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Chapter 13: Viruses Microbiology

STUDY
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Viruses
obligatory intracellular parasites and small;
contain a single type of nucleic acid (DNA or RNA)
Contain a protein coat that surrounds the nucleic acid
Multiply inside living cells by using synthesizing machinery of the cell
cause the synthesis of specialized structures that can transfer the viral nucleic acid to to other cells

have few or no enzymes of their own for metabolism, use machinery of the host to multiply
host range
the spectrum of host cells the virus can infect; some can only infect specific types of cells of cells of only one host species;
focused on viruses that infect humans or bacteria

determined by the virus' requirements for its specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication
must be able to attach to a receptor site (mostly part of the cell wall)
bacteriophages
viruses that infect bacteria
phage therapy
using bacteriophage to treat bacterial infections
viral size
determined with the aid of electron microscopy; different viruses have different sizes, much smaller than bacteria
Viron
complete, fully developed infectious viral particle composed of nucleic acid and surrounded by a protein coat that protects it from the environment and is a vehicle of transmission from one host cell to another (viruses classified by differences in their coats)
Virus' DNA or RNA
can have either, but not both; nucleic acid can be single or double stranded;nucleic acids can be circular or linear
capsid
protein coat that protects the nucleic acid of a virus

structure determined by viral nucleic acid and accounts for most of the mass
capsomers
protein units that compose the capsid
envelope
overs the capsid; consists of a combination of lipids, proteins, and carbs
spikes
may or may not cover envelopes (carbohydrate-protein) complexes that project from the surface of the envelope; some use spikes to attach the host cells; reliable characteristic that can be used as means of identification
nonenveloped viruses
viruses whose capsids are not covered by an envelope
Viruses interaction with antibodies
should kill the virus, but some viruses are able to escape because regions of the genes that code for htese viruse' surface proteins are susceptible to mutations (progeny) of mutant viruses have altered surface proteins, and will not react with them@
different morphology
helical (long rodds), polyhdral viruses, enveloped viruses=sperhical, complex virsues (bacterial viruses, bacteriophages)
Taxonomy of viruses
oldest classificaiton: disease caused; (hard because viruses can cause more than one disease), , so formed ICTV), based on: nucleic acid type, strategy for replication, morphology
virus is genus
viridae is family, and order names end in ales
viral species
group of viruses that share the same genetic information and ecological niche (host range); specific epithets aren't sued.
isolation of viruses
must be grown in a living cell=causes problems
plaques
clearing, visible against the lawn of bacterial growth ; each plaque corresponds to a single virus in the initial suspension; concentrations givein in PFU (plaque forming units)
growth of viruses in living animals
some animal viruses can be cultured only in living animals
inoculate the animal, if shows signs of disease, or kill it to use tissue to be examined; some human viruses cannot be grown in animals (lack of natural models for AIDS slowed the understanding of this disease, prevented experimentation with the disease)
growth of viruses in embryonated eggs
if the viruses grows in the egg, it is fairly convenient and inexpensive form of host for many animal viruses
viral growth is signaled by death of the embryo or cell damage
growth in cell cultures
have replaced embryonated eggs as the preferred type of growth medium; cell cultures consist of cells grown in culture media in the lab; viruses cause "Happy" cells with right osmotic pressure, nutrients, and growth factors) to deteroriate

