What is a virus?

Obligate intracellular parasite

Small: 10-100 nm

Nucleic acid genome
 DNA or RNA
 single- or double-stranded

Protein capsid
Naked DNA virus

Lipid envelope for some animal viruses
Enveloped RNA virus
Viruses don’t divide, they replicate

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
Is a virus a cell?
cells
genetic material
composition
metabolism
membrane
size
viruses
Is a virus alive?
living
non-living
Virus replication
1.
Attachment
2.
Entry
3.
Uncoating
4.
Nucleic acid replication & protein synthesis
5.
Assembly
6.
Exit
Attachment

Virus protein binds membrane receptor

Determines host range
host cell
receptor
virus “spike” protein
HPV
(naked)
host cell
receptor
virus “spike”
protein
influenza virus
(enveloped)
Entry

Naked virus usually enters by endocytosis

Enveloped virus usually enters by fusion
HPV
(naked)
influenza virus
(enveloped)
Uncoating

Genome released from capsid proteins

For naked virus, must also escape vesicle
HPV
(naked)
influenza virus
(enveloped)
Replication

Genome replicated

Viral proteins synthesized by host ribosomes
envelope proteins
inserted into membrane
HPV
(naked)
influenza virus
(enveloped)
Assembly

Viral proteins self-assemble into capsid

Viral proteins package genome
HPV
(naked)
influenza virus
(enveloped)
Exit

Naked virus lyses cell

Enveloped virus “buds” out, taking membrane as envelope
HPV
(naked)
influenza virus
(enveloped)
Antiviral drugs

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
Vaccination

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
antibody production →

anti-flu
antibodies
infection with
actual flu virus
“flu shot”
(killed virus)
≈10 days
time →
1-2 days
Smallpox 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
Vaccination

Polio should be the next disease to be eradicated
1988
350,000 cases
2010
968 cases
Vaccine issues

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
Emerging viral diseases

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