Chapter 6: Viruses, Viroids and Prions

Louis Pasteur

postulated that rabies was caused by a “living thing” smaller than Bacteria (1884)

Dmitri Ivanovski

first proposed the term “virus” (L. “poison”)

Ivanovski and Beijerinck

showed a disease in tobacco was caused by a virus (1890s)

1950s virology

a multifaceted discipline

What are viruses?

noncellular particles with a definite size, shape, and chemical composition. Obligate intracellular parasites

when did viruses originate?

there is no universal agreement on how and when viruses originated

What are viruses considered?

the most abundant microbes on earth

What did Viruses play a role in?

in the evolution of Bacteria, Archaea, and Eukarya

What is the size of viruses?

Ultramicroscopic size, ranging from 20 nanometer up to 450 nm (diameter). Most <0.2um; require electron microscope

How are viruses in nature?

Not cellular in nature; structure is very compact and economical

How do viruses multiply?

By taking control of the host cell’s genetic material and regulating the synthesis and assembly of new viruses

What do viruses lack?

lack enzymes for most metabolic processes and lack machinery for synthesizing proteins

How are viruses active in host cells?

inactive macromolecules outside the host cell and active only inside host cells

what is the basic structure of Viruses?

consists of protein shell (capsid) surrounding nucleic acid core

what can be the nucleic acid in viruses?

is either DNA or RNA but not both. Nucleic acid can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA

what do the molecules on the virus surface do?

impart high specificity for attachment to host cell

Largest viruses

average 500 to 1,00 nanometer (20 to 50 times larger than an average virus)

what are some large viruses?

Megaviruses and Pandaviruses

what do viruses contain?

contain only the parts needed to invade and control a host cell

what is the covering of a virus particle?

• capsid

• envelope (not found in all viruses)

what is found in the central core of a virus particle?

• Nucleic acid molecule(s) (DNA or RNA)

• matrix proteins Enzymes (not found in all viruses)

what do all viruses have?

capsids (protein coats that enclose and protect their nucleic acid)

what is nucleocapsid?

capsid together with the nucleic acid

what external structures do some viruses have?

an envelope; those lacking an envelope are naked

what are capsid made up of?

identical protein subunits called capsomers

what are spikes?

they are protruding protein molecules, essential for the attachment of viruses to the next host cell. found in some viruses.

Virus envelope

• mostly in animal viruses

• acquired when virus leaves the host cell

• has spikes

what do spikes help with?

helps the virus bind to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell

what are the two structural capsid types?

Helical and icosahedral

Helical capsid

continuous helix of capsomers forming a cylindrical nucleocapsid

Icosahedral capsid

three-dimensional, symmetrical polygon, with 20 sides and 12 every spaced corner

Nucleocapsid Assembly

• Rod-shaped capsomers assemble into hollow discs

• The nucleic acid is inserted into the center of the disc

• Elongation of the nucleocapsid progresses from both ends, as the nucleic acid is coiled inside

Arrangements of Icosahedral Capsids

• arrangement of capsomers vary among icosahedral viruses

• they can also vary in number of capsomers

what happens during the assembly of Icosahedral viruses?

the nucleic acid is packed into the center of the icosahedron, forming the nucleocapsid

what do Icosahedral viruses lack?

an outer envelope

Atypical viruses

Poxviruses and bacteriophages

Poxviruses

lack a typical capsid and are covered by a dense layer of lipoproteins

Bacteriophages

some have a polyhedral nucleocapsid along with a helical tail and attachment fibers

Nucleic Acids

• Viral genome- either DNA or RNA but never both

• Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses

• Number of genes varies for each type of virus- few to hundreds: Hiv- 9 genes, pandoraviruses, 2,550

DNA Viruses

Usually double stranded (ds) but may be single stranded (ss), circular or singular

RNA viruses

• Usually single stranded, may be double stranded, may be segmented into separate RNA pieces

• ssRNA genomes ready for immediate transformation are positive sense RNA

• ssRNA genomes that must be converted into proper form are negative sense RNA

what pre-formed enzymes are required for viral replication?

• Polymerases

• Replicases

• Reverse transcriptase

Polymerases

enzymes that synthesize DNA or RNA

Replicases

enzymes that copy RNA

Reverse transcriptase

synthesis of DNA from RNA (HIV virus)

what genes do viruses lack?

viruses lack the genes for synthesis of metabolic enzymes, but they are able to take over the cell’s metabolic resources

How are viruses classified?

main criteria presently used are structure, chemical composition, and genetic makeup

Taxonomy of Viruses list

7 orders, 104 families, and 505 genera of viruses

Nomenclature of viruses

• Families italicized and given the suffix -viridae

• Genera likewise italicized and end in -virus

Viral species

Distinct virus types that share a collection of properties such as host range, pathogenicity, and genetic makeup

Characteristics for placement in a virus family include

• type of capsid, nucleic acid strand number, presence and type of envelope, overall viral size, and area of the host cell in which the virus multiplies

