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Obligate intracellular parasites
–Helical, Polyhedral, Enveloped, Complex
–Most <0.2mm; requires electron microscope
- –Virion – fully formed virus able to
- establish infection
Viruses are made of:
- 1.Nucleic acid – genome
- •May be DNA or RNA; single stranded or
- double stranded
Viral Structure 2
Capsid (protein coat)
•Capsomeres – individual protein unit
Viral Structure 3
Envelope – lipid membrane
•Not found in all viruses
- –All viruses have capsids - protein coats that enclose and protect their nucleic
The capsid together with the nucleic acid
–Some viruses have an external covering called envelope; those lacking an envelope are naked
–Any cellular components
–Genes for replication and infection
–Some have a few enzymes
cell (tissue) cultures
–cultured cells grow in sheets that support viral replication and permit observation for cytopathic effect
viruses that infect bacteria
- •Host cell “lawn”
- –Infection visible as clear plaques (where cells were infected then lysed)
Cytopathic Effect (CPE)
Infection of Cell Culture is obvious due to Cytopathic Effect (CPE)
•Multiplication goes through similar stages as animal viruses.
•Only the nucleic acid enters the cytoplasm - uncoating is not necessary.
- •Release is a result of cell lysis induced
- by viral enzymes and accumulation of viruses - lytic cycle.
6 Steps in Phage Replication
1. Attachment – binding of virus to specific molecule on host cell
2. Entry –genome enters host cell
3-4. Synthesis – bacterial DNA degraded. Viral components produced
5. Assembly - viral components assembled
6. Release –viruses leave cell to infect other cells
Lytic life cycle
(part 1 of lysogenic life cycle)
•Attachment, Entry, DNA circularizes
•Transcription & Translation, Replication of DNA
•Assembly & Release via lysis
Latent life cycle (part 2 of lysogenic life cycle)
doesn’t kill host cell
•Attachment & Entry
•DNA integrates into the host chromosome = prophage
•Bacteria replicates normally
•can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis.
- •The viral genome inserts into bacterial genome and becomes an inactive prophage - the
- cell is not lysed.
- Prophage is retained and copied during normal cell division resulting in the transfer of
- temperate phage genome to all host cell progeny – lysogeny
•Not all phages complete the lytic cycle.
- •Some DNA phages, called temperate
- phages, undergo adsorption and penetration but don’t replicate.
- •When the phage integrates into the
- bacterial chromosome it brings new genes to the bacteria!
•May encode toxins
–binding of virus to specific molecule on host cell
–genome enters host cell
Pinocytosis or Fusion
–the viral nucleic acid is released from the capsid
–viral components are produced
–new viral particles are constructed
- –assembled viruses are released by budding
- (exocytosis) or cell lysis
are replicated and assembled in the nucleus.
•are replicated and assembled in the cytoplasm.
- –Positive-sense RNA contain the message
- for translation.
- –Negative-sense RNA must be converted into
- positive-sense message.
Types of Infection
- 1.Acute – short duration
- •Common cold
- 2.Persistent – virus remains in tissues
- •Complication of measles infection, SSPE (subacute sclerosing panencephalitis)
Types of Infection 2
- 1.Chronic – virus continually released from
- –Hepatitis B virus
- 2.Latent – virus remains dormant in host
- and can reactivate later (replicate and release virus)
- –measles virus – may remain hidden in
- brain cells for many years
- –herpes simplex virus – cold sores and
- genital herpes
- –herpes zoster virus – chickenpox and
misfolded proteins, contain no nucleic acid
–cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases
•Extremely resistant to usual sterilization techniques
- occurs when there is genetic recombination
- between 2 different influenza strains
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