Viruses Big Idea 3: Living systems store, retrieve, transmit, and respond to info essential to life processes.

Essential Knowledge 3C3: Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts.

Structure of Viruses Viruses are not cells  small infectious particles consisting of nucleic acid in a protein coat or a membranous envelope

Viral Genomes Made of either: –Double- or single-stranded DNA, or –Double- or single-stranded RNA

Box #1 What is a virus? How is a virus’ genetic information stored?

RNA Viruses Need Reverse Transcriptase: enzyme that makes double stranded DNA to match w/ host cell Lack replication error-checking mechanisms  higher rates of mutation (HIV!)

Herpes Virus-DNA

Ebola Virus-RNA

Box #2 What is reverse transcriptase?

Capsids and Envelopes A protein shell that encloses viral genome Built from protein subunits called capsomeres

Box #3 Describe the viral structure

Highly efficient  allow for rapid evolution and acquisition of new phenotypes ! Viral Replication

AKA: phages, viruses that infect bacteria Have a long capsid head that encloses DNA A protein tail attaches phage to host and injects DNA inside Bacteriophage

Fig µm

Viruses Need Hosts! Intracellular parasites that have a host range (a limited number of host cells that it can infect)

Box #4 Why do virus’ need a host cell?

Viral Reproductive Cycles Virus VC

Reproductive Cycles of Phages 1 st Rep Cycle: lytic cycle  Death of host cell Virulent phage: virus that only reproduces via lytic cycle

Lytic Cycle Lytic Cycle (Click to see animation) Whe(n a dormant virus is stimulated, it enters the lytic phase: new viruses are formed, self-assemble, and burst out of the host cell, killing the cell and going on to infect other cells Attachment  entry  replication  assembly  lysis & release

Lytic Cycle Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment

2 nd reprod. Cycle: latent/lysogenic cycle No destroying host cell  viral DNA is mixed into host cell’s chromosome  prophage Gives host cell properties like increased pathogenicity in bacteria Reproductive Cycles of Phages

Lysogenic Cycle When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions. An infected cell produces more viral protein and genetic material instead of its usual products. Some viruses may remain dormant inside host cells for long periods, causing no obvious change in their host cells (a stage known as the lysogenic phase). (click to see animation)lysogenic

Lysogenic Lysogenic Cycle vs Lytic CycleLytic Cycle Attachment  entry  replication  assembly  lysis & release

Box #5 Describe the two virus life cycles. How do they differ?

Phages that use both lytic and lysogenic cycles Envelope signal can trigger switch Temperate Phages Lambda Phage  Attacks E. Coli

Fig Phage DNA Phage The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

Evolution of Viruses Maybe plasmids?? (circular DNA in bacteria and yeasts) and transposons, small mobile DNA segments

Mimivirus (double-stranded DNA) is 2 nd largest virus discovered Virus evolved before or after cells?

Megavirus (2 strand DNA) Oct. 2011, largest virus

Box #6 Why have virus’ been able to evolve so quickly?

Harmless derivatives of pathogenic microbes that stimulate immune system to mount defenses against actual pathogen Viral infections cannot be treated by antibiotics Vaccines

Box #7 What are vaccines?

Box #8 What vaccines have you had?

Viroids and Prions: Simple Viroids: circular RNA molecules that infect plants and disrupt growth Prions: slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals

Ex: Prion Mad Cow Disease

Ex: Prion Creutzfeldt Jakob Syndrome

Plant Viruses Most have an RNA genome –Horizontal transmission, entering through damaged cell walls –Vertical transmission, inheriting the virus from a parent

Harmful Viruses Viruses are notorious for the plethora of diseases they cause, including influenza, rabies, AIDS, polio, herpes, ebola, measles, mumps, chicken pox, warts, small pox -->influenza Lewandowsky-LutzLewandowsky-Lutz/ 2 2 Epidermodysplasia verruciformis, herpes Warts

Helpful Viruses Viruses carry out natural "genetic engineering": by incorporating genetic material into its host This is known as transduction, and in some cases it may serve as a means of evolutionary change Certain varieties of flowers have been developed using viruses to alter the genetic code. Dr Patrick Lee uses reovirus to kill brain cancer cells transplanted into laboratory mice, while sparing normal, healthy cells. Clinical trials involving reovirus in people are now underway. Clinical trials Virus Rap

Helpful Viruses A good virus Most of us go out of our way to avoid viruses. But Dr Patrick Lee - formerly of the University of Calgary and now at Dalhousie University - spends a lot of time in the company of a very common virus known as a reovirus. Normally this bug causes nothing more serious than a mild infection. But Dr Lee’s team discovered that the reovirus has the ability to kill brain cancer cells transplanted into laboratory mice, while sparing normal, healthy cells. Clinical trials involving reovirus in people are now underway.Clinical trials

Box #8 What is transduction? How is it helpful?

Understanding Viruses Movie o/360cafa902fb60e55f61/Under standing-Viruses o/360cafa902fb60e55f61/Under standing-Viruses

HIV