Viral populations do not grow through cell division, because they are not cells; instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves. Even though a virus sometimes will not kill the host cell, it can still affect the cell's homeostasis.

Replication of DNA Viruses

A DNA virus is a virus that uses 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 by the host cell. These viral proteins then assemble to form new viral particles.

Replication of RNA Viruses

An RNA virus is a virus that uses ribonucleic acid (RNA) as its genetic material. Their nucleic acid is usually single-stranded RNA but may also be double-stranded RNA. Important human pathogenic RNA viruses include the Severe Acute Respiratory Syndrome (SARS) virus, Influenza virus, and Hepatitis C virus. 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 messenger RNA (mRNA). The viral RNA is translated directly into new viral proteins by the host cell, 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 cell's 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.