Viruses are obligate parasite they can replicate only on living host cell.
Smallest viruses have genetic material enclosed by protein coat capsid.
Whereas other viruses have catalytic regulatory and structural protein and some have host derived envelope.
Replication of virus involves following steps.
- Attachment
- Penetration
- Uncoating
- Replication
- Assembly
- Release of virions.
Attachment
Infection begins with attachment of capsid or envelop protein with specific cell surface receptors on host cell membrane through attachment proteins in capsid or via glycoprotein embeded in viral envelop
Penetration
Attachment process induces conformational changes in viral capsid or lipid envelop of host cell and fuses.
- Plant and animal virus enters by endocytosis in this cell membrane surrounds and engulf the entire virus.
- And some enveloped virus enters in cell when envelop of virus directly fuses with cell membrane
- Bacteriophages injects only nucleic acids in cells, the virus does not enters.
Uncoating
Once inside the cell cell envelop gets destroyed and the genetic material of virus will liberated in the cytoplasm of host cell.
Replication
- Viruses do not grow by cell divisions because viruses are not cells.
- They use the host cell machinery to produce new copies of itself.
- After infecting the host cell virion uses host cells ribosome,enzymes, ATP and other components to replicate.
- Viruses vary in replication process some RNA viruses are translated direct to viral proteins in ribosome of host cell.
- Some DNA viruses first transcribed in viral mRNA then these mRNA are translated in to proteins by host cell.
Replication of RNA Viruses
An RNA virus is a virus that has RNA as its genetic material. Their nucleic acid is usually single-stranded RNA, but may be double-stranded RNA. Animal RNA viruses can be placed into different groups depending on their type of replication.
- Some RNA viruses have their genome used directly as if it were mRNA. The viral RNA is translated directly into new viral proteins after infection by the virus.
- Some RNA viruses carry enzymes which allow their RNA genome to act as a template for the host cell to a form viral mRNA.
- Retroviruses use DNA intermediates to replicate. Reverse transcriptase, a viral enzyme that comes from the virus itself, converts the viral RNA into a complementary strand of DNA, which is copied to produce a double stranded molecule of viral DNA. This viral DNA is then transcribed and translated by the host machinery, directing the formation of new virions. Normal transcription involves the synthesis of RNA from DNA; hence, reverse transcription is the reverse of this process. This is an exception to the central dogma of molecular biology.
Replication of DNA Viruses
A DNA virus is a virus that has DNA as its genetic material and replicates using a DNA-dependent DNA polymerase. The nucleic acid is usually double-stranded DNA but may also be single-stranded DNA. The DNA of DNA viruses is transcribed into mRNA by the host cell. The viral mRNA is then translated into viral proteins. These viral proteins then assemble to form new viral particles.
Assembly of virus
Once sufficient amount of viral genetic material and proteins are synthesized nucleocapsid formation and assembly of virions begins i.e. formation of new virus.
Release of virions
The release of virions from the host cell can occur via two main methods. The lysis method results in the death of the host cell and the viruses that do this are usually referred to as cytolytic viruses.
Viruses with envelopes are more commonly released by budding, allowing the virus to gain a viral phospholipid envelope. These viruses do not usually kill the host cell.
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