• Microscopic appearance (that is rhabdoviruses),

• Anatomical or geographic areas (that is adenoviruses and hantaviruses)

• Effects on the host (that is lentiviruses)

• Acronyms blending several characteristics (that is picornaviruses)

General phases in animal virus multiplication cycle

1. adsorption

2. penetration

3. uncoating

4. synthesis

5. assembly

6. release

Adsorption in animal virus multiplication cycle

binding to specific molecules (receptors) on the host cell with the spikes

Penetration in animal virus multiplication cycle

genome enters the host cell

Uncoating in animal virus multiplication cycle

the viral nucleic acid is released from the capsid

synthesis in animal virus multiplication cycle

viral components are produced, new spikes, capsomers, nucleic acid to form new viruses

Assembly in animal virus multiplication cycle

new viral particles are constructed

release in animal virus multiplication cycle

assembled viruses are released by budding (exocytosis) or cell lysis

Host range

the variety of species that a virus can infect

Tropisms

tissues or cells that a virus can infect

• hepatitis b

• poliovirus

• rabies

what cells can Hepatitis B infect?

human liver cells

what cells can Poliovirus infect?

primate intestinal and nerve cells

what cells can Rabies infect?

various cells of many mammals

what do animal viruses must during penetration?

must penetrate the cell membrane of the host cell and deliver the viral nucleic acid into its interior

how do most viruses enter the cell?

Fusion and Endocytosis

Fusion (envelope viruses)

viral envelope fuses directly with host membrane by rearrangement of lipids

Endocytosis (enveloped or naked)

entire virus is engulfed and enclosed in a vacuole or vesicle. Once inside, the virus is uncoated and viral nucleocapsid or nucleic acid is released

synthesis for animal viruses

varies depending on whether the virus is a DNA or RNA virus

how are DNA viruses replicated?

generally are replicated and assembled in the nucleus

how are RNA viruses replicated?

generally are replicated and assembled in the cytoplasm

what do positive-sense RNA contain?

the message for translation

what happens to negative-sense RNA?

they are converted into positive-sense message

what are mature virus particles constructed from?

the growing pool of parts

what is first laid down during assembly?

the capsid is first laid down as an empty shell that will serve as a receptacle for the nucleic acid strand

what do electron micrograph show that were taken during assembly?

cells with masses of viruses, often enclosed in packets

how do assembled viruses leave the host cell?

by budding or exocytosis, or by cell lysis or rupturing

Budding or exocytosis (enveloped)

nucleocapsid binds to membrane which pinches off and sheds the viruses gradually: cell is not immediately destroyed

Cell lysis or rupturing (nonenveloped and complex)

viruses released when cell dies or ruptures

Virion

a fully formed, extracellular virus particle that is virulent and able to establish infection in a host (the number of virions released by infected cells is variable)

Cytopathic effects

cell damage altering microscopic appearance

• disorientation of individual cells

• gross changes in shape or size

• intracellular changes (inclusion bodies, syncytium)

Persistent infections

cells harbors the virus and is not immediately lysed. can last weeks or host’s lifetime; several can periodically activate- chronic latent state

• measles virus

• Herpes simplex virus

• Herpes zoster virus

Measles virus

may remain hidden in brain cells for many years

Herpes simplex virus

cold sores and genital herpes

Herpes zoster virus

chickenpox and shingles

viral damage

some animal viruses enter the host cell and permanently alter its genetic material resulting in cancer. these viruses are termed oncogenic, and their effect in the cell is called transformation

Transformed cells

an increased rate of growth, altercations in chromosomes, and the capacity to divide foe indefinite time periods resulting in tumors

Oncoviruses

mammalian viruses capable of initiating tumors

• Papillomavirus

• Epstein-Barr virus

Papillomavirus

cervical cancer

Epstein-Barr virus

Burkitt’s lymphoma

Bacteriophages

bacterial viruses (phages). most widely studies are those that infect E.coli- complex structure, DNA

multiplication of bacteriophages

• multiplication goes through similar stages as animal viruses

• only the nucleic acid enters the cytoplasm-uncoating is not necessary

• release is a result of cell lysis induced by viral enzymes and accumulation of viruses- lytic cycle

steps in Phage replication

• adsorption

• penetration

• replication

• assembly

• maturation

• Lysis and release

absorption in phage replication

binding of the virus to specific molecules on host cell

Penetration in phage replication

genome enters host cell

Replication in phage replication

viral components are produced

assembly in phage replication

viral components are assembled

maturation in phage replication

completion of viral formation

Lysis and release in phage replication

The lytic cycle involves full completion of viral infection through lysis and release of virions.

what does bacterial phage occasionally enter during lysis and release?

a reversible state of lysogeny and its DNA is incorporated into the host’s genetic material

Primary goals of viral cultivation in animal viruses

1. Isolate and identify viruses in clinical specimens

2. prepare viruses for vaccines

3. allow detailed research on viral structure, multiplication cycles, genetics, and effects on the host cells