Chapter 13 Viruses, Viroids, and Prions

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Not all pathogens are cellular!

• Many infections of humans, animals, plants, and even bacteria are caused by acellular (noncellular) particles

• Acellular infectious particles include– Viruses– Viroids – prions

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Characteristics of Viruses

• Virus• Much smaller than bacteria(usually measured in

nanometers)• Acellular infectious agent having either DNA or RNA

• They do not have both!• Cause many infections in humans, animals, plants,

and bacteria• Cause most of the diseases that affect the

industrialized world– Examples: common cold, influenza, herpes, SARS, Polio,

HIV

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Figure 13.4 Sizes of selected virions

Red blood cell(10,000 nm in diameter)

E. coli (bacterium)(1000 nm 3000 nm)

Poliovirus(30 nm)

Bacteriophage MS2(24 nm)

Bacteriophage T4(50 nm 225 nm)

Smallpox virus(200 nm 300 nm)

Tobacco mosaic virus(15 nm 300 nm)

Bacterialribosomes(25 nm)

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Characteristics of Viruses

• Cannot carry out any metabolic pathway outside of a cell

• Neither grow nor respond to the environment• Cannot reproduce independently

– Recruit the cell’s (animal, plant, or bacterial cells) metabolic pathways to reproduce

• No cytoplasmic membrane, cytosol (liquid portion of cytoplasm), organelles

• Have extracellular and intracellular state

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Characteristics of Viruses

• Extracellular State (Naked or Enveloped)– Protein coat (capsid) surrounding nucleic acid

(naked)– Some have a phospholipid envelope which

surrounds the capsid (enveloped)– Outermost layer provides protection and

recognition sites for host cells• Intracellular State

– Capsid removed– Virus exists inside the cell as nucleic acid (DNA or

RNA)6

Figure 13.1 Virions-overview

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Differentiating Viruses

• We can differentiate viruses from one another based on their Genetic Material

• Show more variety in genomes than cells– The genetic material a virus contains is the

primary way scientists categorize and classify viruses

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• We can differentiate viruses from one another based on their Genetic Material

– Viral genome may be DNA or RNA, but never both• dsDNA, ssDNA, dsRNA, ssRNA

– ds= double stranded, ss= single stranded• Linear and segmented or single and circular

– Influenza virus genome has 8 linear segments of ssRNA• Much smaller than genomes of cells

– Cells always have double stranded DNA• ssDNA and dsRNA are almost nonexistent in cells

Differentiating Viruses

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Differentiating Viruses

• Hosts of Viruses– Most viruses infect only particular host’s cells

• Species specific– Dog viruses don’t infect humans

– May be so specific they infect only particular kind of cell in a particular host

• HIV attacks helper T lymphocytes in humans but does not infect muscle or bone cells

– Generalists – infect many kinds of cells in many different hosts

• Rabies10

Figure 13.3 Hosts of viral infections-overview

Tobacco mosaic virus infected leaf on left

Bacteria (blue/gray) under attack from a bacteriophage (pink)

Human WBC cytoplasmic membrane with HIV particles (blue) attached

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Differentiating Viruses

• Host specificity– Due to viral surface proteins which have a precise affinity

(attraction) for complementary proteins on the host cell membranes

Differentiating Viruses

• Hosts of Viruses– All types of cells can be infected by a virus

• Archaeal, bacterial, plant, protozoan, fungal, animal– Most research focuses on animal and bacterial

viruses• A virus that infects a bacterium is called a

bacteriophage or phage

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Naked vs. Enveloped

Differentiating Viruses

• Capsid Morphology– Capsids

• Provide protection for viral nucleic acid• Means of attachment to host’s cells• Composed of proteinaceous subunits called

capsomeres– Capsomere made of single or multiple types of proteins

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Differentiating Viruses

• Viral Shapes– Three basic shapes

• Helical (capsomeres bond in a spiral fashion)• Polyhedral (close to spherical, geodesic dome)• Complex (many different shapes, bullet shaped

rabies virus)

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Figure 13.5 The shapes of virions-overview

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Figure 13.6 Bacteriophage T4-overview

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Differentiating Viruses

• The Viral Envelope– Acquired from host cell during viral replication or

release• Envelope is portion of membrane system of host

– Composed of phospholipid bilayer and proteins• Some proteins are virally coded proteins (spikes)

– Envelope proteins play a role in host recognition

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Figure 13.7 Enveloped virion-overview