can be grown in primary or continuous cell lines
cytopahtic effect
CPE, cell deterioration can be deteced adn counted in the same way plaques can
primary cell lines
derived from tissue slices, tend to die out after a few generations
diploid cell lines
from human embryoes, can be maintained for 100 generations and used for culturing viruses that need a human host
continious cell lines
are used for viruses rountinely grown in a lab; these are transformed (cancerous) cells that can be maintained through an indefinite number of generations, and they are sometimes called immortal cell lines
HELA cell
was isolated from a cancer of woman, from 1951 still used, have lost original characteristics, but have no interfered with use of the cells for viral propagation
cannot have microbial contamination
viral identification
not easy; viruses cannot be seen wtih electron microscope
serogolical methods liek western blotting is very common ; identified with use of antibodies
Viral enzymes for replicaiton
can only work within the host cell; viral enzymes are almost entirely concerned with replicating or processing viral nucleic acid; enzymes needed for protein synthesis, ribosomes, tRNA, and energy production are supplied by the host cells
one-step growth curve
multiplication of viruses; most infected cells die as a result of infection, so no new virions will be produced
lytic cycle
ends with lysis and death of the host cell
1. Attachment: (change collision between phage particle and bacteria-attachmetn or adsorption occurs)
2. penetration: T-even bacteriophage injects its DNA into the bacterium (bacteriophages tail reaelses phage lysozyme which breaks down a portion of cell wall)
3. biosynthesis: once the bacteriophage DNA has reached the cytoplasm of the cell, the biosynthesis of of viral nucleic acid and protein occurs; host protein synthesis is stopped by virus degradation of ht host DNA, and viral proteins repress translation
during eclipse period, seperate phage and virus can be found
4. maturation: bacteriophage DNA and capsids are assembled into complete virions
5. Release: the final stage of viral multiplication is the release of virions from the host cell; lysis
lysogneic
host remains alive; lambda; can use lytic cycle but can slo incorporate their DNA into the host cell's DNA to being lysogenic cycle
1. penetration occurs
2. phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage, lysogenic bacterium reproduces normally and completes many cell divisions
4. occassionally the prophage may excise from the bacterial chromosome by another recombination event, initiating the lytic cycle
results of lysogeny
1. immune to reinfection by the same phage
2. phage conversion (host cell may exhibit new properties)
3. makes specialized transduction possible
lysogeny
phage reamins latent (inactive)
phage conversion
the host cell may exhibit new properties (bacteria DNA with viral DNA in it)
specialized transduction
lysogenic phage can transfer bacterial genes from one cell to another through transduction; any genes can be transferred in generalized transducation and specific genes can be transferred in specialized transduction
multiplicaiton of animal viruses
differ from bacteriophages by: mechanism of entering host cell; mechanisms of maturation and release are different, and assembly of the new viral components are different
attachment
have attachment sites that attach to complementary receptor sites on the host cell's surface; receptor sites are GLYCOPROTEINS OF THE PLASMA MEMBRANE
dont' have tails like bacteriophages; distrubuted over the surface of the virus; sites themselves vary from one group of viruses to another

attachment is complete when many sites are bound
receptor sites are inhertied characteristics of the host
Entry
following attachment, entry occurs; enter by pinocytosis (active cellular process by which nutrient and ohte rmolecules are brought into a cell)=form a vesicle
or eveloped viruses enter by fusion (viral envelope fuses with the plasma membrane and releases the capsid into the cell's cytoplasm (HIV penetrates this way)
bacteriophages vs. animal viruses
attachment
entry
uncoating
biosynthesis
chornic infection
release
bacteriophages/animal viruses
attachment: tall fibers attach to cell wall proteins/attachment sites are plasma proteins and glycoproteins
entry: viral DNA injected into host cell/capsid enters by endocytosis or fusion
uncoating: not required/enzymatic removal of capsid proteins
Biosythesis: in cytoplasm/in nucleus (DNA viruses) or cytoplasm (RNA viruses)
chronic infection: lysogeny/latency, slow viral infection, cancer
relase: host cell lyses/enveloped viruses bud out/noneveloped viruses rupture plasma membrane
uncoating
viruses disappear during the eclipse period of an infection because they are taken apart inside the cell; uncoating is the seperating of viral nucleic acid form its protein coat once the virion is enclosed within the vesicle; capsid digested when the cell attempts to digest hte vesicle's contents, or the nonenveloped capsid may be released into the cytoplasm of the host cell
Biosynthesis of DNA viruses
transcirption of phasge DNA produces mRNa coding for ptoeins necessary for phage multiplication; phage DNA is replicated and capsid proteins are produced
use host cell enzymes to sythesize their capsid and other proteins in the cytoplasm
adenoviridae
cause acute respiratory disease (common cold)
proxviridae
all diseases include skin lesions (poxviruses, smallpox)
herpesviridae
100 are known, spreading apperance of cold sores; HHV, HHV-1
Papovavirdae
named for warts and tumors and cytoplasmic vacuoles produced by these viruses
Overview of replication of all animal viruses
Virion attaches to host cell, enters cell and its DNA is uncoated, portion of the viral DNA is transcribed (making mRNA), viral DNA is replicated and some viral proteins are made, late translation capsid proteins are synthesized, virions mature, virons released
hepadnaviridae
cause hepatitis and contain DNA; hep B, synthesize DNA by copying RNA using viral reverse transcriptase
where do RNA viruses replicate?
in the host cell's cytoplasm; differences lie in how mRNA and viral RNA are produced
Picornaviridae
polioviruse; single stranded RNA viruses; smallest, RNA within the virion called the sense strand becuase it can act as mRNA; viral RNA is translated into two principle proteins, which inhibit the host cell's synthesis of RNa dn protein and which form an eznyme called RNA-dependent RNA polymerase

