Chapter 6 Topics Structure Classification Multiplication Cultivation and

Chapter 6 • Topics – Structure – Classification – Multiplication – Cultivation and replication – Nonviral infectious agents – Treatment

Structure • • • Size and morphology Capsid Envelope Complex Nucleic acid

The size of viruses compared to yeast, bacteria, and the molecular protein hemoglobin. Fig. 6. 1 Size comparison of viruses with a eucaryotic cell and bacteria.

Negative staining, positive staining, and shadow casting are methods of viewing viruses. Fig. 6. 2 Methods of viewing viruses 4

There are two major structures of viruses called the naked nucleocapsid virus and the enveloped virus. Fig. 6. 4 Generalized structure of viruses

Capsid • Protective outer shell that surrounds viral nucleic acid • Capsid spikes • Composed of capsomer subunits • Two types of capsids – Helical – Icosahedral

Helical capsid • Naked helical virus – Ex. Tobacco mosaic virus – Nucleocapsid is rigid and tightly wound into a cylinder-shaped package • Enveloped helical virus – Ex. Influenza, measles, rabies – Nucleocapsid is more flexible

Helical capsids have rod-shaped capsomers that form hollow discs that resemble a bracelet. Fig. 6. 5 Assembly of helical nucleocapsids

Comparison between a naked helical plant virus and an enveloped helical human virus. Fig. 6. 6 Typical variation of viruses with helical Nucleocapsids.

Icosahedron capsid • Three-dimensional, 20 -sided with 12 evenly spaced corners • Variation in capsomer number – Polio virus 32 capsomers – Adenovirus 240 capsomers

The structure and formation of an adenovirus. Fig. 6. 7 The structure and formation of an icosahedral virus.

Icosahedral viruses can be naked or enveloped. Fig. 6. 8 Two types of icosahedral viruses 12

Envelope • Lipid and proteins • Envelope spikes • During release of animal viruses, a part of the host membrane is taken • Enable pleomorphic shape of the virus – Spherical – filamentous

Function of the capsid/envelope • Protect nucleic acid from the host’s acidand protein-digesting enzymes • Assist in binding and penetrating host cell • Stimulate the host’s immune system

Complex viruses • Structure is more intricate than helical and icosahedral viruses • Pox virus – Several layers of lipoproteins – Course surface fibrils • Bacteriophage – Polyhedral head – Helical tail – Fibers for attachment

` Fig. 6. 10 Detailed structure of complex viruses 16

Comparison of the morphology (helical, icosahedral, complex) of a naked virus, enveloped virus and a complex virus. Fig. 6. 9 Morphology of viruses

Nucleic acid • Viruses contain either DNA or RNA • Possess only the genes to invade and regulate the metabolic activity of host cells • Ex. Hepatitis B (4 genes) and herpesviruses (100 genes) • No viral metabolic genes, as the virus uses the host’s metabolic resources

Examples of medically important DNA viruses.

Examples of medically important RNA viruses.

Classification • • • Structure Chemical composition Genetic makeup Host relationship Type of disease

Three orders of viruses have been developed for classification. Table 6. 4 Examples from the three orders of viruses. 22

Classification of important human viruses. Table 6. 5 Important human virus families, genera, common names, and types of viruses. 23

Multiplication • • • Adsorption Penetration Uncoating Synthesis Assembly Release

Adsorption to the host cell of a enveloped spike virus and naked capsid spike virus. Fig. 6. 12 The mode by which animal viruses adsorb to the host cell membrane

Penetration of animal viruses occur by endocytosis or fusion between the viral envelope and the host cell membrane. Fig. 6. 13 Two principal means by which animal viruses penetrate.

Uncoating and synthesis of viruses rely on the host’s metabolic systems. Fig. 6. 11 General feature in the multiplication cycle of an enveloped animal virus.

A mature virus can obtain an envelope by budding off the host cell. Fig. 6. 15 Maturation and release of enveloped viruses

Cytopathic effects • Damage to the host cell due to a viral infection – Inclusion bodies – Syncytia – Chronic latent state – Transformation

Examples of syncytia and inclusion bodies. Fig. 6. 16 Cytopathic changes in cells and cell cultures infected by viruses 30

Bacteriophage • Bacterial virus • Multiplication is similar to animal viruses except for the penetration (inject DNA), release (lyses) and prophage (lysogeny) stages

T-even bacteriophage penetrate the host cell by specifically binding and injecting their DNA into the host cell. Fig. 6. 18 Penetration of a bacterial cell by a T-even bacteriophage. 32

After viral multiplication inside the host cell, viral enzymes will weaken the host cell membrane, rupture the cell (lyses), and release numerous virions. Fig. 6. 19 A weakened bacterial cell, crowed with viruses. 33

Lysogeny is when the bacteriophage can insert its DNA into the bacterial host genome. Fig. 6. 20 The lysogenic state in bacteria

Comparison of bacteriophage and animal virus multiplication. Table 6. 7 Comparison of bacteriophage and animal virus multiplication. 35

Cultivation and Replication • In vivo methods – Laboratory animals – Embryonic bird tissues • In vitro methods – Cell or tissue culture

The early developing bird embryo contains a protective case, providing an ideal environment for viral propagation. Fig. 6. 21 Cultivating animal viruses in a developing bird embryo 37

A monolayer of monkey kidney cells is a cell culture enabling the propagating viruses. Fig. 6. 22 Appearance of normal and infected cell cultures 38

Noncellular Infectious Agents • Prions • Satellite viruses • Viroids

Prions • Protein particle with no nucleic acid, no envelope, no capsid • Diseases – Creutzfeldt-Jakob – “mad cow disease”

Satellite viruses • Dependent on other viruses for replication • Ex. Delta agent, which is only expressed in the presence of hepatitis B virus

Viroids • Plant pathogens – Tomatoes, potatoes, cucumbers. • 1/10 th the size of normal viruses • Naked strands of RNA, no capsid