VirusesVirusesVirusesViruses

Chapter 19 in Campbell’s 8th Edition

Chapter 19 in Campbell’s 8th Edition

• The Single biggest threat to man’s continued dominance on the planet is the virus.

– Joshua Lederberg, Ph.D.– Nobel laureate

• The Single biggest threat to man’s continued dominance on the planet is the virus.

– Joshua Lederberg, Ph.D.– Nobel laureate

Editorial CommentEditorial Comment

• Despite the fact that viruses lack a nervous system and just about everything else in cells, they are geniuses at what they do!

• Despite the fact that viruses lack a nervous system and just about everything else in cells, they are geniuses at what they do!

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• A bacteria cell infected by viruses

• Are the viruses alive?

• A bacteria cell infected by viruses

• Are the viruses alive?

Relative Sizes of MicroorganismsRelative Sizes of Microorganisms

• Photosynthetic bacteria- 5-50µm

• Influenza- 0.0005µm

• Poliovirus- 0.00001µm

• HIV- I have to look this up!

• Photosynthetic bacteria- 5-50µm

• Influenza- 0.0005µm

• Poliovirus- 0.00001µm

• HIV- I have to look this up!

Quick review: Quick review:

• ALL living things….• ALL living things….

Discovery of VirusesDiscovery of Viruses

• Several people, many years of work, but in the end…unlike bacteria that infect plants and animals, the mysterious substance could not be grown in test tubes or in petri dishes.

• Several people, many years of work, but in the end…unlike bacteria that infect plants and animals, the mysterious substance could not be grown in test tubes or in petri dishes.

Reality check: virus sizeReality check: virus size

• Millions of viruses can fit on the head of ONE pin

• Millions of viruses can fit on the head of ONE pin

Viral GenomesViral Genomes

• Can be DNA (double or single stranded)

• Or RNA (double or single stranded

• Usually a single nucleic acid either linear or circular

• Can be DNA (double or single stranded)

• Or RNA (double or single stranded

• Usually a single nucleic acid either linear or circular

Capsids and EnvelopesCapsids and Envelopes

• Capsid- protein shell enclosing genome

• Built from protein subunits called capsomeres

• Multiple shapes possible

• Capsid- protein shell enclosing genome

• Built from protein subunits called capsomeres

• Multiple shapes possible

Helical

icosahedral

Icosahedral head + tail

Envelope + glycoprotein spikes +RNA in helical capsids

Viral envelopesViral envelopes

• Made from host cell membrane phospholipids and proteins + viral proteins

• Made from host cell membrane phospholipids and proteins + viral proteins

Viruses are obligate intracellular parasites

Viruses are obligate intracellular parasites

• Need a host cell in order to reproduce• Each virus has specific host range• Viral surface proteins match up to receptor

molecules on cells• West Nile- horses, human, birds,

mosquitoes• Measles-humans only

• Need a host cell in order to reproduce• Each virus has specific host range• Viral surface proteins match up to receptor

molecules on cells• West Nile- horses, human, birds,

mosquitoes• Measles-humans only

• “Life” cycle of a virus

• “Life” cycle of a virus

Differences b/n Viral replication and “normal” cell replication

Differences b/n Viral replication and “normal” cell replication

• RNA lacks a “spellcheck” system very high mutation rate

• Related viruses in the same place can mix (HIV does this)

• Rapid evolution of virus species

• RNA lacks a “spellcheck” system very high mutation rate

• Related viruses in the same place can mix (HIV does this)

• Rapid evolution of virus species

Viral Reproduction 1 aka “serial killer mode”

Viral Reproduction 1 aka “serial killer mode”

• Lytic cycle- kills host cell• Virulant phage- virus that ONLY uses the

lytic cycle• Have been used in some cases as

“medicines” for bacterial infections• Restriction enzymes w/in bacteria can kill

them by slicing the DNA/RNA

• Lytic cycle- kills host cell• Virulant phage- virus that ONLY uses the

lytic cycle• Have been used in some cases as

“medicines” for bacterial infections• Restriction enzymes w/in bacteria can kill

them by slicing the DNA/RNA

Pic 19.5Pic 19.5

Viral Reproduction 2 aka “stalker mode”

Viral Reproduction 2 aka “stalker mode”

• Lysogenic cycle- replicates the virus w/o destroying the host cell

• Prophage-when the viral genetic info is added into the host cell DNA

• Lysogenic cycle- replicates the virus w/o destroying the host cell

• Prophage-when the viral genetic info is added into the host cell DNA

Viral Reproduction 3Viral Reproduction 3

• Temperate phages use both lytic and lysogenic cycles at different times w/in a bacterial cell

• Lambda phage we are using in the lab is a temperate phage

• Temperate phages use both lytic and lysogenic cycles at different times w/in a bacterial cell

• Lambda phage we are using in the lab is a temperate phage

Pic 19.6Pic 19.6

Medical/Scientific FactMedical/Scientific Fact

• Difference b/n E.coli in human intestines (helpful) and O157:H7 E.coli that causes food poisoning is presence of prophages in the latter

• Difference b/n E.coli in human intestines (helpful) and O157:H7 E.coli that causes food poisoning is presence of prophages in the latter

Viral envelopesViral envelopes

• Animal viruses have an outer membrane used to enter the host

• Glycoproteins on surface of virus bind to receptors on the cell

• Animal viruses have an outer membrane used to enter the host

• Glycoproteins on surface of virus bind to receptors on the cell

QUICK review of HIVQUICK review of HIV

• Retrovirus• Uses reverse transcriptase• Can be dormant for years• Strikes immune system- GENIUS!

