Lesson 7 Viruses, Viroids, and Prions February 19, 2015.

Lesson 7Viruses, Viroids, and Prions

February 19, 2015

General Characteristics of Viruses

• Obligatory intracellular parasites—require living host cells in order to multiply

• Some features of a virus are:– Contain DNA or RNA• Do not contain both

– Contains an encapsulating protein coat• Some viruses have spikes

General Characteristics of Viruses

• Uses host cell machinery to replicate– Why is this problematic for humans????

• Have a varied host range (cells that it infects)– determined by specific host attachment sites and

cellular factors– Viruses that infect bacteria are called

bacteriophages

Figure 13.1 Virus sizes.

Adenovirus90 nm

Bacteriophage T4

225 nm

Rabies virus170 × 70 nm

Chlamydia elementary body300 nm

Rhinovirus30 nm

Bacteriophagesf2, MS224 nm

Poliovirus30 nm

Prion200 × 20 nm

Vaccinia virus300 × 200 × 100 nm

Viroid300×10 nm

Ebola virus970 nm

Plasma membraneof red blood cell10 nm thick

E. coli(a bacterium)

3000 × 1000 nm

Tobacco mosaic virus250 × 18 nm

Bacteriophage M13800 × 10 nm

Human red blood cell10,000 nm in diameter

Virion Structure

• Virion—complete, fully developed viral particle– Nucleic Acid• DNA or RNA (double stranded or single stranded)• Positive-strand RNA(uses host’s RNA polymerase) vs

Negative stranded RNA (encodes own RNA polymerase)

– Capsid—protein coat that protects nucleic acid• Capsomeres—small protein subunits of the capsid• Arrangement of capsomeres is virus-specific

Figure 13.2 Morphology of a nonenveloped polyhedral virus.

Capsomere

Nucleic Acid

Capsid

A polyhedral virus Mastadenovirus

Virion Structure

– Envelope—surrounds the capsid • Comprised of lipids, proteins, and carbohydrates• Envelopes are comprised of the host cell’s plasma membrane

(extrusion)• Some viruses are non-enveloped

– Capsid protects genetic material from nucleases

– Spikes—carbohydrate-protein complexes that protrude from the envelope.

• Viruses can escape the immune system by mutating the surface proteins!!!!

• Influenza (flu-vaccine not 100% effective due to mutations)

Taxonomy of Viruses

• Family names end in -viridae• Genus names end in -virus– Family Herpesviridae, genus Simplexvirus

• Viral species: a group of viruses sharing the same genetic information and ecological niche (host) – Common names are used for species– Subspecies are designated by a number• Human Immunodeficiency Virus subspecies I (HIV-1)

Growing Viruses• Viruses MUST be grown in living cells– Why is this?– Bacteriophages form plaques on lawn of bacteria• Plaques—clearing of bacteria. Concentration of viral

suspension measured by plaque-forming units (PFUs)

– Animal viruses may be grown in living animals or in embryonated eggs or in cell cultures• Continuous cell lines (HeLa cells)• Influenza for flu vaccine are grown in embryonic eggs

(primary cell lines)

Figure 13.6 Viral plaques formed by bacteriophages.

Plaques

Figure 13.7 Inoculation of an embryonated egg.

Air sac

ShellAmniotic cavity

Chlorioallantoicmembrane

Chlorioallantoic membrane inoculation

Amniotic inoculation

Allantoic inoculation

Yolk sac inoculation

Shell membrane

Albumin Allantoic cavity

Yolk sac

Virus Identification

• Cytopathic Effects—visual effects seen in viral infected cell– Cell rounding– Cell lysis– Cell clumping (multiple nuclei in one cell)

• Serological Tests– Detect antibodies against viruses in a patient (ELISA)– Use antibodies to identify viruses in neutralization tests, viral

hemagglutination, and Western blot• Nucleic Acids

– RFLPs– PCR

Viral Multiplication

• In order for a virus to multiply it must invade a host cell and use the host’s metabolic machinery

• Host cell can be ruptured to release virus (Lytic cycle) or survive to continue producing viruses indefinitely (Lysogenic Cycle)

• Unlike bacteria, viruses have a one-step growth curve

Figure 13.10 A viral one-step growth curve.

Num

ber o

f viri

ons

Time (days)

Acute infection

Virio

ns re

leas

ed fr

om h

ost c

ell

Eclipseperiod

Results of Multiplication of Bacteriophages

• Viral infection can be categorized into two different processes1. Lytic cycle

• Phage causes lysis and death of host cell

2. Lysogenic cycle• Prophage DNA incorporated in host DNA• Phage conversion—host cell exhibiting new properties

– Some bacteria are only pathogenic when they contain prophage DNA• Shiga toxin in E. coli and exotoxin of S. aureus that causes Toxic

Shock Syndrome

The Lytic Cycle• Attachment: phage attaches by tail fibers to

host cell• Penetration: phage lysozyme opens cell wall;

tail sheath contracts to force tail core and DNA into cell

• Biosynthesis: production of phage DNA and proteins

• Maturation: assembly of phage particles• Release: phage lysozyme breaks cell wall

Figure 13.11 The lytic cycle of a T-even bacteriophage.

