Bacteriophages


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The micrograph shows hexagonal bacteriophage capsids attached to a host bacterial cell by slender stalks.
Bacteriophages attached to a host cell (transmission electron micrograph). In bacteriophage with tails, like the one shown here, the tails serve as a passageway for transmission of the phage genome. (credit: modification of work by Dr. Graham Beards; scale-bar data from Matt Russell)

OpenStax Biology 2e

Most bacteriophages are dsDNA viruses, which use host enzymes for DNA replication and RNA transcription. Phage particles must bind to specific surface receptors and actively insert the genome into the host cell. (The complex tail structures seen in many bacteriophages are actively involved in getting the viral genome across the prokaryotic cell wall.) When infection of a cell by a bacteriophage results in the production of new virions, the infection is said to be productive. If the virions are released by bursting the cell, the virus replicates by means of a lytic cycle. An example of a lytic bacteriophage is T4, which infects Escherichia coli found in the human intestinal tract. Sometimes, however, a virus can remain within the cell without being released. For example, when a temperate bacteriophage infects a bacterial cell, it replicates by means of a lysogenic cycle, and the viral genome is incorporated into the genome of the host cell. When the phage DNA is incorporated into the host-cell genome, it is called a prophage. An example of a lysogenic bacteriophage is the λ (lambda) virus, which also infects the  Ecoli  bacterium. Viruses that infect plant or animal cells may sometimes undergo infections where they are not producing virions for long periods. An example is the animal  herpesviruses, including herpes simplex viruses, the cause of oral and genital herpes in humans. In a process called latency, these viruses can exist in nervous tissue for long periods of time without producing new virions, only to leave latency periodically and cause lesions in the skin where the virus replicates. Even though there are similarities between lysogeny and latency, the term lysogenic cycle is usually reserved to describe bacteriophages.

The bacteriophage lytic cycle begins when the phage attaches via a slender stalk to the host cell. Linear DNA from the viral head is injected into the host cell. The phage DNA circularizes, remaining separate from the host DNA. The phage DNA replicates, and new phage proteins are made. New phage particles are assembled. The cell lyses, releasing the phage. The bacteriophage lysogenic cycle begins the same way as the lytic cycle, with phage infecting a host cell. However, the phage DNA becomes incorporated into the host genome. The cell divides, and phage DNA is passed on to daughter cells. Under stressful conditions, the phage DNA is excised from the bacterial chromosome and enters the lytic cycle.
A temperate bacteriophage has both lytic and lysogenic cycles. In the lytic cycle, the phage replicates and lyses the host cell. In the lysogenic cycle, phage DNA is incorporated into the host genome, where it is passed on to subsequent generations. Environmental stressors such as starvation or exposure to toxic chemicals may cause the prophage to excise and enter the lytic cycle. Source: OpenStax Biology 2e

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e