exam 1 (viruses) Flashcards
Viruses
Infective particles 20 nm-1000 nm (1 nm =
1/1000 of a μm, or 1/1,000,000 of a mm)
Simple structure—genetic material, protein
capsid
All types of organisms have viruses that
infect them
The host specificity and pathogenicity of
each virus is variable
In multicellular organisms, viruses often
specialize on specific cell types
Over 4000 “species” named
Perhaps the most abundant “group” on earth
(there can be billions in 1 liter of seawater!)
Are viruses living organisms?
Have genes, reproduce, evolve
Are not typical cells, do not
metabolize, require a host cell to
reproduce
Main structural components of
viruses
1. Small genome (4 to 1000 genes)— DNA or RNA, single or double stranded 2. Capsid—protein shell of varying shapes 3. Some have accessory structures, mostly relating to gaining entry to host cells Viral envelopes (mostly animal viruses)—host cell and viral components
General Types of Viruses
1. Helical (rod-shaped)—e.g. many plant viruses, Ebola virus 2. Icosahedral (polyhedral)—e.g. adenoviruses that infect animal respiratory tracts 3. Enveloped (either of above types)— mostly found in animal viruses. Envelopes come from host cell membrane 4. Complex polyhedral (bacteriophages)
Generalized Stages in Viral
Infection
1. Binding to host cell and entry (mechanism variable) No envelope—bind with recognition proteins, endocytosis Envelope—fuse with plasma membrane 2. Viral genome released and replicated by host enzymes 3. Synthesis of viral capsid proteins using host enzymes 4. Self-assembly of many new viruses 5. Exit cell (kills host cell) infect other host cells
The lytic cycle (host cell dies)
of a bacteriophage
- Attach to bacteria using tail fibers
- Enzyme digests hole in cell wall
- DNA is injected
- Hydrolysis of bacterial DNA
- Viral DNA and proteins are replicated
using viral and host enzymes - Viral components self assemble
- Release—enzyme degrades bacterial
cell wall, phages released killing the
cell
Phages kill enormous numbers of
bacteria—can be important in
controlling many bacterial
populations
However, bacteria have evolved
ways of resisting viral infection
For example, the use of restriction
enzymes that cut up viral DNA;
CRISPR-Cas system
These have been adopted as useful
tools of biotechnology
Possible Effects of Animal
Virus Infections
- No effect
- Cause cell death at emergence
- Viral proteins may be toxic
- Infected cells release inflammatory
or other agents that produce
symptoms—fever, etc. - Oncoviruses cause cells to become
cancerous (HPV human
papillomavirus, Epstein Barr,
hepatitis B and C)
Human Immunodeficiency
Virus (HIV)
A retrovirus with ssRNA as a genome Glycoproteins on its envelope allow it to bind to receptors on specific cell types In the host cell, its RNA is converted to DNA which is then incorporated into the host cell’s DNA as a provirus
Pathogenic Viruses
Immune system attempts to fight infection Some vaccines are effective Once infected, there are few cures Certain drugs can be effective at controlling the infection Viral mutation and/or recombination with other viruses is common— complicates the effectiveness of vaccines and treatments
Where do “emerging viruses”
(e.g. H1N1, West Nile, HIV,
Ebola) come from?
- Mutation, recombination of existing viruses
- Spread out from small, local
populations - Spread from animals (e.g. pigs,
birds, monkeys) to humans (most cases)
Some emerging viruses have been
responsible for pandemics of the
past (e.g Spanish flu of 1918 that
killed about 40 million)
Do we all carry around viral
DNA in our genome, intact
viruses?
It has been estimated that 8% of our genome is of viral origin! Most from retroviruses; some still functional, many have mutated Functions mostly not known— implicated in cancers, protection from disease Some thought to have enabled the evolution of the mammalian placenta and help embryo develop a variety of tissues
