CHAPTER 12 - Viruses, Viroids, and Prions

Viruses, Viroids, and Prions

contagium vivum fluidum

a contagious fluid.

virus

• are alive when they multiply in the host cells they infect

• Contain a single type of nucleic acid, either DNA or RNA.

• Contain a protein coat that surrounds the nucleic acid.

• Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells.

Viroids

RNA only

Virusoids

RNA with protein coat

Satellites

– nucleic acid only

prions

protein only

Wendell Stanley

an American chemist, isolated tobacco mosaic virus, making it possible for the first time to carry out chemical and structural studies on a purified virus

Dmitri Ivanowski

in 1892, he showed that tobacco mosaic disease (TMD) was transmitted via plant extracts

obligatory intracellular parasites

viruses are —, they absolutely require living host cells in order to multiply.

host range

is the spectrum of host cells the virus can infect.

host tropism

most viruses are able to infect specific types of cells of only one host species

bacteriophages, or phages.

Viruses that infect bacteria

virus’s requirements for its specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication

The particular host range of a virus is determined by the

the outer surface of the virus must chemically interact with specific receptor sites on the surface of the cell

For the virus to infect the host cell,

phage therapy

using bacteriophages to treat bacterial infections

20 to 1000 nm

size of viruses

virion

is a complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat outside a host cell.

nucleic acids and differences in the structures of their coats

Viruses are classified by their

true

t or f. viral genes are encoded by either DNA or RNA— but never both.

capsid

The nucleic acid of a virus is protected by a protein coat called

capsomeres

Each capsid is composed of protein subunits

envelope

covers viruses, which usually consists of some combination of lipids, proteins, and carbohydrates

spikes

which are carbohydrate-protein complexes that project from the surface of the envelope

hemagglutination

resulting clumping in influenza virus

nonenveloped viruses

Viruses whose capsids aren’t covered by an envelope

regions of the genes that code for these viruses’ surface proteins are susceptible to mutations

how can some viruses escape antibodies?

electron microscopy and a technique called X-ray crystallography.

The structure of these capsids has been revealed by

helical virus, polyhedral virus, enveloped virus, complex virus

different morphology of viruses

Helical Viruses

• resemble long rods that may be rigid or flexible.

• The viral nucleic acid is found within a hollow, cylindrical capsid that has a helical structure

• Rabies and ebola

Polyhedral Viruses

capsid is in the icosahedron, a regular polyhedron with 20 triangular faces and 12 corners

• ex. adenovirus

Enveloped Viruses

are roughly spherical

• ex. influenza virus

enveloped helical or enveloped polyhedral viruses

When helical or polyhedral viruses are enclosed by envelopes, they are called —

Complex Viruses

• viruses that have complicated structures

• bacteriophage

viral species

• is a group of viruses sharing the same genetic information and ecological niche (host range)

• are designated by descriptive common names, such as human immunodeficiency virus (HIV), with subspecies (if any) designated by a number (HIV-1).

• International Committee on Taxonomy of Viruses (ICTV)

classify viruses based on host range, virion morphology, and genome type

suspensions of bacteria in liquid media or in bacterial cultures on solid media

Bacteriophages can be grown either in

plaque method

A bacteriophage sample is mixed with host bacteria and melted agar. The agar containing the bacteriophages and host bacteria is then poured into a Petri plate containing a hardened layer of agar growth medium. The virus-bacteria mixture solidifies into a thin top layer that contains a layer of bacteria approximately one cell thick.

plaque

number of clearings, visible against a lawn of bacterial growth on the surface of the agar

plaque-forming units (PFU)

the concentrations of viral suspensions measured by the number of plaques are usually expressed in terms of

living animals, embryonated eggs, or cell cultures

three methods are commonly used for culturing animal viruses.

Animal inoculation

may be used as a diagnostic procedure for identifying and isolating a virus from a clinical specimen.

embryonated egg inoculation

A hole is drilled in the shell of the embryonated egg, and a viral suspension or suspected virus-containing tissue is injected into the egg’s fluid.

by embryo cell damage, or by the formation of typical pocks or lesions on the egg membranes

how is viral growth signaled in an embryonated eggs

cell cultures

have replaced embryonated eggs as the preferred type of growth medium for many viruses.

cell culture lines

are started by treating a slice of animal tissue with enzymes that separate the individual cells

western blotting and cytopathic effects

methods of viral identification

cytopathic effect (CPE)

cell deterioration, can be detected and counted in much the same way as plaques caused by bacteriophages on a lawn of bacteria and reported as PFU/ml.

primary or continuous cell lines

Viruses may be grown in

Primary cell lines

derived from tissue slices, tend to die out after only a few generations.

diploid cell lines

cell lines developed from human embryos can be maintained for about 100 generations and are widely used for culturing viruses that require a human host.

continuous cell lines

cell lines used when viruses are routinely grown in a laboratory

• These are transformed (cancerous) cells that can be maintained through an indefinite number of generations, and they’re sometimes called immortal cell lines

Western blotting

most commonly used means of viral identification.

