 Non-living entities  Can infect organisms of every domain  Commonly referred to by organism they infect  Viruses that infect bacteria: Bacteriophage

 Virus architecture › Virus particle called virion › Consists of nucleic acid surrounded by protein coat  Capsid › Shapes  Isometric  Helical  Complex › Two types of virion  Naked – without envelope  Enveloped – surrounded by lipid membrane

 Viral genome › DNA or RNA  NEVER BOTH › Linear or circular › Single-stranded or double- stranded  Replication cycle overview › Only multiply inside metabolizing cell › Contains information to make viral proteins, assure replication and move in and out of host cells › Viruses live in two phases  Extracellular phase  Metabolically inert  Intracellular phase  Metabolically active

 Penetration › Entrance of the virus OR its nucleic acid in the host cell  Plant and bacteria viruses inject the nucleic acid into the host through the cell wall  Animal viruses enter the cell whole  Phagocytosis  Membrane fusion occurs with enveloped viruses  Viruses enter with an uncoating step

 Release › Host cell bursts and releases viruses to the outside environment  Viruses are now extracellular › As viruses leave the host cell, the envelope is picked up  The envelope is made of a portion of the host cell plasma membrane which becomes the lipid envelope of the virus

 Lysogeny › Replication of a temperate virus  This is a non-productive cycle › Lysogeny begins like the lytic cycle  Adsorption  Penetration, then;  Incorporation into genome › HIV is a lysogenic RNA virus in humans › λ phage is lysogenic DNA in bacteria

 Incorporation › Prophage: Viral nucleic acid incorporates onto the host chromosome › The viral DNA is replicated only when the host cell replicates › Cell eventually “pops” off the host chromosome and returns to the lytic cycle

 Lysogenic conversion › Prophage can confer new properties on cell › Phage DNA not completely suppressed  Genes coding for trait are expressed  Infected cells have new characteristics  Streptococcus pyogenes manufactures toxin resulting in scarlet fever

 DNA may be transferred by a bacteriophage to a bacteria in a process called transduction.  1. Generalized Transduction: In this type any bacterial gene can be transferred  2. Specialized Transduction: In this type only a few specific genes can be transferred

 Number of different bacteria that phage can infect termed host range › Usually limited to single bacterial species for a single phage  Factors limit host range › Two most important  Phage must be able to attach to host receptors  Restriction-modification system the host cell must overcome

 Receptors on bacterial surface › Vary in chemical structure and location  Usually on bacterial cell wall › Sites can be altered by two mechanisms  Receptor sites can be altered by mutation  Lysogenized bacteria can alter cell surface  Results in alteration of receptor site

 Restriction-modification system › Restriction enzyme that codes for endonuclease › Modification enzyme attaches methyl group to DNA recognized by restriction enzyme