Download The Viruses General Characteristics: ¨ virus means poison

- The Viruses
:General Characteristics
virus means poison; someone once called them "a piece of bad news wrapped
";in a protein
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)obligate intracellular parasites (can reproduce/replicate only inside a host cell
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)not cells; debate over whether or not they are considered “alive” (see below
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consist of nucleic acids (DNA or RNA) in a protein coat, called a capsid (no cell
)membrane
they insert themselves into a host cell & direct the host cell's metabolic
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machinery to make more virus; the virus supplies information (the plan) in the form
of its nucleic acid - raw materials and driving force (ATP & reducing power) are
.supplied by the host cell
all cellular organisms can be attacked by viruses; however, viruses are very
.specific for the organisms & cells they infect
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?Are Viruses Alive
Characteristics of living things: reproduction, metabolism, organized as cells,
contain all organic molecules (lipids, enzymes, nucleic acids, carbs), evolution &
.adaptation to changing environments
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Viruses have some of these char's: they can evolve, they contain some
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macromolecules, they direct their own reproduction; However, they are not cells they do not have cytoplasm, a cell membrane, organelles, ribosomes, or a
nucleus. They have DNA or RNA, unlike prokaryotic and eukaryotic cells, which have
both. In addition, they lack a metabolism of their own (they cannot produce ATP,
etc.) - raw materials and driving force (ATP & reducing power) are supplied by the
.host cell
)I. HOW ARE THEY CLASSIFIED? (4 ways: size, structure, host range, life cycles
.A. Size - range from about 1/10th to 1/3rd the size of a small bacterial cell
B. Structure - basic structure of a virus is a nucleic acid surrounded by a protein
capsid; a membrane envelope may also be present outside of the capsid, but this is
acquired from host cell. A complete viral particle (= capsid + nucleic acid + envelope
.if it is present) is called a virion
Nucleic Acid - Viruses can store their genetic info. in different types of nucleic .1
acid (each virus has only type). Viruses can have DNA or RNA. Their nucleic acid can
be double stranded (ds) or single stranded (ss); they can even have double stranded
.RNA! RNA viruses can have a (-) sense strand or a (+) sense strand of RNA
sense RNA acts like mRNA and can be translated into proteins by the host cell’s )+(
ribosomes. (-) sense RNA does not make sense to the host cell’s ribosomes. After
the virus containing this type of RNA enters the host cell, a complementary (+) sense
strand is made from its (-) sense strand. Only (+) sense strand RNA can be read by
!the host cell’s ribosomes
Capsids - protein coat that surrounds the nucleic acid; the constituent protein .2
molecules making up the capsids are called capsomeres; there are 3 basic shapes
!!based on how the capsomeres are arranged. See diagrams of these shapes
.a.
helical - proteins fit together as a spiral to form a rod-shaped structure
b. polyhedral - proteins are arranged in equilateral triangles that fit together to
form a geodesic dome-shaped structure; some appear almost spherical; you may
.have seen architectural structures that have this shape
c. complex - combination viruses with a helical portion (tail) attached to a
polyhedral portion (head); ex. many bacteriophages; may also have a tail sheath
(participates in injecting the viral nucleic acid into the host cell), plate, pins, & tail
.)fibers (help virus attach to host cell
Viral Envelopes - pieces of the host cell's cell membrane that the virus acquires .3
as it emerges from its host cell; the virus pushes out of the cell membrane, forming a
bud that encloses the virus - then the bud pinches off behind, resealing the cell - as a
result the host cell is not lysed. Glycoprotein spikes from the host cell’s glycocalyx
may stick out of the envelope. Viruses that lack envelopes are called naked
viruses. Because envelopes are acquired from host’s cell membranes, viruses may
be hidden from attach by the host’s immune system. Envelopes also help viruses
infect new cells by fusion of the envelope with the host’s cell membrane. On the
other hand, enveloped viruses are damaged easily by physical and chemical
.antimicrobial agents
C. Host Range - defined as the spectrum of organisms a virus attacks; viruses
exhibit considerable specificity for hosts and even cells within that host; viral
specificity is determined by whether or not a virus can attach to a cell. Attachment
depends on the presence specific receptor sites on the surface of host cell and on
specific attachment structures on the viral capsid or envelope. Examples of receptor
.sites are proteins, LPS’s, glycolipids, or glycoproteins
D. Life Cycles of Bacteriophages (viruses that infect bacteria – means “bacteria
)”eating
:Replication [= Lytic Cycle] See diagram
Events
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a.
