Chapter 17-Virus

• What do you know about virus?

Virus characteristics

• Small

• Only view with an electron microscope

LE 18-2

Virus

Bacterium

Animalcell

Animal cell nucleus0.25 µm

Virus characteristics

• Not alive

• Obligate intracellular parasites. Infect cells-turn into a virus factory.

• Computer virus analogy

Virus Characteristics

• Contain only protein and nucleic acid (either DNA or RNA)

• Nucleic acid can be SS or DS-makes up virus core

• Core surrounded by a protein capsid. Shape of capsid gives the virus its characteristic shape. Some virus have envelope (animal virus only-produced by budding)

• Some virus have enzymes.

LE 18-4a

Capsomereof capsid

RNA

18 250 mm

Tobacco mosaic virus20 nm

LE 18-4b

Capsomere

Glycoprotein

70–90 nm (diameter)

DNA

Adenoviruses50 nm

LE 18-4d

80 225 nm

DNAHead

TailsheathTailfiber

Bacteriophage T450 nm

Figure 18.02x2 Phages

LE 18-4c

Glycoprotein

80–200 nm (diameter)

RNA

Capsid

Influenza viruses50 nm

Membranousenvelope

LE 18-9

Capsid

Viral envelopeGlycoprotein

Reversetranscriptase

RNA(two identicalstrands)

Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid

Virus characteristics

• Virus are host specific

• A. animal virus

• B. plant virus

• C. bacterial virus (phage)

• Sometimes a virus can mutate so it can infect a new host (avian flu)

Figure 18-01

Virus characteristics

• Virus cause diseases

• Rabies

• Influenza

• Colds

• Etc.

Figure 18-03

LE 18-11

Young ballet students in HongKong wear face masks toprotect themselves from thevirus causing SARS.

The SARS-causing agent is acoronarvirus like this one(colorized TEM), so named forthe “corona” of glyco-proteinspikes protruding form theenvelope.

Figure 17.8

(a) 2009 pandemic H1N1 influenza A virus

(b) 2009 pandemic screening

1 m

Virus Life cycle

Simplified virus life cycle (lytic). Virulent virus. What kind of cell can be infected by a virus (any-plant, animal, bacteria) Typical cycle-20-30 minutes.

• A. Attachment (specific-received e-mail)• B. Entry (don’t open that e-mail)• C. Synthesis• D. Assembly (spontaneous)• E. Release

LE 18-5

DNAVIRUS

Capsid

HOST CELL

Viral DNA

Replication

Entry into cell anduncoating of DNA

Transcription

Viral DNA

mRNA

Capsidproteins

Self-assembly ofnew virus particlesand their exit from cell

Enveloped Virus life cycle

LE 18-8

RNA

ER

Capsid

HOST CELL

Viral genome (RNA)

mRNA

Capsidproteins

Envelope (withglycoproteins)

Glyco-proteins Copy of

genome (RNA)

Capsid and viral genomeenter cell

New virus

Template

LE 18-10

HOST CELL

ReversetranscriptionViral RNA

RNA-DNAhybrid

DNA

NUCLEUS

ChromosomalDNA

Provirus

RNA genomefor thenext viralgeneration

mRNA

New HIV leaving a cell

HIV entering a cell

0.25 µm

HIVMembrane ofwhite blood cell

Treatment of viral diseases

Infectious cycle becomes an uncontrolled chain reaction. How can you deal with it?

A. Antibiotics? NoB. Immune system. Can’t respond quick

enough first time you see virus (immunization). Colds, flu.

C. Antiviral drugs-AIDS (AZT, protease inhibitors); Tamiflu (neuramindase inhibitor)

D. Restriction enzymes-if you’re a bacteria.

Why do virus infections cause disease symptoms?

• Damage to tissue-cell destruction (some cells regenerate (epithelium of upper respiratory tract), some can’t (nerves-polio)

• Cause infected cells to produce toxins (infected cell becomes transformed)

• Viral cancer-oncogenes. Hep B-liver cancer; Herpes-Burkitt’s lymphoma; papovavirus-cervical cancer

• Immune system-inflammation, lymphokines

Emerging virus

• Typically not new virus-Existing virus that:a. Evolve and infect individuals who are only

resistant to ancestral virus (influenza B and C)b.Spread from small populations to larger. AIDSc.Environmental disturbances can increase

emergence. Roads into tropical rainforest, global warming, airplanes.

d.Spread from species to species. Hanta virus-deer mice to humans. Influenza A- Avian virus. Porcine virus.

Triple Re-assorted H1N1Influenza virus

• Three processes contribute to the emergence of viral diseases– The mutation of existing viruses, which is

especially high in RNA viruses– Dissemination of a viral disease from a small,

isolated human population, allowing the disease to go unnoticed before it begins to spread

– Spread of existing viruses from animal populations; about three-quarters of new human diseases originate this way

Lysogenic life cycle (temperate virus)

Characteristics of the Lysogenic life cycle

• Steps Provirus inserts into infected cell’s genome. Repressor gene usually represses viral gene expression. Induction.

• Differences in lytic and lysogenic life cycle

LE 18-7

Phage

Phage DNA

The phage attaches to ahost cell and injects its DNA.

Phage DNAcircularizes

Bacterial chromosome

Lytic cycle

The cell lyses, releasing phages.Lytic cycleis induced

or Lysogenic cycleis entered

Certain factorsdetermine whether

Lysogenic cycle

Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.

The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.

Prophage

Many cell divisionsproduce a large population of bacteria infected withthe prophage.

Daughter cellwith prophage

Phage DNA integrates into thebacterial chromosomes, becoming aprophage.

New phage DNA and proteins aresynthesized and assembled into phages.

Consequences of Lysogeny

• none-(viral genetic info lies dormant)

• Transformation-makes cell cancerous (viral gene is an oncogene)

• Induction-Herpes-

Prions

• Infectious proteins

• Proteins are misfolded-cause other proteins to be misfolded

• Misfolded proteins impair cellular function

• “Mad cow” disease

LE 18-13

Normalprotein

New prion

Prion Original prion

Many prions

Origin of Virus?

There are three main hypotheses regarding the origins of virus:

a. The progressive, or escape, hypothesis states that viruses arose from genetic elements that gained the ability to move between cells;

b.The regressive, or reduction, hypothesis asserts that viruses are remnants of cellular organisms;

c. The virus-first hypothesis states that viruses coevolved with their current cellular hosts.

Progressive hypothesis

Genetic material moved from cell to cell via injured cell surfaces?

Evolution of capsid proteins may have facilitated infection of undamaged cells.

Candidates for sources of viral genomes?

a. plasmids?

b. Transposons? (Mobile genetic elements)

Mirror image rna sequences

Often associated with rna virus

AAAAAAUUUUUXXXXXAAAAAAUUUUUU

RNA folds back on itself

DS RNA recognized by a system of enzymes and proteins (the COP)

The COP then destroys an DS m-rna with these mirror image sequences

RNai

Fig. 18-13

miRNA-proteincomplex(a) Primary miRNA transcript

Translation blocked

Hydrogenbond

(b) Generation and function of miRNAs

Hairpin miRNA

miRNA

Dicer

3

mRNA degraded

5

RNAi Video

• http://www.nature.com/nrg/multimedia/rnai/animation/index.html