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Bacteriophages

The micrograph shows hexagonal bacteriophage capsids attached to a host bacterial cell by slender stalks.
This transmission electron micrograph shows bacteriophages attached to a bacterial cell. (credit: modification of work by Dr. Graham Beards; scale-bar data from Matt Russell)

Bacteriophages are viruses that infect bacteria ( [link] ). 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    ( [link] ). 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    ( [link] ), 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 E . coli bacterium. Viruses that infect plant or animal cells may also 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. Latency will be described in more detail below.

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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.

Which of the following statements is false?

  1. In the lytic cycle, new phage are produced and released into the environment.
  2. In the lysogenic cycle, phage DNA is incorporated into the host genome.
  3. An environmental stressor can cause the phage to initiate the lysogenic cycle.
  4. Cell lysis only occurs in the lytic cycle.

Animal viruses

Animal viruses, unlike the viruses of plants and bacteria, do not have to penetrate a cell wall to gain access to the host cell. Non-enveloped or “naked” animal viruses may enter cells in two different ways. As a protein in the viral capsid binds to its receptor on the host cell, the virus may be taken inside the cell via a vesicle during the normal cell process of receptor-mediated endocytosis. An alternative method of cell penetration used by non-enveloped viruses is for capsid proteins to undergo shape changes after binding to the receptor, creating channels in the host cell membrane. The viral genome is then “injected” into the host cell through these channels in a manner analogous to that used by many bacteriophages. Enveloped viruses also have two ways of entering cells after binding to their receptors: receptor-mediated endocytosis, or fusion    . Many enveloped viruses enter the cell by receptor-mediated endocytosis in a fashion similar to some non-enveloped viruses. On the other hand, fusion only occurs with enveloped virions. These viruses, which include HIV among others, use special fusion proteins in their envelopes to cause the envelope to fuse with the plasma membrane of the cell, thus releasing the genome and capsid of the virus into the cell cytoplasm.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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