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