Many Epidemics of viral diseases occured before anyone understood the nature of their causative agents

Early Attempts to prevent viral diseases

Lady Wortley Montagu ( 1700s)- Inoculated children with material from small pox lesion, got idea from turkish women

Edward Jenner ( 1798) - tried to prevent small pox by exposure to cow pox

Discovery of viruses 1800s

* Charles Chamberland

* Dimitri Ivanowski

* invented a porcelein bacterial filter used later in the discovery of viruses

* Demonstrated that causative agent of tobacco mosaic diease passed through bacterial filters, thought agent was toxin

Discovery of Viruses

* Martinus Beijerinick

Felix d'Helle

did what 3 things

Firmly estabished the existance of bacteriophages

Devised method for enumerating them

demonstrated that bactreriophages only reproduce in live bacteria

Simple organism

DNA or RNA not both

unable to reproduce outside of living cell

obligate intercellular parasite

Complex organism

Both DNA and RNA

carry out cell division

some are obligate intracellular parasites

a capsid

1

either DNA of RNA never both

make viral proteins

assure replication

move in and out of host cell

extracellular phase

metaboliclly inert

intracellular phase

metabolically active

phage morphology

nucleic acid properties

- absorbtion/ attachment

-penetration

- synthesis

-assembly

-release

penetration

and uncoating during penetration

entrance of the virus or its nucleic acid into the host cell

viral genome is released

The lytic cyles:

absorbtion / attachment

The phage collide by chance with the bacteria

the base plate with its tail spikes settles in on the surface of the bacterium

specific surface structure on host of which viruses attach they are specific for each virus

some can be lipopolysacharides, techoic acids..etc

Synthesis of the lytic cycle

Sequential steps

duplication of viral components is a sequential process

 1. Virus will inhibit activity of the host DNA

-Viruses produce enzymes to destroy host DNA

2. Viral DNA takes over anf begins transcribing mRNA

-early mRNA synthesis

- Late mRNA synthesis

endolysin- attack peptidoglycan

holin- produces lesions in the cell membrane

lytic cycle- virus replicates in the cell and the cell eventually burts

lysogeny- the virus integrates viral genome into host DNA

lysogens

lysogeny begins like the lytic cycle

attachment/absorbtion

penetration...

double stranded DNA phage

 its genome circularizes upon entry into the host

incorporation depends on the production and competition of two regular proteins:

Lambda repressor

Cro protein

involved in regulating lytic cycle genes

blocks the synthesis of the lambda repressor

then the viral nucleic acid will incorporate into the host chromosome, and integrase will catalyse integration

the viral DNA is replicated only when the host cell replicates, this allows for the population of the bacterial cells to carry viruses

the virion is extruded from the cell

the capsid proteins are stored in the bacterial membrane

as the DNA is extruded, the virion is assembled

this occurs continuously and the infected cells are the carriers

Transduction

bacteria DNA that is transferred from donor to recipitant via bacteriophage two types

generalized and specialized

any part of the bacterial genome can be transferred

occurs during the lytic cycle of virus replication

during viral assembly, fragments of host DNA are mistankenly packed into phage head forming generalized transducing particles

induction occurs and begins viral replication

the excised genome contains a portion of the bacterial chromosome

phage must be able to attach to host receptors

restriction-modification system of the host cell must be overcome

host ranges for phages receptor sites can be altered by two mechanisms

mutation

lysogenized bacteria can alter cell surface, which results in alteration of receptor site