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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