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Viral Replication• Dependent on hosts’ organelles and enzymes to

produce new viral particles• Lytic replication

– Replication cycle usually results in lysis and death of host cell

• Basic stages of lytic replication cycle1. Recognition and Attachment2. Entry 3. Chromosome degraded4. Synthesis5. Assembly6. Release

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Figure 13.12 Three mechanisms of entry of animal viruses-overview

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Figure 13.8 The lytic replication cycle in bacteriophages-overview

Attachment

Tail sheath

Bacteriophagegenome

Outer membrane

Peptidoglycan

Cytoplasmicmembrane

Bacterialchromosome

Attachment

Entry

Lytic replicationcycle of bacteriophage

Release

Assembly

Phage proteins

Entry

Phage DNA

Bacterialchromosomedegraded

Synthesis

Assembly

Base Tail Sheath DNA Capsid Mature head Tail fibers Mature virion

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Viral Replication

• Lysogeny– Modified replication cycle – Infected host cells grow and reproduce normally

for generations before they lyse– Inactive bacteriophage is called a prophage– Induction occurs and the prophage is excised from

the host chromosome• Induction can occur through DNA damaging chemicals,

UV light, X rays• After induction the lytic cycle will occur

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Figure 13.11 The lysogenic replication cycle in bacteriophages: phage lambda and E. coli

Attachment

Lambda phage

Entry

Lyticcycle

Release

Assembly

Induction

Synthesis

Prophagein chromosome

Further replications andcell divisions

Lysogeny

Replication ofchromosomeand virus;cell division

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Transduction

Viral Replication

• Replication of Animal Viruses– Same basic replication pathway as

bacteriophages1. Recognition and Attachment (capsid or envelope proteins

recognize host cell receptors)2. Entry (fusion with cell membrane or endocytosis)3. Synthesis (DNA virus in the nucleus, RNA virus in the cytoplasm)4. Assembly5. Release (budding, exocytosis, lysis)

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Viral Replication

• Replication of Animal Viruses– Enveloped viruses cause persistent infections

• Released from cell by budding– Naked viruses are released by exocytosis or lysis

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Figure 13.14 The process of budding in enveloped viruses

Envelopedvirion

Budding ofenveloped virus

Viral capsid

Viral glycoproteins

Cytoplasmicmembraneof host

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Viral Replication

• Replication of Animal Viruses– Latency of animal viruses (chicken pox and

herpes virus)• Called latent viruses or proviruses

– When animal viruses remain dormant in host cells» May be prolonged for years with no viral activity

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The Role of Viruses in Cancer

• Viruses cause 20–25% of human cancers– Some viruses carry copies of oncogenes as part of

their genomes• Oncogenes are involved in cell division and are usually repressed

(not activated) and no cancer results

– Some promote oncogenes already present in host– Specific viruses are known to cause human cancers

• Kaposi’s sarcoma (HIV)• Cervical cancer (HPV)

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Are Viruses Alive?

• Infectious agents with both living and non-living characteristics– Living characteristics:

• Reproduce, but only in living host cells• Can mutate

– Nonliving characteristics:• Acellular: no cytoplasm or organelles• Cannot carry out metabolism on their own• Have DNA or RNA but not both

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Other Parasitic Particles: Viroids and Prions

• Characteristics of Viroids– Extremely small, circular pieces of RNA that

are infectious and pathogenic in plants– Similar to RNA viruses, but lack capsid– No known animal diseases are known to be

caused by viroids

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Figure 13.21 One effect of viroids on plants

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Characteristics of Prions

• Proteinaceous infectious agents• Are ONLY protein• Cause spongiform encephalopathies:

– Mad cow – Scrapie– Kuru– Creutzfeld-Jakob syndrome

• Resistant to proteases, UV light, heat, disinfectants• Prions only destroyed by incineration or autoclaving in NaOH

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Prions

• Characteristics of Prions– Prion diseases

• Fatal neurological degeneration, and loss of brain matter• Large vacuoles form in brain

– Characteristic spongy appearance

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Chronic wasting disease

Prions• Characteristics of Prions

– Proteinaceous infectious agents– Cellular PrP protein

• Made by all mammals• Normal structure with -helices called cellular PrP

– Prion PrP• Disease-causing form with -pleated sheets called prion

PrP– Prion PrP changes shape of cellular PrP so it

becomes prion PrP

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Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP)-overview

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Shared characteristics of viruses, viroids, and prions

• Simple compared to cells• Lack cell membranes• Composed of 1 or a few organic molecules• Lack most of the characteristics of life

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