ssRNA
+ sense: Positive‐sense; − sense: negative‐sense; dsRNA: double‐stranded RNA; N/A: not applicable; NssRNA: negative‐sense single‐stranded RNA; RNP: ribonucleoprotein; RT: reverse transcriptase; ssRNA: single‐stranded RNA.
Viral genomes
The nucleic acid core can be DNA for some types of viruses, RNA for others. This genetic material may be single or double stranded and may be linear or circular, but it is always the same for any given type of virus. The type of genetic material (i.e., whether DNA or RNA) is an important factor in the classification of any given virus into groups. Thus, although all free‐living cells utilize only double‐stranded DNA (dsDNA) as genetic material, some viruses can utilize other types of nucleic acid.
Viruses that contain DNA as genetic material and utilize the infected cell's nucleus as the site of genome replication share many common patterns of gene expression and genome replication along with similar processes occurring in the host cell.
The viruses that use RNA as their genetic material have devised some way to replicate such material, since the cell does not have machinery for RNA‐directed RNA replication. The replication of RNA viruses requires expression of specific enzymes that are not present in the uninfected host cell.
Although virus genes encode the proteins required for replication of the viral genome and these proteins have similarities to cellular proteins with roughly analogous functions, viral and cellular proteins are not identical. Viral replication proteins are enzymes involved both in nucleic acid replication and in the expression and regulation of viral genetic information. Viruses also encode enzymes and proteins involved in modifying the cell in which the virus replicates, in order to optimize the cell for virus replication.
Viral capsids
The capsid