• I wish we had more time to really discuss this!!!!

• Retrovirus• Uses reverse transcriptase• Can be dormant for years• Strikes immune system- GENIUS!

• I wish we had more time to really discuss this!!!!

Evolution of VirusesEvolution of Viruses

• Most likely evolved AFTER cells since they need a host

• Plasmids- small circular DNA molecules- can live outside of cell’s genome

• Transposons- DNA segments that can change location WITHIN cell’s genome

• Mobile genetic elements

• Most likely evolved AFTER cells since they need a host

• Plasmids- small circular DNA molecules- can live outside of cell’s genome

• Transposons- DNA segments that can change location WITHIN cell’s genome

• Mobile genetic elements

Problems Studying Evolution of viruses

Problems Studying Evolution of viruses

• Some viral genes identical to host or mixed in with the host

• Some viruses do show similar genomic patterns- good adaptations for natural selection

• Some viral genes identical to host or mixed in with the host

• Some viruses do show similar genomic patterns- good adaptations for natural selection

mimivirusmimivirus

• Relatively new• Largest virus known so far• Double stranded DNA• 1.2million bases in DNA (100x larger than

the flu)• Contains directions for DNA repair, protein

folding and synthesis of sugars!

• Relatively new• Largest virus known so far• Double stranded DNA• 1.2million bases in DNA (100x larger than

the flu)• Contains directions for DNA repair, protein

folding and synthesis of sugars!

Viruses vs. Animals- who will win?Viruses vs. Animals- who will win?

• Most animal cells have some ability to heal, repair, and fight off viruses

• Immune system’s task is to watch out for them as well

• These are why viruses are not always fatal

• Most animal cells have some ability to heal, repair, and fight off viruses

• Immune system’s task is to watch out for them as well

• These are why viruses are not always fatal

Vaccines & antiviral drugsVaccines & antiviral drugs

• Small doses of the virus to train the immune system to kill it

• AZT interferes with HIV’s reverse transcriptase…stops the virus from reproducing

• Small doses of the virus to train the immune system to kill it

• AZT interferes with HIV’s reverse transcriptase…stops the virus from reproducing

Emerging viruses- examplesEmerging viruses- examples

HIV 1959 Congo, 1980’s San Francisco, now worldwide

Ebola 1976 Central Africa

West Nile 1999 North America, now in all 48 contiguous states

SARS 2002 Southern China

H1N1 aka Swine Flu 2008/2009

How do new viruses “appear”?How do new viruses “appear”?

• 1. mutations of existing viruses

• 2. spreading of the virus outward from a small isolated population of hosts

• 3. spread of existing viruses from different species

• 1. mutations of existing viruses

• 2. spreading of the virus outward from a small isolated population of hosts

• 3. spread of existing viruses from different species

Basic termsBasic terms

• Epidemic- general outbreak of an illness

– Influenza in BMHS (hypothetical example)

• Pandemic- global outbreak of an illness

– 1918 Spanish flu

• Epidemic- general outbreak of an illness

– Influenza in BMHS (hypothetical example)

• Pandemic- global outbreak of an illness

– 1918 Spanish flu

Tracking and Studying the FluTracking and Studying the Flu

• 3 types Influenza A, B, C• B and C are solely human parasites and do

not cause epidemics• A caused 3 epidemics in the last century-

1918 Spanish Flu killed ~40million people (H1N1)

• Named for the 2 viral surface proteins

• 3 types Influenza A, B, C• B and C are solely human parasites and do

not cause epidemics• A caused 3 epidemics in the last century-

1918 Spanish Flu killed ~40million people (H1N1)

• Named for the 2 viral surface proteins

Viruses vs. Plants- who will win?Viruses vs. Plants- who will win?

• Signs of viral infections: • Bleached/brown spots on leaves/fruit• Damaged flowers/roots• Stunted growth• Viruses FIRST discovered from plants with

these symptoms

• Signs of viral infections: • Bleached/brown spots on leaves/fruit• Damaged flowers/roots• Stunted growth• Viruses FIRST discovered from plants with

these symptoms

How do viruses invade plants?How do viruses invade plants?

• Horizontal transmission- from an external source after damage to cells from animals, storms, etc

• Vertical transmission- inherits virus from parent- usually in asexual plant propogation

• Horizontal transmission- from an external source after damage to cells from animals, storms, etc

• Vertical transmission- inherits virus from parent- usually in asexual plant propogation

Smaller and more dangerous than a virus…

Smaller and more dangerous than a virus…

• Viroids- circular RNA molecules, a few hundred nucleotides long, infect plants, affect regulation of plant growth

• A single molecule is all that is needed to spread a disease!

• Viroids- circular RNA molecules, a few hundred nucleotides long, infect plants, affect regulation of plant growth

• A single molecule is all that is needed to spread a disease!

Even scarier!Even scarier!

• Prions- infectious proteins, appear in degenerative brain diseases like mad cow disease, transmitted via food (person eats mad-cow-steak)

• Slow acting, long incubation period (~10 years!)

• ALMOST INDESTRUCTABLE- even high heat doesn’t kill them

• Prions- infectious proteins, appear in degenerative brain diseases like mad cow disease, transmitted via food (person eats mad-cow-steak)

• Slow acting, long incubation period (~10 years!)

• ALMOST INDESTRUCTABLE- even high heat doesn’t kill them

Replication of prionsReplication of prions