produces antisense strand, which serves as a template to produce addition + or sense strands
Togaviridae
togavriuses, also have a single + (sense) strand, enveloped viruses, their name is from covering; after a - strand is made from + strand, two types of mRNA transcribed from - strand (short strand for envelope proteins) and a longer strand for capsid proteins)
Rhabdoviridae
rabiesvirus; bullter shaped; contain single - strand of RNa; alos have RNA-dependnet RNA polymerase that uses the - strand as a template to produce the + strand (+ strand serves as mRNa and as a template for synthesis of new viral RNA)
RReoviridae
named for their habits: the respiratory and enteric systems of humans

has double stranded RNA;
Retrovirdae
HIV, these viruses carry out reverse transcriptase which uses the viral RNA as a template to produce complementary double-stranded DNA; this enzyme also degrades the original viral RNA
AFter maturation what occurs?
release; budding is one example; nonenveloped viruses are released through ruptures in the host cell membrane q
virus and cancer
often go unrecognized because: most of the particles of viruses infect cells do not cause cancer; cancer might not develop until long after the viral infection; and cancer isn't contagious like viruses are

earliest relationship was with chicken leukemia ah chickn sarcome were transferred to healthy animals by viruses
Cancer cells
almost anything that alters the genetic material of eukaryotes has the potential to make normal cells cancerous; thse cancer-causing alterations to cellular DNA affects of of hte genes called oncogenes
oncogenes
can be activated to abnormal functioning by a variety of agents like chemicals, high energy radiation, and viruses,
oncogenic virus
viruses capable of inducing tumors
10% of cancers are virus-induced
tranformation
the acquire properties that are distinct form the properties of the uninfected cells or form infected cells that do no form tumors, after being transformed by viruses, many tumor cells contain a virus specific antigen called T antigen
latent viral infections
a virus can remain in equilibrium with the host adn not actually produce disease for a long period of time; oncogenic viruses are examples of these
herpes is an exmaple=immunosurpressed=herpes comes back;
ex. herpes, chickenpox, shingles
persistent or chronic viral infection
occurs gradually over a long period; persisttent viral infections are fatal;
measels can cause SSPE subaute scleorisng panceophaltiis
Prions
infectious protein
result from altered proteins, mutation in the normal gene for PrPc or contact with an altered protein
proteinaceous infectious particle
nine animal diseases fall into this: made cow disease
all nine are neurological diseases called spongiform encephalopathy that cause large vacuoles to develop in the brain
Plant viruses
enter the plant via wounds or invasive parasites, some also multiple in insect or vector cells
viroids are infectious pieces of RNA that cause some plant diseases