Replication of Animal Virus
Replication (Reproduction) of virus takes place in cell; cytoplasm, nucleus or both. Uncoating of virus, its nucleic acid replication and the assembly of viral protein and nucleic acid components occur correspondingly.
It involves the attachment of virus on cell membrane which is specific and irreversible. Molecular entities on the surface of cells act as receptor and capsid protein of naked or envelopes of enveloped virus interacts.
Penetration and Uncoating:
The engulfment of whole virions followed by uncoating of capsids occur in naked virus, process called viropexis. In enveloped virus, viral lipoprotein envelope fuse with host cell surface membrane releasing viral nucleocapsid material into cytoplasm of host cell. Uncoating again occurs within the host cell.
Transcription, Replication and Biosynthesis:
After uncoating, the viral nucleic acid is freed from the capsid and is accessible to enzymes required to translate, transcribe and replicate it. Transcription of viral nucleic acid into m-RNA takes place except in RNA virus; whose viral RNA directly acts as m-RNA. RNA virus carrying -ve strand first transcribes itself to +ve strand which functions as m-RNA.
This biosynthetic process of virus specific molecules can be divided into early and late events.
Early m-RNA codes for early enzymes required for nucleic acids replication and for proteins that inhibit synthesis of cellular macromolecules and breakdown cellular polyribosomes in cytoplasm required for making them available in transcription of viral genes. Utilizing the enzyme, the DNA gets synthesized.
Since late m-RNA is not synthesized until after viral DNA replication has begun, it is transcribed from progeny genomes. Thus, late protein synthesis may be defined as taking place after nucleic acid replication. Late m-RNA specifies structural proteins (capsid formation) of virions. Translation of m-RNA into proteins takes place in host cell cytoplasm.
When a critical number of various viral components has been synthesized, the components (viral specific molecules) are assembled into mature virus particle in nucleus (and/or) cytoplasm of infected cell (depending on type of virus). DNA virus usually assemble in nucleus (exception: pox virus) and RNA virus usually assemble in cytoplasm.
Release of the completed virions from host cell is the final step in virus multiplication. In some animal viruses, host cell lyse, releasing the virions. The naked viruses are generally released in burst like fashion from cell as they lyse. Enveloped viruses are released from budding through special areas of host cell membrane coded by the virus and in doing so virus acquires the portion of host membrane. In some viruses, budding occurs in nuclear membrane, taking its membrane as its envelope and releasing through the channel.