Adsorption - the virion attaches itself to a specific receptor site on the
.surface of the host cell
b. Penetration - the viral nucleic acid penetrates the host cell
c. Uncoating - removing the capsid & envelope; basically 2 ways it
:can happen
during penetration, the virion disassembles so that only the viral nucleic
acid enters host cell
the entire virion enters the host cell & uncoating
occurs later
).2
d. Viral Synthesis (Latent Period) (also called biosynthesis) - more viral
components
.nucleic acids & proteins for capsids) are synthesized by the host cell(
d. Maturation (Assembly) - components are assembled into new viruses
f. Release (Burst Period) - hundreds of intact virions exit host cell;
:2 ways
).1
If the virus is of the naked type, an encoded protein, lysozyme, dissolves the cell ).1
membrane &/or cell wall of the host cell, causing the cell to lyse & releasing the
.hundreds of viruses inside it
If the virus is to be an enveloped virus, it pushes out the cell membrane, forming ).2
a bud that encloses the virus - then the bud pinches off behind, resealing the host
.cell; as a result the host cell is not lysed
Lysogenic Cycle (Lysogeny or Temperance) - Temperence involves the capacity .2
of certain viruses to set up long-term relationships with their host cells - the virus
remains latent for many cellular generations by becoming integrated into a host
cell's chromosome (the integrated viral DNA is called a prophage). In this case no
new viral components are synthesized & the host cell is not harmed. The virus may
remain latent for long periods of time before initiating a lytic cycle. The problem
with this type of cycle is that the viral nucleic acid that becomes integrated into the
host cell's chromosome gets replicated along with the host cell's chromosome and is
passed to daughter cells during cell division. In the prophage state, some viral genes
are expressed, which may slightly change the host cell's phenotype (ex. only
lysogenic strains of Corynebacterium diphtheriae cause the disease diphtheria
because the disease-causing toxin is encoded in the prophage of the infecting
virus). Something (ex. temperature change) may trigger prophages to go into the
lytic cycle. Released virions cannot infect cells that are carrying the same prophage .it makes the cell immune to attack by a virion of the same phage
II.
TAXONOMY
Family names all end in viridae ; family names are often converted into English
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(ex. Retroviridae are called retroviruses). Genus names end in virus - species names
.are English words
)Ex. Retroviridae, Lentivirus, Human Immunodeficiency Virus (HIV
Groupings reflect only common characteristics and are not intended to
.represent evolutionary relationships
III. ANIMAL VIRUSES
A. Cultivating Animal Viruses & Diagnosing Viral Illnesses
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At one time animal viruses had to be cultivated & counted by infecting
.animals
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In the 1930's it was discovered that embryonated chicken eggs could be used
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to culture animal viruses; embryonated eggs are inoculated with dilutions of a virus
sample to determine the highest dilution that kills the embryo; this procedure was
.more economical & efficient than using adult animals
In the 1950's cell culture & tissue culture methods were developed. This
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solved the problem of viral specificity. Ex. Before cell cultures it was impossible to
culture viruses in mice or chicken eggs that only infected humans (ex. HIV);
continuous cell lines are usually derived from cancerous tissue & grow indefinitely in
culture; regular cell lines grow increasingly slowly after 20-30 subcultures &
eventually lose their ability to support viral replication; the most famous c.c.l. is the
.)HeLa cell line (named after Helen Lack, the donor - from cervical cancer
Important Note: Physicians rely on symptoms to diagnose most viral
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illnesses. Culturing viruses takes too long & antibodies in the blood can usually be
.detected only after patient has recovered
Viral infections sometimes affect human cells in ways that can be seen under
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the microscope. For ex. the measles virus causes the membranes of neighboring
cells to fuse, creating giant, multinucleated cells. Some virus-infected cells can be id.
because they contain inclusion bodies, collections of viral components such as
capsids and nucleic acid, waiting to be assembled into new viral particles. For
example, the rabies virus produces inclusion bodies called negribodies in infected
nerve cells (this is what we look for in suspected cases of rabid animals - have to look
.)for negribodies in brain - animals have to be euthanized
B. Replication of Animal Viruses (Lytic cycle) - proceeds through similar stages as
.bacteriophage replication
Adsorption - Proteins in cell membrane act
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