating (disassembly). The viral genome is uncoated
by removing the capsid proteins, allowing the genome
(nucleic acids)to be available for transcription and viral
multiplication.

4. 4. Genome replication. Viruses must use host cellular
machinery to replicate and make functional and structural
proteins. The first step in viral gene expression is mRNA
synthesis. Different viruses follow different pathway
depending on the nature of their nucleic acids and the part of
the cell in which they replicate
• DNA viruses generally replicate in the nucleus and use
host cell DNA- dependent RNA polymerase to synthesis
their mRNA
• Most RNA viruses replicate in the cytoplasm where they
do not have access to the host cell DNA- dependent RNA
polymerase. They therefore carry their own polymerase
within the virus particle
Steps of viral replication
cont.’d

5. Steps continued
5. Assembly and Release. Progeny virus particles are
assembled by packaging the viral nucleic acid within
the capsid proteins. Virus particles are then released
from the cell by either of two processes
1. Non-enveloped viruses are release by rupture of the
cell membrane and release of the mature particles
2. Enveloped viruses are released by budding through the
outer cell membrane (except herpesviruses which
acquires their envelope from the nuclear membrane
rather than from the outer cell membrane)

6. RNA replication
Phase II:Replication of viral nucleic acid and protein synthesis
Once uncoating has taken place, synthesis of viral NA starts.
This occurs as three different stages with differences between
different families of the viruses.
Early transcription and translation: The proteins derived from
this stage is mostly the enzymes required for virus replication.
Replication of Nucleic acid:
Late transcription and translation :The proteins produced
during this stage are structural proteins.

7. RNA viruses that do not
have a DNA phase
Viruses that replicate via RNA intermediates need an
RNA-dependent RNA-polymerase to replicate their
RNA, but animal cells do not seem to possess a suitable
enzyme. Therefore, this type of animal RNA virus
needs to code for an RNA-dependent RNA polymerase.
No viral proteins can be made until viral messenger
RNA is available; thus, the nature of the RNA in the
virion affects the strategy of the virus:

8. Plus-stranded RNA viruses
In these viruses, the virion (genomic) RNA is the
same sense as mRNA and so functions as mRNA.
This mRNA can be translated immediately upon
infection of the host cell
Examples:
poliovirus (picornavirus)

9. Negative-stranded RNA
viruses
The virion RNA is negative sense (complementary to mRNA)
and must therefore be copied into the complementary plus-
sense mRNA before proteins can be made. Thus, besides
needing to code for an RNA-dependent RNA-polymerase,
these viruses also need to package it in the virion so that
they can make mRNAs upon infecting the cell.
Examples:
influenza virus (orthomyxovirus)

10. Double-stranded RNA
viruses
The virion (genomic) RNA is double stranded and so cannot
function as mRNA; thus these viruses also need to package
an RNA polymerase to make their mRNA after infection of
the host cell.
Example:
rotaviruses (belong to reovirus family)

11. RNA viruses that copy their
RNA to DNA
These are the retroviruses. In this case, their virion
RNA, although plus-sense, does not function as
mRNA immediately on infection since it is not
released from the capsid into the cytoplasm. Instead,
it serves as a template for reverse transcriptase and
is copied into DNA. Reverse transcriptase is not
available in the cell, and so these viruses need to
code for this enzyme and package it in virions.