Viruses

Viruses are intracellular parasites made up of a genome of double- or single-stranded DNA (or RNA) within a capsid, or protein coat. Some viruses have viral envelopes, lipid membranes that encase the capsid and assist in invading a host.

 

Virus Replication

Viruses cannot replicate without a host. The host range refers to the specific cell type (bacterial, plant, or animal) that each type of virus may infect. Viruses generally replicate via one of two modes, both of which are best understood in bacterial viruses (or phages).

1. Lytic cycle:

   1. The phage enters the host’s cell, releases its DNA from the capsid, and then uses the host’s cellular equipment to replicate.

   2. The virus’s genetic material is transcribed, and viral proteins (such as the capsid proteins) are translated from a viral RNA template.

   3. Once the virus replicates and reassembles itself, the host cell lyses, releasing the phage copies. The cycle repeats.

2. Lysogenic cycle:

   1. The phage enters the host and incorporates its genome into the host’s genome at certain chromosomal locations.

   2. The viral genome lies dormant and is replicated each time that the host itself replicates (with every cell division).

   3. Environmental cues cause the phage genome to extricate itself from the host’s DNA and begin the lytic cycle.

 

Retroviruses

Retroviruses are animal viruses that reproduce in a manner similar to a phage lysogenic cycle aside from the fact that that the viral genome is RNA-based. Reverse transcriptase transcribes the RNA genome into a DNA template that is incorporated into the host’s genome at specific sites.