VirusesViruses Chapter 19 in Campbell’s 8 th 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 Comment Despite the fact that viruses lack a nervous system and just about everything else in cells, they are geniuses at what they do!

b b A bacteria cell infected by viruses Are the viruses alive? A bacteria cell infected by viruses Are the viruses alive?

Relative Sizes of Microorganisms Photosynthetic bacteria- 5-50µm Influenza µm Poliovirus µm HIV- I have to look this up! Photosynthetic bacteria- 5-50µm Influenza µm Poliovirus µm HIV- I have to look this up!

Quick review: ALL living things….

Discovery 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.

Reality check: virus size Millions of viruses can fit on the head of ONE pin

Viral 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 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 envelopes Made from host cell membrane phospholipids and proteins + viral proteins

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

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” 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 19.5

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 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 19.6

Medical/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

Viral 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 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 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 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

mimivirus 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? 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 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- examples HIV1959 Congo, 1980’s San Francisco, now worldwide Ebola1976 Central Africa West Nile1999 North America, now in all 48 contiguous states SARS2002 Southern China H1N1 aka Swine Flu2008/2009

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 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 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 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 Spanish Flu killed ~40million people (H1N1) Named for the 2 viral surface proteins

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? 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… 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! 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 prions