-
Viruses definition
Obligate intracellular parasites
Viral structure:
–Capsid
–Genome
–Envelope
-
Viral shapes
–Helical, Polyhedral, Enveloped, Complex
–Most <0.2mm; requires electron microscope
- –Virion – fully formed virus able to
- establish infection
-
Viral Structure
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
•Spike proteins
•Not found in all viruses
-
Capsids
- –All viruses have capsids - protein coats that enclose and protect their nucleic
- acid.
-
nucleoscapsid
The capsid together with the nucleic acid
-
Capsids 2
–Some viruses have an external covering called envelope; those lacking an envelope are naked
-
Viruses lack
–Any cellular components
–Mitochondria
–Ribosomes
-
Viruses have
–Genetic information
–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
-
Bacteriophage
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)
-
Bacteriophages
•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
-
Induction
•can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis.
-
Prophage
- •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
-
temperate phages
•Not all phages complete the lytic cycle.
- •Some DNA phages, called temperate
- phages, undergo adsorption and penetration but don’t replicate.
-
Phage conversion
- •When the phage integrates into the
- bacterial chromosome it brings new genes to the bacteria!
•May encode toxins
-
Adsorption
–binding of virus to specific molecule on host cell
-
Penetration
–genome enters host cell
Pinocytosis or Fusion
-
Uncoating
–the viral nucleic acid is released from the capsid
-
Synthesis
–viral components are produced
-
Assembly
–new viral particles are constructed
-
Release
- –assembled viruses are released by budding
- (exocytosis) or cell lysis
-
DNA viruses
are replicated and assembled in the nucleus.
-
RNA viruses
•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
- host
- –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
- shingles
-
Prions
misfolded proteins, contain no nucleic acid
–cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases
•Extremely resistant to usual sterilization techniques
-
Antigenic shift
- occurs when there is genetic recombination
- between 2 different influenza strains
|
|