Ch. 17 Warm-up Why do many scientists classify viruses as non-living? Draw the basic structure of a virus. Label and define capsid, viral envelope and nucleic acid.

Ch. 17 Warm-up Draw the lytic/lysogenic cycle. What stage of the lytic-lysogenic cycle is a virus virulent? Temperate? What determines a host range?

Viruses Chapter 17

Bacteria vs. Viruses Bacteria Virus Prokaryotic cell Most are free-living (some parasitic) Relatively large size Antibiotics used to kill bacteria Not a living cell (genes packaged in protein shell) Intracellular parasite 1/1000 size of bacteria Vaccines used to prevent viral infection Antiviral treatment

Viruses Very small (<ribosomes) Components = nucleic acid + capsid Genetic material: DNA or RNA (double or single-stranded) Capsid: protein shell Some viruses also have viral envelopes that surround capsid

Viruses Limited host range Reproduce quickly within host cells Entry = attach to host cell membrane receptors through capsid proteins or glycoproteins on viral envelope (animal) Eg. human cold virus (rhinovirus)  upper respiratory tract (mouth & nose) Reproduce quickly within host cells Can mutate easily RNA viruses: no error-checking mechanisms

Simplified viral replicative cycle

Viral Reproduction Lytic Cycle: Use host machinery to replicate, assemble, and release copies of virus Virulent phages: Cells die through lysis or apoptosis Lysogenic (Latent) Cycle: DNA incorporated into host DNA and replicated along with it Bacteriophage DNA = prophage Animal virus DNA = provirus UV radiation, chemicals: lysogenic  lytic cycle Temperate Phage: uses both methods of replication

Bacteriophage Virus that infects bacterial cells

Lytic Cycle of T4 Phage

Lytic Cycle vs. Lysogenic Cycle

Bacterial defenses against phages: Natural selection: bacterial surface proteins mutate and prevent phage entry Restriction enzymes: destroy foreign DNA CRISPR-Cas9 system: enzymes that identify and cut invading phage DNA The CRISPR system = bacterial immune system

Animal viruses have a membranous envelope Host membrane forms around exiting virus Difficult for host immune system to detect virus

Retrovirus RNA virus that uses reverse transcriptase (RNA template  DNA) Newly made viral DNA inserted into chromosome of host (provirus) Host transcribes provirus to make new virus parts Example: HIV (Human Immunodeficiency Virus)

HIV = Retrovirus

HIV: Origin: Chimpanzee virus Infects white blood cells (helper T) HIV+: provirus (DNA inserted), latent AIDS: <200 WBC count, opportunistic infections

Other Human Viruses Herpes virus Smallpox Herpes Simplex Virus 1 (HSV-1) Herpes Simplex Virus 2 (HSV-2) Eradicated in 1979 due to worldwide vaccination campaigns

RNA viruses = High mutation rates

Emerging Viruses Zika Virus Ebola Virus H1N1 Influenza A Virus Viruses that suddenly become apparent Caused by existing viruses (combo of viruses) Human diseases can originate from animals Zika Virus H1N1 Influenza A Virus Ebola Virus West Nile Virus

Current Outbreaks Epidemic: widespread outbreak Pandemic: global outbreak Chikungunya Virus Zika Virus Measles Virus

Drugs for Prevention/Treatment Vaccine: weakened virus or part of pathogen that triggers immune system response to prevent infection Ex. HPV, MMR, HepA, Flu shot Antiviral Drugs: block viral replication after infection Ex. Tamiflu (influenza), AZT (HIV)

Plant Viruses Similar structure and mode of replication as animal viruses Horizontal transmission: infected by outside source Vertical transmission: inherited from parent No cures for most viral plant diseases Eg. tobacco mosaic virus (TMV)

Prions Misfolded, infectious proteins that cause misfolding of normal proteins Eg. scrapie (sheep), mad cow disease (BSE), Creutzfeldt-Jakob disease (humans), kuru (humans – New Guinea)

Diseases caused by prions Prions act slowly – incubation period of at least 10 years before symptoms develop Prions are virtually indestructible (cannot be denatured by heating) No known cure for prion diseases Kuru in New Guinea

Prion Neurodegenerative Diseases Alzheimer’s Disease Parkinson’s Disease

Amoeba Sisters: Viruses: Virus replication and the mysterious common cold