Obligate intracellular parasite Small: 10-100 nm Nucleic acid genome

DNA or RNA single- or double-stranded

Protein capsid Lipid envelope for some animal viruses

What is a virus?

Enveloped RNA virus

Naked DNA virus

No metabolism outside a host cell Requires host nucleotides, amino acids, enzymes, energy

Genome directs host cell to make virus proteins Copies of genome + proteins assembled into new viruses

Viruses don’t divide, they replicate

+

Is a virus a cell?

size

membrane

metabolism

composition

genetic material

virusescells

Is a virus alive?

living non-living

1. Attachment

2. Entry

3. Uncoating

4. Nucleic acid replication & protein synthesis

5. Assembly

6. Exit

Virus replication

Virus protein binds membrane receptor Determines host range

Attachment

HPV(naked)

influenza virus(enveloped)

virus “spike” protein

virus “spike”protein

host cellreceptor

host cellreceptor

Naked virus usually enters by endocytosis Enveloped virus usually enters by fusion

Entry

influenza virus(enveloped)

HPV(naked)

Genome released from capsid proteins For naked virus, must also escape vesicle

Uncoating

HPV(naked)

influenza virus(enveloped)

Genome replicated Viral proteins synthesized by host ribosomes

Replication

HPV(naked)

influenza virus(enveloped)

envelope proteinsinserted into membrane

Viral proteins self-assemble into capsid Viral proteins package genome

Assembly

HPV(naked)

influenza virus(enveloped)

Naked virus lyses cell Enveloped virus “buds” out, taking membrane as envelope

Exit

influenza virus(enveloped)

HPV(naked)

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

Antiviral drugs

“Relenza” and relatives Influenza virus Prevents new budding viruses from

detaching and spreading Reduces duration of flu by ~2 days

Antiviral drugs

HAART “cocktail” HIV virus Blocks 2 key viral enzymes Extends life Improves quality of life

Antiviral drugs

Our best weapon against viruses so far

Vaccination

Inject safe form of viral proteins (antigens) Immune system produces antibodies and memory cells Fast response to actual virus prevents disease

Vaccination

“flu shot”(killed virus)

Y YY

Y

anti-fluantibodies

anti

body p

roduct

ion →

time →≈10 days 1-2 days

infection withactual flu virus

Y YY

Y

YYY

Y YY

Y

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

Polio should be the next disease to be eradicated

2010968 cases

Vaccination

1988350,000 cases

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?

Species jump HIV probably evolved from a chimpanzee virus SARS coronavirus may have started as a bat virus

Spread from isolated population Public attention/media

Emerging viral diseases