Lysogeny or Latency by Integration

When an infecting virus generates many virus particles and destroys the cell, this is known as lytic growth because the cell is burst or lysed. When instead, the virus divides in step with the host chromosome, this is known as lysogeny and a cell containing such a virus is a lysogen. The term latency means the same as lysogeny but is usually used when referring to animal cells. In Chapter 16 we discussed the close relationships between plasmids and viruses. Some gene creatures can choose to live either as a plasmid or as a virus. Some plasmids are probably derived from viruses that have lost the ability to grow lytically. Conversely, some viruses may have evolved from plasmids that obtained the genes for lytic growth, either from another virus or, over a longer period, from the host cell.

Lysogeny or latency means that the virus has decided to divide in step with the host cell instead of killing it. It does not necessarily mean the virus has decided to live as a plasmid. Many cases of lysogeny or latency are caused by integration of the virus DNA into a host cell chromosome. Such an integrated virus is known as a provirus (or prophage in the case of bacterial viruses). The virus DNA becomes a physical part of the chromosome and is replicated when the chromosome divides.

The bacterial virus lambda (l), which infects the bacterium E. coli, recognizes and integrates into a special sequence of DNA on the chromosome of its host cell, known as attl (l attachment site). Integration occurs by site-specific recombination as described in Ch. 14. This allows lambda to occasionally pick up and carry bacterial genes as described in the chapter on bacterial genetics (Ch. 18). Some animal viruses, such as the herpes viruses, also insert themselves into the chromosomes of their host cells. Some have special recognition sites, while others insert at random. Retroviruses, attl (l attachment site) Recognition sequence on the chromosome of Escherichia coli where bacteriophage lambda integrates lambda (l) Virus that infects the Escherichia coli and may integrate into a special sequence of DNA on the bacterial chromosome latency Type of virus infection in which the virus becomes largely quiescent, makes no new virus particles and duplicates its genome in step with the host cell. Same as lysogeny but used of animal viruses lysogen A cell containing a lysogenic virus lysogeny Type of virus infection in which the virus becomes largely quiescent, makes no new virus particles and duplicates its genome in step with the host cell. Same as latency but used of bacterial viruses lytic growth Type of infection in which a virus generates many virus particles and destroys the cell prophage Bacteriophage genome that is integrated into the DNA of the bacterial host cell provirus Virus genome that is integrated into the host cell DNA

PHAGE WITH 32P IN DNA

PHAGE WITH 35S IN PROTEIN

Phage with 32P in DNA

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Phage with 35S in protein coat

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