3.31 Flashcards
Viruses (4)
• simple organization • DNA or RNA genome • unable to reproduce outside of living cells • obligate intracellular parasites
Cellular Organisms (4)
• complex organization • DNA genome and RNAs • carry out cell division • some are obligate intracellular parasites
FIGURE 1 • Theories of viral
origin. Three theories have been
proposed to account for the origin of
viruses.
A. According to the theory of reductive origin, viruses evolved by degenerative evolution from intracellular parasitic cells. B. According to the theory of intracellular origin, viruses evolved from functional parts of cells that acquired an ability to reproduce themselves uncontrolled by the cell. C. The theory of independent origin holds that viroid nucleic acids could have evolved outside of cells during the "RNA world" and acquired the ability to infect cells.
— virus is bigger by far than ant other known virus, and rivals bacteria
pandora
~1000 nm
Multicomponent viruses genomes are segmented and the
segments are distributed into
separate viral particles
Cultivation of viruses requires inoculation of
living host cell (4)
- suitable animals
- embryonated eggs
- tissue (cell) cultures
- cytopathic effects (CPEs)
tissue (cell) cultures (3)
– monolayers of animal cells
– plaques
• localized area of cellular destruction and lysis
cytopathic effects (CPEs) (1)
– microscopic or macroscopic degenerative changes or
abnormalities in host cells and tissues
HERPES: — CAPSID SYMMETRY
ICOSAHEDRAL
bacteriophage — capsule
T4
Viruses are classified using a
combination of properties: (4)
• Type and structure of nucleic acid used for viral genome • Presence or absence of envelope • Type of capsid symmetry • Replication strategy
Virus Replication (5)
–adsorption –penetration and uncoating –synthesis of viral proteins and nucleic acids –assembly of virions –release
Adsorption of Virions
viral surface proteins and/or enzymes
mediate attachment to specific host
receptors
Penetration and Uncoating
• three mechanisms
– injection of nucleic acid
– fusion of envelope with host membrane
– endocytosis
Assembly of Virus Capsids (4)
- capsid proteins
- assembly of naked viruses
- assembly of enveloped viruses
- site of morphogenesis varies
Assembly of Virus Capsids
• capsid proteins
– encoded by late genes
Assembly of Virus Capsids
• assembly of naked viruses
– empty procapsids formed then nucleic acid
inserted
Assembly of Virus Capsids
• assembly of enveloped viruses
– in most cases, similar to assembly of naked
viruses
Virion Release
• naked viruses
– usually by lysis of host cell
Virion Release
• enveloped viruses
– formation of envelope and release usually
occur concurrently
• virus-encoded proteins incorporated into host
membrane
• nucleocapsid buds outward and is surrounded
by modified host membrane
FIGURE 6.23 • Bacteriophage reproduction: lysis and lysogeny.
B. Lysis
occurs when the phage genome reproduces progeny phage particles, as many as possible,
and then lyses the cell to release them. In phage lambda, lysogeny can occur when the
phage genome integrates itself into that of the host. The phage genome is replicated along
Nith that of the host cell. The phage DNA, however, can direct its own excision by
expressing a site-specific DNA recombinase. This excised phage chromosome then initiates
a lytic cycle.
prophage (host infected): phage C1
(Clostridium botulinum)
bacterial product from prophage gene:
Botulnum toxin
c1
progphage (host infected): Beta phage (Corynebacterium
diphtheriae)
bacterial product from prophage gene:
Diphtheria toxin
tox
progphage (host infected):
Epsilon 34
(Salmonella enterica)
bacterial product from prophage gene:
LPS synthesis
rfb
HERV-W
syncytin-1 (retroviral env protein, placental fusion)
HERV-E
apolipoprotein c1 (liver fx)
FIGURE 6.24 - Bacteriophage
replication cycle:
slow release. In the slow-release replication cycle, a filamentous phage produces phage particles without lying the cell. The hos continues to reproduce itself, but more slowly than uninfected cells do because many of its resources are being used to make phages.
Bacterial Defenses (against bacteriophages) (3)
Genetic resistance - mutations in bacterial genes (e.g. receptor)
Restriction endonucleases - enzymes that cut invading DNA
CRISPR - a bacterial immune system
FIGURE 6.25
CRISPR defense of bacterial cell.
A piece of phage DNA
gets copied as a “spacer” into the host genome. If the bacterium survives
infection, later reinfection by the same kind of phage causes
transcription of the spacers into CRISP RNA. A processed spacer (crRNA)
joins the Cas complex to recognize and cleave the phage DNA.
FIGURE 6.26 - The gut bacteriophage community.
Bacteriophages enter the intestine, where they infect
intestinal bacteria. Most intestinal bacteria carry prophages. Phages also modulate the immune system
Phages may limit the bacterial numbers to levels that the human immune system can
tolerate. Lysogenized bacteria may use quorum sensing to detect host cell populations and
“decide” whether to start a lytic cycle.
Phage particles may modulate immune system activity by suppressing T-cell activation and
tumor formation
Phages may attack biofilms. Biofilms of pathogens such as Pseudomonas aeruginosa may be
eroded by phage infection.
Phage therapy being explored as a potential
antibiotic treatment
