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Life Cycle

Viral Genomes

Either DNA or RNA, never both.

DNA viruses can be further divided into

RNA viruses occur in four distinct groups:

  1. Those with a genome that consists of single-stranded antisense RNA; that is, RNA that is the complement of the message sense. This is also called negative-stranded RNA. Examples: measles, Ebola [Link]
  2. Those with a genome that consists of single-stranded sense RNA; that is, the RNA has message sense (can act as a messenger RNA — mRNA). This is also called positive-stranded RNA. Examples: poliovirus [Link]
  3. Those with a genome made of several pieces of double-stranded RNA. Example: reovirus. [Link]
  4. Retroviruses. Their RNA (also single-stranded) is copied by reverse transcriptase into a DNA genome within the host cell. Example: HIV-1 [Link]

DNA Viruses

1. Genome is a molecule of double-stranded DNA


The essential elements of the infective cycle of DNA bacteriophages consist of:

Hepatitis B

The genome of hepatitis B ("serum hepatitis") is also dsDNA, but its mode of replication is different from the other dsDNA viruses.

2. Genome is single-stranded DNA


RNA Viruses

1. Negative-stranded RNA viruses: genome consists of one or more molecules of single-stranded "antisense" RNA


Method of replication

Note that this strategy

2. Positive-stranded RNA: genome is a molecule of single-stranded "sense" RNA


Method of replication

3. Genome consists of several molecules of double-stranded RNA


Method of replication

The virus particle contains enzymatic machinery that transcribes each of the dsRNA molecules into a mRNA (complete with cap) and exports these into the cytosol of the infected cell.

4. Retroviruses

These viruses contain a reverse transcriptase that copies their RNA genome into DNA.

Link to an illustrated discussion of the life cycle of retroviruses.


Latent Viruses

Most of the infective cycles described for the various viruses end in the death of the host cell. Bacterial cells literally burst, a process called lysis, and similar infective cycles are called lytic cycles.


In some cases, though, the events of the lytic cycle are not completed. An E. coli cell infected by a DNA bacteriophage may resume its normal existence, including reproducing itself.

Where has the virus gone?

It is still there and, in fact, is present in the descendants of the bacterium. That these cells still harbor the virus can be demonstrated by irradiating the cells with ultraviolet rays or treating them with certain chemicals. Such treatment restores the normal lytic cycle. The phage is said to have been "rescued" — hardly the case for its host!

The stable relationship between a bacteriophage and its host is called lysogeny. The viral DNA actually becomes replicated when the host's DNA is replicated prior to each cell division. During lysogeny, the phage is called a prophage.

In some cases, the prophage DNA becomes inserted into the chromosome of its host. In fact, when the phage is "rescued", the released virions may contain some host genes as well as their own. When these virions infect new hosts, they insert these bacterial genes into them. This process of genetic transfer, a virus-mediated transformation, is called transduction.

What does the prophage do while it is a part of its host genome? It can express certain of its genes. For example, the gene that encodes diphtheria toxin is the property of a prophage in the diphtheria bacillus, not of the bacillus itself.

Some animal viruses can also establish latent infections. Simian virus 40 (SV40) is a DNA virus that produces

Although a human cell with harboring SV40 shows no outward sign of the provirus, its presence can be detected by:

Latent infections may also cause the cell to become cancerous. The cell has become transformed. (In these cases, the word fulfills both of its biological meanings:

Link to a discussion of the properties of transformed cells.

In humans,



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18 August 2016