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VIRUS - Viruses are infectious particles that consist of a DNA or RNA molecule packaged in a protein capsid.  A virus can multiply only inside a host cell, whose genetic mechanisms it subverts for its own reproduction.  Both the structure of the virus and its mode of replication differ widely between viruses.  The usual outcome of a viral infection is the lysis (destruction) of the infected cell, with the release of viral particles.  Some viruses, however, can instead become integrated into the host chromosome, where their genes are replicated along with those of the host cell.  Because viruses sometimes carry host DNA sequences from one species to another, they make possible an occasional mixing of different gene pools during their evolution.  Virulent viruses usually kill the cells they infect.  Other viruses, although also often able to kill cells, frequently have a more subtle effect.  These viruses are called temperate.  Virus infected bacteria which appear un-infected but have the hereditary ability to produce phage, are called lysogenic.  (Phage - indicating something that devours another)  The lambda genome has two set of genes, one controlling lytic growth, the other lysogenic growth.  Upon infection, genes promoting both lytic growth and lysogenic integration are expressed.  Which pathway succeeds is determined by the competing action of these early gene products and by the influence of host factors.  The lambda phage is a well studied temperate phage which can grow in synchrony with its host (E.coli) in its lysogenic phase, or go into a lytic phase, when its genome is replicated many times by a rolling circle mechanism.  The lytic cycle is that part of the life cycle of a temperate phage in which it multiplies rapidly, destroying its host and releasing many copies into the medium.  The pathway genes in the lambda immunity region are:  cro, Or, and cI.  The lambda system provides one of the best studied examples of a genetic switch.  Two back-to-back promoters in the region of cI and cro control the genetic switch.  When cI is present it activates its own synthesis and blocks transcription of cro.  When cI is inactivated, transcription of cro can occur, resulting in the lytic cycle.  The cI protein combines with the operator, Or.  Transcription is the process by which an RNA polymerase produces single-stranded RNA complementary to one strand of DNA or, rarely RNA.  Polymerase are enzymes producing a polynucleotide sequence, complementary to a pre-existing template polynucleotide.  DNA polymerase requires a primer from which to start polymerization whereas RNA polymerase does not.  Polymerization is the combination of several molecules to form a more complex molecule, usually by addition or a condensation process.  It is sometimes a reversible process.

VIRONS -PIRONS - http://www.miamisci.org/youth/unity/Unity1/Lubens/pages/viroidprion.html

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