Polio
Smallpox
MumpsPotyvirus infection
Tobacco mosaic virus infection
Bacteriophage VIRUSES CAUSE DISEASE
Herpes simplex type 2 infection
What is a virus?-Obligate intracellular parasite-Small: 10-100 nm-Structure
-Nucleic acid genome-Protein capsid-Lipid envelope for some animal viruses
Enveloped viruses
“spike”
capsid
Nonenveloped viruses
Virus replication: Problem
What is needed for virus replication:Protein synthesisDNA replication
PROBLEM:Small genome
Virus replication: Solution- Virus infects a host cell in order to replicate -no metabolism outside the host cell
-requires host nucleotides, amino acids, enzymes, energy- Viral genome directs host cell to make viral proteins- Copies of genome + proteins assembled into new viruses
What is a virus?
Viruses are noncellular, nonliving infectious particles consisting of a genome surrounded by a protein coat.
Is a virus a cell?
1-10 μm for prokaryotes10-100 μm for eukaryotes
size
always presentessential for transport
membrane
enzymesbreak down sugars for energysynthesize DNA, proteins, etc.
metabolism
nucleic acids (DNA and RNA)1000s of proteinscarbohydrateslipids
composition
dsDNAgenetic material
virusescells
10-100 nm (similar to ribosome size)
no functional membraneno transport
no active enzymesno energy generationno synthesis of molecules
nucleic acid (DNA or RNA)capsid proteins (1-10s)lipid envelope (some animal viruses)
DNA or RNA, ds or ss
A virus does not qualify as a cell.
What is a virus?
Viruses are noncellular, nonliving infectious particles consisting of a genome surrounded by a protein coat.
• Some characteristics of living things• Most consider viruses non-living
Is a virus alive?living non-living
What is a virus?
Viruses are noncellular, nonliving infectious particles consisting of a genome surrounded by a protein coat.
1. Attachment2. Entry3. Uncoating4. Nucleic acid replication & protein
synthesis5. Assembly6. Exit
Virus replication
Attachment
Adenovirus(naked) Influenza virus
(enveloped)
virus “spike” protein
virus “spike”proteinhost cell
receptorhost cellreceptor
-Virus protein binds membrane receptor-Determines host range
Entry
Influenza virus(enveloped)
Adenovirus(naked)
-Naked virus usually enters by endocytosis/ followed by endosomal membrane lysis-Enveloped virus usually enters by fusion with the plasma membrane or the endosomal membrane
Replication
Adenovirus(naked) influenza virus
(enveloped)
envelope proteinsinserted into membrane
-Genome replicated-Viral proteins synthesized by host ribosomes
Assembly
Adenovirus(naked) Influenza virus
(enveloped)
-Viral proteins self-assemble into capsid-Viral proteins package genome
-Naked virus lyses cell
-Enveloped virus “buds” out, taking membrane as envelope
Exit
influenza virus(enveloped)
Adenovirus(naked)
Viruses are very diverse
Bacteriophage T4
(bacterial virus):naked
dsDNA genomecomplex capsid
Tobacco mosaic virus(plant virus):nakedssRNA genomehelical capsid
Influenza virus:enveloped
ssRNA genomehelical capsid
Poliovirus(replicating in cell):nakedssRNA genomeicosahedral capsid
How are viruses classified?
• Enveloped vs Nonenveloped• Genetic material
– DNA (single stranded or double stranded)
– RNA (single stranded or double stranded)
• Capsid structure– Helical (rod shape)– Icosahedral– Complex
Helical: Marburg virus
Icosahedral: Adenovirus
Complex:Bacteriophage
How is viral infection treated?
– For a bacterial infection, a doctor can prescribe antibiotics which kill the bacteria.
– For a viral infection: rest, fluids, aspirin, etc.
Why are there so few options for treating a
viral illness?
• Useful drugs must be selectively toxic:– Kill the disease-causing organism– Leave host cells unharmed
• Antibiotics exploit differences between proks and euks:– Unique cell wall carbohydrates in bacteria– Unique structures of bacterial ribosomes– Prokaryotic RNA polymerase
• Viruses replicate in our own cells, using our own machinery
Antiviral drugs
Acyclovir-Herpes family: herpes, chicken pox, shingles, etc.-Blocks viral DNA synthesis-Reduces duration and severity of infection
Targets for antiviral drugs
Relenza-Influenza virus-Prevents new budding viruses from detaching and spreading-Reduces duration of flu by ~2 days
Targets for antiviral drugs
• HAART “cocktail”– HIV virus– Blocks 2 key viral enzymes– Extends life– Improves quality of life
Antiviral drugs
• Inject safe form of viral proteins (antigens)• Immune system produces antibodies and memory cells• Fast response to actual virus prevents disease
Vaccination
• Smallpox killed 300,000,000 in the 20th century
• Edward Jenner developed vaccination in 1796
• Vaccination allowed eradication of the disease– Last case in 1977
Smallpox vaccination
• Vaccine development difficult for some diseases (e.g., HIV)• Difficulty of universal distribution• Side effects, real and imagined• Public resistance to vaccination• Sensational, irresponsible media coverage• Not an economic priority for many drug companies• Regulatory issues: >10 years to license a new vaccine
Vaccine issues
• Mutation– New influenza virus strains (need a shot every year)– “Swine flu” or “Bird flu” becomes human flu pandemic?
Emerging viral diseases
influenza hemagglutinin
• Species jump– HIV probably evolved from a chimpanzee virus– SARS coronavirus may have started as a bat virus
Emerging viral diseases
SARS coronavirus