Chapter 13 Characterizing 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 infectious particles include– _____________– _____________– _____________

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General 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

described in chapters 3, 5, 6, and 7

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Shared Characteristics of Viruses

• Virus– Minuscule (usually measured in nanometers),

acellular infectious agent having either ____ or ______

• _______________________– Cause many infections in humans, animals,

plants, and bacteria– Cause most of the diseases that plague the

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

SARS, Polio, HIV

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Shared Characteristics of Viruses

• __________carry out any metabolic pathway outside of a cell

• Neither _________________to the environment

• Cannot _____________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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Shared Characteristics of Viruses

• Extracellular State– Protein coat (________) surrounding nucleic acid– Some have a phospholipid ____________which

surrounds the capsid– Outermost layer provides ____________and

_____________sites for host cells• Intracellular State

– Capsid removed– Virus exists as nucleic acid

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Figure 13.1 Virions-overview

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Differentiating Viruses

• We can differentiate viruses from one another based on their _________________

• 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• _________________________________

– 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 __________have ___________________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

• _________________– 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

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

• Rabies10

Differentiating Viruses

• Host specificity– Due to viral surface ____________which have a precise

__________ (attraction) for complementary proteins on the host cell membranes

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

• Hosts of Viruses– ____________________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

________________or phage• Bacteriophages out number all bacteria, archaea, and

eukaryotes put together!

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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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Differentiating Viruses

• Capsid Morphology– Capsids

• Provide ________________for viral nucleic acid• Means of _________________to host’s cells• Composed of proteinaceous subunits called

_______________– Capsomere made of single or multiple types of proteins

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Differentiating Viruses

• Viral Shapes– Three basic shapes

• ___________(capsomeres bond in a spiral fashion)• ___________(close to spherical, geodesic dome)• ___________(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 ________________– Acquired from host cell during viral replication or

release• Envelope is portion of _____________system of _______

– Composed of ____________bilayer and _________• Some proteins are virally coded glycoproteins (spikes)

– Envelope’s proteins and glycoproteins often play 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• ___________________

– Replication cycle usually results in ______and ______of host cell

• Basic stages of lytic replication cycle1. Recognition and Attachment2. Entry3. Chromosome degraded4. Synthesis5. Assembly6. Release

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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

• __________________– Modified replication cycle – Infected host cells _______________________for

generations before they lyse– Inactive bacteriophage is called a _____________– ______________occurs and the prophage is

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

UV light, X rays• After induction the _______________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 Attachment2. Entry3. Chromosome degraded4. Synthesis5. Assembly6. Release

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Viral Replication

• Replication of Animal Viruses– Attachment of animal viruses

• Chemical attraction between glycoproteins or proteins on the virus exactly fitting _______________________on animal cells

– Animal viruses do not have tails or tail fibers like bacteriophages do

• Have glycoprotein spikes or other attachment molecules that mediate attachment

• http://highered.mcgraw-hill.com/classware/ala.do?alaid=ala_1697939

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Figure 13.12 Three mechanisms of entry of animal viruses-overview

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Viral Replication

• Replication of Animal Viruses– Synthesis of animal viruses

• Requires different strategy depending on its nucleic acid

– DNA viruses often enter the nucleus– RNA viruses often replicate in the cytoplasm

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Viral Replication

• Replication of Animal Viruses– Assembly and release of animal viruses

• Most DNA viruses assemble in nucleus• Most RNA viruses develop solely in cytoplasm

– Number of viruses produced depends on type of virus and size and initial health of host cell

– Enveloped viruses cause ____________________• Released from cell by _______________

– Naked viruses are released by __________or _____

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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 __________viruses or proviruses

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

– Different than ___________• Some latent viruses do not become incorporated into host

chromosome• _______________________________________________________

– Incorporation of provirus into host DNA is _________• HIV

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The Role of Viruses in Cancer

• Viruses cause ___________ of human cancers– Some 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– Some interfere with tumor repression– Specific viruses are known to cause human cancers

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

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Culturing Viruses in the Laboratory

• Viruses must be _____________________________ ________________________________________

• Culturing Viruses in Mature Organisms– In bacteria– In plants and animals

• Culturing Viruses in Embryonated Chicken Eggs– Inexpensive, among the largest of cells, free of

contaminating microbes, and contain a nourishing yolk

• Culturing Viruses in Cell (Tissue) Culture

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Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate

Bacterial lawn

Viral plaques

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Are Viruses Alive?

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

• •

– Nonliving characteristics:• • •

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Not all viruses are bad!• A previously unknown virus has been found which

attackes tiny marine alga that multiplies and forms algal blooms– Algal blooms can often be seen from space!

• Deplete water of oxygen• Harming fish and other marine life

• A bacteriophage has been discovered which transfers genes for photosynthetic machinery into cyanobacteria increasing the rate of photosynthesis– Up to 10 million of these viruses have been found in a single milliliter of

seawater!– Much of the oxygen we breath may be attributed to the action of this virus

on cyanobacteria37

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Other Parasitic Particles: Viroids and Prions

• Characteristics of ____________– Extremely small, _______________________

__________________________– Similar to RNA viruses, but ________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• • Cause spongiform encephalopathies:

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

• _____________to proteases, UV light, heat, disinfectants

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Prions

• Characteristics of Prions– Prion diseases

• Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter

• Large vacuoles form in brain – Characteristic spongy appearance

• Spongiform encephalopathies– Prions only destroyed by incineration or autoclaving

in 1 N NaOH

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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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