About Viral Life Cycle

What is viral replication?
Viral replication is the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. For the virus, the purpose of viral replication is to allow production and survival of its kind. By generating abundant copies of its genome and packaging these copies into viruses, the virus is able to continue infecting new hosts. Replication between viruses is greatly varied and depends on the type of genes involved.

Baltimore Classification
Viruses are classed into 7 types of genes, each of which have their own families of viruses, which in turn have differing replication strategies themselves. David Baltimore, a Nobel Prize-winning biologist, devised a system called the Baltimore Classification System to classify different viruses based on their unique replication strategy. There are seven different replication strategies based on this system (Baltimore Class I, II, III, IV, V, VI, VII).

Class I: double-stranded DNA (dsDNA)
- Replication exclusively nuclear; very dependent on host cell factors
- Replication in cytoplasm; viral genome contains all factors for genome replication and transcription
Examples: Herpes virus, pox virus, adenovirus

Class II: single-stranded DNA (ssDNA)
- Replication of genome in nucleus
- dsDNA formed to make new single-stranded daughters
- Extreme parasitism
Example: Parvoviridae

Class III: double-stranded RNA (dsRNA)
- Genome in several fragments
- Replication, transcription, translation regulated separately
- Monocistronic mRNA
- All activity in cytoplasm
Example: Reovirus

Class IV: single-stranded positive RNA [(+)ssRNA]
- Majority of animal and plant viruses
- Group 1 – polycistronic mRNA. Polyprotein formed and cleaved
- Group 2 – complex transcription process. 2 rounds of translation before formation of genomic RNA.
Examples: Hand food mouth disease, hepatitis A virus, hepatitis C virus

Class V: single-stranded negative RNA [(-)ssRNA]
Group 1 –
- Non-segmented genome
- Transcription of –ve RNA by RNA-dependent RNA polymerase to give monocistronic mRNA
- Ambisense organisation
Group 2 –
- Orthomyxoviruses (segmented genome)
- Monocistronic mRNA in nucleus
- Virus transcriptase in nucleocapsid
Example: Influenze virus

Group VI: reverse RNA
- (+)ssRNA with DNA intermediate
- Diploid
- Reverse transcription of viral RNA to dsDNA by viral RT
- Integration of dsDNA into host genome
- Viral RNA not used as mRNA
Example: HIV

Group VII: reverse DNA
- dsDNA with RNA intermediate
- not well understood
- overlapping reading frames
Example: hepatitis B virus

An animation about viral replication..
http://www.liquidjigsaw.com/animation/virus.htm

References:
http://en.wikipedia.org/wiki/Viral_replication
http://www.web-books.com/MoBio/Free/Ch1E2.htm