- Ribosomes for protein synthesis
- RNA/DNA polymerase (in some instances)
- Transporter proteins (to get into/out of nucleus, for eukaryotic viruses)
- Other host factors that help in replication
We can build off of the table in the Virus Genomes section, as the type of genome that a virus is gives a lot of information on how it must be replicated in a cell (and what proteins are used by the host).
|Virus Type||Genome||What Viral Enzymes Necessary?||Site of Replication|
|I||dsDNA||None, but uses its own DNA polymerase to improve replication.||Nucleus (except for poxviruses, which replicate in cytoplasm)|
|II||ssDNA (+)||Requires own DNA polymerase that recognizes ssDNA to make dsDNA||Nucleus|
|III||dsRNA||Makes its RNA dependent RNA polymerase to replicate and make its own mRNAs.||Cytoplasm|
|IV||ssRNA (+)||Has to make RNA dependent RNA polymerase to make antigenomes for replication templates.||Cytoplasm|
|V||ssRNA (-)||Has to make its own RNA dependent RNA polymerase.||Cytoplasm|
|VI||ssRNA (+)||Has to have reverse transcriptase (RNA dependent DNA polymerase)||Reverse transcription in cytoplasm, but has to integrate into host genome in nucleus to replicate|
The type of genome also dictates where the virus will replicate. Viruses that are RNA often replicate in the cytoplasm, while DNA viruses replicate in the nucleus.
A DNA virus won't have an RNA polymerase gene to mediate transcription, so it has to be in the nucleus – where the host RNA polymerase is. On the other hand, RNA viruses make their own RNA polymerase, so they don't need to be in the nucleus. Also, if they were to get into the nucleus, they might be misused by enzymes that modify RNAs (called splicing enzymes) or degrade them, which are not found in the cytoplasm. That being said, some viruses, including influenza, enter and replicate in the nucleus because they require splicing enzymes for replication.
Virus replication usually occurs in two stages: early and late. These two stages are defined by the types of viral proteins made in each stage. These proteins are:
- Early proteins – polymerase, replication co-factors, transcription factors
- Late proteins – capsid, envelope proteins, some polymerases, host shutoff proteins (for some viruses)
Some viruses, such as hepatitis C virus, don't become latent, but instead maintain a low level infection for a long period of time in the body. This is called a chronic infection. That means there is a very low amount of actively replicating virus in the body that is never completely cleared by the immune system. Scientists argue about many viruses, such as HIV, which are believed to have a latent stage of infection, but instead might actually be a very low-level chronic infection. Scientists tend to argue about these things, because they had very lonely childhoods watching other kids play the "blob" game.