Chapter 28.1: Microbiology-VIRUS!
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Viruses
• Viruses are noncellular, nonliving particles
and therefore are not included in the
classification scheme.
• Comparable in size to a large protein
macromolecule, many viruses can be
purified, crystallized, and stored as
chemicals.
Structure of Viruses
• A virus has an outer capsid composed of
protein subunits, and an inner core of
nucleic acid.
• An outer membranous envelope may be
acquired when the virus buds from the cell.
• It may also include enzymes for nucleic acid
replication.
• Viruses are classified by type of nucleic
acid, viral shape and size, and by presence
of an outer envelope.
Adenovirus
• Parasitic Nature
• Viruses are obligate intracellular parasites.
• Viruses are very specific for the type of
cells they infect (e.g., HIV only infects
certain kinds of blood cells).
• Viruses are likely derived from the very
host they infect, and therefore evolved
after cells evolved.
• Some viruses, such as the flu virus, can
mutate rapidly.
Replication of Viruses
• Viruses are specific to a particular host cell
because they bind to a particular plasma
membrane receptor.
• After viral nucleic acid enters the host cell,
it takes over the metabolic machinery of
the host cell so that more viruses are
produced.
•
• Replication of Bacteriophages
• Bacteriophages are viruses that parasitize
bacteria.
• Some undergo two cycles, a lytic cycle and
a lysogenic cycle.
• The lytic cycle is divided into five phases:
attachment, penetration, biosynthesis,
maturation, and release.
• In the lysogenic cycle, the infected
bacterium does not immediately produce
viruses but may do so sometime in the
future; the phage has a latent period and is
called a prophage during this time.
Lytic and lysogenic cycles
• Replication of Animal Viruses
• Entire animal virus penetrates host cell by
endocytosis.
• Once inside, the virus is uncoated to
remove the envelope and capsid.
• The viral genome, either DNA or RNA, is
now free and biosynthesis proceeds.
• The assembled viruses bud from the cell
and acquire envelopes.
•
• Retroviruses are RNA animal viruses that
have a DNA stage.
• They have an enzyme called reverse
transcriptase that carries out RNA →
cDNA transcription.
• Following replication, cDNA integrates into
the host genome until viral reproduction
occurs.
Reproduction of HIV, a retrovirus
• Viral Infections
• Viruses cause infectious diseases of
plants and animals, including humans.
• Some crop diseases are attributed not to
viruses but to naked strands of RNA called
viriods.
• Some diseases in humans and animals
are attributed to prions which are protein
particles.
• Mad cow disease (BSE) in Britain is
believed to be a prion disease.
Lab Ex 19: Platyhelminthes
(flatworms)
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Flatworms
• Flatworms are characterized by the tissue
level of organization and a sac body plan.
• These acoelomates (NO coelom) have
three germ layers, and have all organs
except respiratory and circulatory organs.
• The flat body facilitates diffusion of oxygen
and other molecules from cell to cell.
• Planarians
• Planarians are freshwater, free living,
flatworms.
• Flame cells function in excretion.
• The small brain extends to a ladder
arrangement of nerves.
• Light-sensitive organs (eyespots) are in
the head; planarians exhibit cephalization.
• One organism has both male and female
sex organs – they are hermaphroditic.
Planarian
• Parasitic Flatworms
• Flukes and tapeworms are two classes of
parasitic flatworms; both have intermediate
hosts.
• Flukes are oval to elongate and have suckers at
the anterior end.
• Blood flukes cause schistosomiasis; other
flukes infect the digestive tract, bile duct, and
lungs.
• A tapeworm has an anterior scolex with hooks
and suckers to hold itself inside the gut.
Schistosomiasis
Lab Ex 20: Aschelminthes
(roundworms)
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Roundworms
• Roundworms have the tube-within-a-tube
plan; they are prevalent in soil and some
parasitize animals and plants.
• The pseudocoelom is a body cavity
incompletely lined with mesoderm.
• The fluid-filled interior forms a hydrostatic
skeleton.
• Most species of roundworms have separate
males and females.
Coelom structure and function
• Ascaris
• Ascaris larvae are swallowed and burrow
through the intestinal wall and make their way
through various organs until they reach the
lungs.
• In the lungs, they grow in size for 10 days, then
move up to the throat, and are then swallowed.
• After they mature in the intestine, females
produce eggs that pass out with feces.
Roundworm anatomy
• Other Roundworms
• Trichinosis is a roundworm infection from
eating undercooked pork containing encysted
Trichinella larvae.
• The filarial worm is carried by mosquitoes and
causes elephantiasis by blocking lymphatic
drainage.
• Pinworms are common infections in children.
• Hookworm is a more serious infection seen in
the southern United States.