Chapter 5: Viruses Flashcards
Seawater can contain ______ viruses per mL
100 million viruses per mL
Louis Pasteur
Postulated that a “living thing” smaller than bacteria was causing certain diseases; also proposed term “virus” (latin for poison)
Living characteristics of viruses (2)
- They reproduce at a fantastic rate, but only in living host cells;
- They can mutate
Non-living characteristics of viruses (2)
- They are acellular,
- that is, they contain no cytoplasm or cellular organelles;
- They carry out no metabolism on their own and must replicate using the host cell’s metabolic machinery.
Parvoviruses
smallest viruses; 20 nm in diameter
Mimiviruses
largest viruses; 450 nm in length
Virus particles contain __________ DNA or RNA
either; not both
Viral Components (3)
- Nucelic Acid
- Capsid
- Envelope
Capside
protein shell that surrounds the nucleic acid
Nucleocapsid
the capsid together with the nucleic acid; naked viruses consist only of this
Envelope
external covering of a nucleocapsid; usually a modified piece of the host’s cell membrane
Spikes
allow viruses to dock with host cell; can be found on BOTH naked and enveloped viruses
Virion
a fully formed virus able to establish an infection in the host cell
Capsomeres/Protomers
identical protein subunits that make up the capsid
Enveloped Virus
a virion with an envelope (most viruses that infect animals are enveloped)
Naked Virus
a virion without an envelope
How do envelopes form?
When viral glycoproteins and oligosaccharides associate with the cell membrane of the host cell
All envelopes have a _________
Phospholipid bilayer
Genome packaging plays an important role in ________
the infection
3 ways genomes are packaged:
- Directly in the capsid-inner side of the protein coat;
- Enclosed in special proteins-nucleic acid binding protein;
- Enclosed in proteins from the host cell
Variola Virus
small pox; double-stranded DNA
Herpes Simplex II
Herpes; double-stranded DNA
Parvovirus
Erythema Infectiosum; single-stranded DNA
Poliovirus
- Poliomyelitis;
- Single-stranded RNA;
- (+) polarity
Influenza Virus
- Influenza;
- Single-stranded RNA;
- (-) polarity
Rotavirus
- Gastroenteritis;
- Double-stranded RNA
HIV
- AIDS;
- Single-stranded RNA reverse transcriptase
Tobacco Mosaic Virus (describe shape)
hollow tubes with protein walls
Influenza Virus (describe shape)
an enveloped virus with a helical nucleocapsid
Icosahedral Viruses (describe shape)
derived from 20 triangular vases that make up the capsid; has 12 points of symmetry
Polymerases
synthesize DNA or RNA
Replicases
copy RNA
Reverse transcriptase
synthesizes DNA from RNA
Lytic Infection
the host cell fills with virions and explodes; result is cell death
Lysogenic (Latent) Infection
the viral genome becomes incorporated into the host cell’s DNA; can remain this way for an extended period; results in host cell living
6 steps in lytic cycle:
- adsorption
- penetration
- uncoating
- synthesis
- assembly
- release
A Pony Undergoes Some Awesome Racing
Length of replication cycle in Polioviruses
8 hours
Length of replication cycle in Herpesviruses
36 hours
Adsorption
A virus can invade its host cell only through making an exact fit with a specific host molecule/Receptor sites/Specific surface structures on host to which viruses attach. These can be proteins, lipopolysaccharides, teichoic acids, etc.
Penetration
The flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid
Penetration through endocytosis
entire virus is engulfed by the cell and enclosed in a vacuole or vesicle
Uncoating
(1) enzymes in the vacuole dissolve the envelope and capsid; (2) virus fuses with the wall of the vesicle; (3) viral nucleic acid is released into the cytoplasm
Direct Fusion
occurs with influenza and mumps viruses; (1) envelope merges directly with the cell membrane; (2) nucleocapsid is released to the cell’s interior
DNA Virus Synthesis:
replicated and assembled in the nucleus
RNA Virus Synthesis:
replicated and assembled in the cytoplasm
Retrovirus Synthesis:
Once inside the host cell cytoplasm the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern
Retrovirus
a single-stranded RNA virus that stores its nucleic acid in the form of an mRNA genome and targets a host cell as an obligate parasite
Assembly
puts together the new viruses using the “parts” manufactured in the synthesis process: new capsids and new nucleic acids
The number of viruses released by infected cells is controlled by these 2 factors
(1) Virus size; (2) Health of the host cell
Poxvirus releases ___ virions
3,000-4,000
Poliovirus releases ___ virions
100,000
Oncogenic Virus
Virus capable of initiating tumors; 20% of cancers are caused by these
Transformation
effect on the cell caused by oncogenic viruses; (1) increased rate of growth; (2) alterations in chromosomes; (3) changes in the cell’s surface molecules; (4) capacity to divide for an indefinite period
2 ways viruses cause cancer
- virus carries genes that directly cause cancer
- virus produces proteins that induce a loss of growth regulation in the cell
2 cancer-causing viruses
- papillomaviruses (cervical cancer)
- Epstein-Barr virus (lymphoma)
Bacteriophage
Virus that infects bacteria; often make the bacteria more pathogenic
Parts of Bacteriophage (7)
- icosahedral capsid head
- central tube
- collar
- base plate
- tail pins
- fibers
Prions
an infectious protein, specifically a protein in a misfolded form; a common feature of spongiform encephalopathies;
Creutzfeldt-Jacob disease
infects the human nervous system; causes gradual degeneration and death; prions
Bovine spongiform encephalopathy (BSE)
contracted when consuming contaminated beef; infected develop a variant of Creutzfeldt-Jacob disease
Viroids
Virus-like agents that parasitize plants; 1/10 the size of viruses; naked strands of RNA (lack capsids)
Interferon
naturally occurs in human cells to stop virus replication; is used with some success in treating and preventing viral infections