• , the virus is detected and identified by its reaction with antibodies

restriction fragment length polymorphisms (RFLPs) and the polymerase chain reaction (PCR)

modern molecular methods of identifying viruses

Viral enzymes

are almost entirely concerned with replicating or processing viral nucleic acid.

host cell enzymes

are used for energy production, synthesizing viral proteins, synthesizing viral nucleic acids

it must invade a host cell and take over the host’s metabolic machinery

how do viruses multiply

one-step growth curve

demonstrates multiplication of viruses

• The data are obtained by infecting every cell in a culture and then testing the culture medium and cells for virions and viral proteins and nucleic acids.

the lytic cycle or the lysogenic cycle

Bacteriophages can multiply by two alternative mechanisms:

lytic cycle

• ends with the lysis and death of the host cell

lysogenic cycle

• host cell remains alive

Virulent bacteriophages

always cause the lytic cycle, which ends with the destruction of the bacterial cell

Temperate bacteriophages

lysogenic phages do not immediately initiate the lytic cycle, but rather, their DNA remains integrated into bacterial cell chromosome, generation after generation

T-even bacteriophages

example of the lytic cycle.

• The length of DNA contained in these bacteriophages is only about 6% of that contained in E. coli, yet the phage has enough DNA for over 100 genes.

attachment, penetration, biosynthesis, maturation, and release

multiplication cycle of these phages occurs in five stages

Attachment

• an attachment site on the virus attaches to a complementary receptor site on the bacterial cell.

Penetration

Phage penetrates host cell and injects its DNA

Biosynthesis

Phage DNA directs synthesis of viral components by the host cells

Maturation

Viral components are assembled into virions

Release

Host cell lyses, and new virions are released

1. The lysogenic cells are immune to reinfection by the same phage.

2. The host cell may exhibit new properties (phage conversion), e.g. produces toxin encoded by a prophage gene

3. It makes specialized transduction possible.

Three Important Results of Lysogeny

Specialized transduction

when a prophage is excised from its host chromosome, it can take with it a bit of the adjacent DNA from the bacterial chromosome

• a virus needs live host cells but must stop synthesis of host proteins, so the viral genes are translated

• early proteins translated from the viral genome may block transcription, existing mRNA, or inprogress translation

Multiplication of Animal Viruses

attachment, entry, uncoating, biosynthesis, maturation, and release

6 stages of Multiplication of Animal Viruses

• receptor-mediated endocytosis

• fusion of the viral envelope and cell membrane

2 mechanisms of the entry of animal viruses into host cells.

• Adenoviridae

• Poxviridae

• Herpesviridae

• Papovaviridae

• Hepadnaviridae

The Biosynthesis of DNA Viruses

• Piconaviridae

• Togaviridae

• Rhabdoviridae

• Reoviridae

The Biosynthesis of RNA Viruses

Retroviridae

The Biosynthesis of RNA Viruses that Use DNA

Parvoviridae

DNA, single-stranded

Herpesviridae Papovaviridae Poxviridae

DNA, double-stranded

Hepadnaviridae

DNA, reverse transcriptase

Picornaviridae Togaviridae

RNA, + strand

Rhabdoviridae

RNA, - strand

Reoviridae

RNA, double-stranded

Retroviridae

RNA, reverse transcriptase

Influenza virus

is one of the few RNA viruses that replicates in the nucleus of cells.

viral glycoproteins; engulfed; budding

In influenza virus infection, — attach the virus to a host epithelial cell. As a result, the virus is —. Viral RNA and viral proteins are made and assembled into new virions that are released by —.

oncogenes

can be activated to abnormal functioning by a variety of agents, including viruses

genetic material

— of oncogenic viruses integrates into the host cell’s DNA and replicates along with the host cell’s chromosome

transformation

tumor cells undergo

tumor-specific transplantation antigen (TSTA)

many tumor cells contain— on their cell surface

human papillomavirus (HPV)

virtually all cervical and anal cancers are caused by

Epstein-Barr virus (EBV)

was isolated from Burkitt’s lymphoma cells in 1964 by Michael Epstein & Yvonne Barr

hepatitis B virus (HBV)

caused liver cancer

human papillomavirus (HPV), Epstein-Barr virus (EBV), hepatitis B virus (HBV)

example of DNA Oncogenic Viruses

sarcoma viruses, mammary tumor viruses, feline leukemia virus (FeLV)

example of RNA Oncogenic Viruses

reverse transcriptase

the ability of retroviruses to induce tumors is related to their production of a —, the provirus becomes integrated into the host cell’s DNA

promoters

turn on oncogenes or other cancercausing factors

tumors

in the early 1900s,—were regressed in patients with concurrent viral infections

antitumor activity

in the 1920s, experimentally-induced viral infections in cancer patients suggested

oncolytic viruses

selectively infect and kill tumor cells or cause an immune response against tumor cells