Certain DNA Molecules may Behave as Viruses or Plasmids

There are several similarities between the behavior of plasmids and viruses. In fact, some circles of DNA can choose to live either as a plasmid or as a virus. The bacterial virus P1 is a good example. It can indeed behave as a virus, in which case it destroys the bacterial cell, replicates by rolling circle mode and manufactures large numbers of virus particles to infect more bacterial cells. This is known as lytic growth since the host cells are "lysed" (derived from the Greek for broken).

Alternatively, P1 can choose to live as a plasmid and divide in step with the host cell. In this case, the circular P1 DNA uses bi-directional replication like a typical plasmid (Fig. 16.26). Each descendant of the infected bacterial cell gets a single copy of P1 DNA. The cell is unharmed and no virus particles are made. This state is known as lysogeny and a host cell containing such a virus in its plasmid mode is called a lysogen.

Changing conditions may stimulate a lysogenic virus to return to destructive virus mode. This tends to happen if the host cell is injured, in particular if there is severe

Flp recombination target (or FRT site) Recognition site for Flp recombinase FRT site Flp recombination target, the recognition site for Flp recombinase lysogen Host cell containing a lysogenic virus lysogeny State in which a virus replicates its genome in step with the host cell without making virus particles or destroying the host cell. Same as latency, but generally used to describe bacterial viruses lytic growth Growth of virus resulting in death of cell and release of many virus particles

P1 phage

Lysis

Lysogeny

Chromosome divides Cell divides and p1 DIVIDES

FIGURE 16.26 Lysis versus Lysogeny

Some plasmids, such as the P1 plasmid of bacteria, have a dual personality. P1 can exist in a lysogenic state as a plasmid, using bi-directional replication to divide when the host cell divides. P1 can also grow as a virus and destroy the cell. During such lytic growth, P1 divides by the rolling circle mechanism, creating a large number of copies. It then packages genome-sized units into new virus particles and lyses the bacterial cell.

damage to the host cell DNA. The virus decides to make as many virus particles as possible before the cell dies. If, on the other hand, the host cell is growing and dividing in a healthy manner, the virus will most likely decide to lie dormant and divide in step with its host. Further aspects of virus behavior are covered in the following chapter, Ch. 17.

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