Ch.6 Intro to Viruses, Viroids, and Prions

__________ identified pathogens causing tuberculosis, cholera, and anthrax

Robert Koch

Fredrich Loeffler and Paul Frosch

• (viruses) transferrable from infected individuals

• passes through filter that can stop bacteria

• filterable agent cause of certain disease

what are viruses?

obligate intracellular parasites

how do viruses replicate?

only by usurping host cell’s machinery

what are bacterial viruses called?

phages

viruses are _______ in size

ultramicroscopic

what is the structure of a virus like?

not cellular in nature, structure is very compact and economical

viruses cannot _____ fulfill the characteristics of life

independently

viruses are _____ outside the host cell and ______ only inside host cells

inactive; active

what is the genome of a virus?

either DNA or RNA, cannot be both

what do the molecules on the virus surface do?

impart high specificity for attachment to host cell

how do viruses multiply?

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

viruses _____ enzymes for most metabolic processes

lack

viruses lack ______ for synthesizing proteins

machinery

what type of microscope can view viruses?

only electron microscopes

what is in the covering of a virus?

capsid and envelope

what is found in the central core of a virus?

nucleic acid molecules and matrix proteins/enzymes

capsids

protein coats that enclose and protect their nucleic acid, all viruses have them

nucleocapsid

the capsid together with the nucleic acid

capsomers

identical protein subunits that make up each capsid

helical capsid

continuous helix of capsomers forming a cylindrical nucleocapsid

icosahedral capsid

3-D, symmetrical polygon, with 20 sides and 12 evenly spaced corners

complex capsid

icosahedral head, joined to a helical sheath, joined to baseplate adorned with spikes and tail fibers

nucleocapsid assembly

• rod-shaped capsomers assemble into follow 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

in icosahedral capsids what happens during assembly?

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

when is the envelope acquired?

when the virus leaves the host cell

spikes

exposed proteins on the outside of the envelope that are essential for attachment of virus to the host cell

functions of the envelope

• protects the nucleic acid when the virus is outside the host cell

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

poxviruses

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

bacteriophages

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

single stranded genomes can be….

“+ sense” or “- sense”

what do the nucleic acids carry?

genes necessary to invade host cell and redirect cells’ activity to make new viruses

DNA virus

usually double stranded but may be single stranded

RNA virus

usually single stranded, may be double stranded, may be segmented into separate RNA pieces

positive-sense RNA

ssRNA genomes are ready for immediate translation

negative-sense RNA

ssRNA genomes that must be converted into proper form

special polymerases

synthesize DNA or RNA

pre-formed enzymes required for viral replication

1. special polymerases

2. replicases

3. reverse transcriptase

replicases

copy RNA

what happens if replicates aren’t present in the cell?

cell doesn't have to do RNA to RNA copying

reverse transcriptase

synthesis of DNA from RNA

what happens if there are no reverse transcriptase?

cells only go DNA to RNA, not the other way

main criteria to classify viruses

structures, chemical composition, and genetic makeup

families

italicized and end in -virdiae

genera

italicized and end in -virus

characteristics for placement in a virus family

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

• Microscopic appearance (ex. Rhabdoviruses)

• Anatomical or geographic areas (ex. Adenoviruses)

• Effects on the host (ex. Lentiviruses)

• Acronyms blending several characteristics (ex. picornaviruses)

phases of animal virus multiplication cycle

• attachment

• penetration

• uncoating

• biosynthesis

• assembly

• release

attachment

binding of virus to specific molecules on the host cell

penetration

genome enters the host cell

uncoating

the viral nucleic acid is released from the capsid

biosynthesis

viral components produced

assembly

new viral particles constructed

release

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

host range

viruses are restricted on the host cells they can effect

a virus collides with a host cell and attaches to _____ receptor sites on the membrane

specific

a potential host cell must have the _________ for the virus to attach to

correct surface protein

how do animal viruses deliver the viral nucleic acid into the interior of the host cell?

by penetrating the cell membrane

fusion

viral envelope fuses directly with host membrane by rearrangement of lipids

endocytosis

entire virus is engulfed and enclosed in a vacuole or vesicle (enveloped or naked)

how do DNA viruses replicate?

they are replicated and assembled in the host cell’s nucleus

how do RNA viruses replicate?

they are replicated and assembled in the host cell’s cytoplasm

___________ RNA contains the message for translation

positive-sense

__________ RNA must first synthesize positive-sense RNA

negative-sense

where are mature virus particles constructed from?

growing pool of parts

how do assembled viruses leave the host cell?

budding or cell lysis

budding or exocytosis (enveloped)

nucleocapsid binds to the membrane which pinches off and sheds the viruses gradually; cell isn’t immediately destroyed

cell lysis or rupturing (non enveloped and complex)

viruses released when cell dies and ruptures

cytopathic effects

cell damage altering microscopic appearance

• disorientation of individual cells

• gross changes in shape or size

• intracellular changes

types of viral infections

acute, persistent, and latent

acute infections

cell harbors virus just long enough to replicate it, it dies rapidly, and results in acute symptoms in the host

once an acute infection is cleared what happens to the host?

the host is immune to that strain of the virus

persistent infections

cell harbors the virus and is not immediately lysed; continually replicating have the virus for life

latent infections

can last weeks or the host’s lifetime; several can periodically reactivate

oncogenic viruses

enter the host cell and permanently alter its genetic material resulting in cancer

what is the effect of oncogenic viruses on the cell?

transformation

what happens to transformed cells?

they have an increased rate of growth, alterations in chromosomes, and the capacity to divide for indefinite periods of time resulting in tumors

oncoviruses

mammalian viruses capable of initiating tumors

bacteriophages

bacterial viruses

in a bacteriophage is uncoating necessary?

it is not; only the nucleic acid enters the cytoplasm

attachment in phage replication

binding of virus to make specific molecules on host cell

penetration in phage replication

genome enters host cell

replication in phage replication

viral components 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

temperate phages (DNA phages)

undergo absorption and penetration but don’t replicate

The viral genome inserts into bacterial genome and becomes an inactive prophage

the cell is not lysed

 lysogeny

Prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny

induction

can occur resulting in activation of lysogenic prophage followed by a viral replication and cell lysis

lysogenic conversion

phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology

primary goals of viral cultivation

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 host cells

“in vitro”

Cell (tissue) cultures: cultured cells support viral replication and permit observation of cytopathic effects

“in vivo”

bird embryos and live animal inoculation

bird embryos

intact and self-supporting unit, complete with its own sterile environment and nourishment, with embryonic tissues that support viral multiplication

live animal inoculation

the animal is exposed by injection of a viral preparation or specimen into the brain, blood, muscle, body cavity, skin, or footpads

medical importance of viruses

• Viruses are the most common cause of acute infections

• Several billion viral infections per year

• Some viruses have high mortality rates

• Possible connection of viruses to chronic afflictions of unknown cause

• Viruses are major participants in the earth's ecosystem

detection and treatment of animal viral infections

• More difficult than other agents

• Consider overall clinical picture

• Take appropriate sample

  • Infect cell culture: look for characteristics cytopathic effects

  • Screen for parts of the virus

  • Screen for immune response to virus (antibodies)

• Antiviral drugs can cause serious side effects

prions

misfolded proteins, contain no nucleic acid

satellite viruses

dependent on other viruses for replication

adeno-associated virus

replicates only in cells infected with adenovirus