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 #1What 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 #2What is reverse transcriptase?

Capsids and Envelopes

• A protein shell that encloses viral genome• Built from protein subunits called capsomeres

Box #3Describe 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. 19-1

0.5 µm

Viruses Need Hosts!• Intracellular parasites that have a host

range (a limited number of host cells that it can

infect)

Box #4Why do virus’ need a

host cell?

Viral Reproductive Cycles

Virus VC

Reproductive Cycles of Phages

• 1st Rep Cycle: lytic cycle Death of host cell• Virulent phage: virus that only reproduces via

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

Attachmententryreplicationassemblylysis & release

Lytic Cycle

Phage assembly

Head Tail Tail fibers

Assembly

Release

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

4

5

3

• 2nd 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 CycleWhen 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 Cycle vs Lytic Cycle

Attachmententryreplicationassemblylysis & release

Box #5Describe 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. 19-6

PhageDNA

Phage

The phage injects its DNA.

Bacterialchromosome

Phage DNAcircularizes.

Daughter cellwith prophage

Occasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.

Cell divisionsproducepopulation ofbacteria infectedwith the prophage.

The cell lyses, releasing phages.

Lytic cycle

Lytic cycleis induced or Lysogenic cycle

is entered

Lysogenic cycle

Prophage

The bacterium reproduces,copying the prophage andtransmitting it to daughter cells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.

New phage DNA and proteinsare synthesized andassembled into phages.

Evolution of Viruses

• Maybe plasmids?? (circular DNA in bacteria and yeasts) and transposons,

small mobile DNA segments

• Mimivirus (double-stranded DNA) is 2nd largest virus discovered

• Virus evolved before or after cells?

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

Box #6Why 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 #7What are vaccines?

Box #8What 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 VirusesViruses are notorious for the

plethora of diseases they cause, including influenza, rabies, AIDS, polio, herpes, ebola, measles, mumps, chicken pox, warts, small pox -->

Lewandowsky-Lutz/ 2Epidermodysplasia verruciformis,

herpes Warts

Helpful VirusesViruses 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.

Virus Rap

Helpful Viruses

A good virusMost 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.

Box #8What is transduction?

How is it helpful?

Understanding Viruses Moviehttp://www.schooltube.com/video/360cafa902fb60e55f61/Understanding-Viruses

HIV

https://www.youtube.com/watch?v=HhhRQ4t95OI