Attachment: Phage attaches to host cell.

Biosynthesis: Phage DNA directs synthesis of viral components by the host cell.

Penetration: Phage penetrates host cell and injects its DNA.

Release: Host cell lyses, and new virions are released.

1

2

3

Maturation: Viral components are assembled into virions.

4

5

Bacterial cell wall

Bacterial chromosome

Capsid DNA

Capsid (head)

Sheath

Tail fiber

BaseplatePinCell wall

Plasma membrane

Sheath contracted

Tail core

Tail DNA

Capsid

Tail

Tail fibers

Figure 13.12 The lysogenic cycle of bacteriophage λ in E. coli.

Phage DNA(double-stranded)

Bacterial chromosome

Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle.

Lysogenic cycle

Cell lyses, releasingphage virions.

Lytic cycle

New phage DNA andproteins are synthesizedand assembled into virions.

Prophage

Many celldivisions

Lysogenic bacteriumreproduces normally.

4A

Phage attachesto host cell andinjects DNA.

5

4B

3B Phage DNA integrates within thebacterial chromosome by recombination, becoming a prophage.

1

2

3A

Phage DNA circularizes and enterslytic cycle or lysogenic cycle.

OR

Figure 13.15 Replication of a DNA-Containing Animal Virus.

1 ATTACHMENT Virion attaches to host cell.

A papovavirus is a typicalDNA-containing virus thatattacks animal cells.

Nucleus

Cytoplasm

Host cell 2 ENTRY and UNCOATING Virion enters cell,

and its DNA is uncoated.

Viral DNA

Capsid proteins

3 A portion of viral DNA istranscribed, producingmRNA that encodes“early” viral proteins.

mRNA

BIOSYNTHESIS Viral DNA is replicated, and some viral proteins are made.

4

Late translation;capsid proteins aresynthesized.

5

Capsid proteins

MATURATIONVirionsmature.

6

7 RELEASEVirions arereleased. DNA

Capsid

Papovavirus

Enveloped vs. Non-Enveloped Virus

Differences between Animal Virus and Bacteriophage Infection

Latent vs. Persistent Infections

• Latent viral infections are infections where the viral DNA is integrated into the host– May not produce disease for a long period of time– Reactivated via immunosupression or exogenous

stimulus• Herpesviruses

– Reactivated by fever or sunburn (fever blister)

• Varicellovirus (chickenpox)– Acquired in childhood but remains dormant in nerve cells– Changes in T-cell response activate the virus (Shingles)

Latent vs. Persistent Infections

• Persistent viral infections occurs gradually over a long period– Also called chronic viral infections– Viruses gradually are produced over a long period

of time– Usually fatal• HIV and measles virus are examples of persistent

infections

Latent and Persistent Viral Infections

Prions

• Proteinaceous infectious particle• Initially discovered in sheep and called scrapie– Infectivity in sheep’s brain reduced by proteases and not

radiation• Inherited and transmissible by ingestion,

transplanted nerve tissue, and surgical instruments– Spongiform encephalopathies: Creutzfeldt-Jakob

disease, kuru, mad cow disease• Caused by the conversion of a normal host

glycoprotein PrPC into an infectious form PrPSC

Figure 13.22 How a protein can be infectious.

1 PrPc produced by cells is secreted to the cell surface.

2 PrPSc may be acquired or produced by analtered PrPc gene.

PrPSc

PrPSc reacts with PrPc

on the cell surface.4 PrPSc converts the PrPc to

PrPSc.

The new PrPSc converts more PrPc.

5 The new PrPSc is taken in, possibly by receptor-mediated endocytosis.

6

PrPc

Lysosome

PrPSc accumulates inendosomes.

7 PrPSc continues to accumulate as the endosome contents are transferred to lysosomes. The result is cell death.

8

Endosome

3

Creutzfeldt-Jakob disease

Plant Viruses and Viroids• Plant Viruses– Plant cells are normally protected via their cell

walls therefore viruses enter through wounds or via insects

– Insects can serve as reservoirs for some plant viruses

– Causes many diseases to economically important crops

Viroids

• Short pieces of infectious RNA– 300-400 nucleotides long– Do not encode proteins– Possibly derived from introns

• Lacks a protein coat• Internally paired (forms stem-loops) that

protect it from degradation• Example: Potato spindle tuber disease

Figure 13.23 Linear and circular potato spindle tuber viroid (PSTV).