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VIRUSES Chapter 19
What is a virus?
• A virus is a submicroscopic infectious particle composed of a protein coat (capsid) and a nucleic acid core (either DNA or RNA).
• Viruses are similar in size to a large protein macromolecule, generally smaller than 200 nm in diameter.
Discovery of Viruses
• Search for cause of tobacco mosaic disease led to viruses
• The elusive virus was crystallized in 1935 by Stanley
Fig. 19-2
RESULTS
1 2 3Extracted sapfrom tobaccoplant withtobaccomosaic disease
Passed sapthrough aporcelain filter knownto trapbacteria
Rubbed filteredsap on healthytobacco plants
4 Healthy plantsbecame infected
Viral Capsids• Capsids are built from protein
subunits called capsomeres• May be rod-shaped (helical viruses),
polyhedral (icosahedral viruses) or more complex
• Some viruses have membranous envelopes that help them infect hosts (flu virus
• Bacteriophages, also called phages, are viruses that infect bacteria
Fig. 19-3
RNA
Capsomere
Capsomereof capsid
DNA
Glycoprotein
18 250 nm 70–90 nm (diameter)
Glycoproteins
80–200 nm (diameter) 80 225 nm
Membranousenvelope RNA
Capsid
HeadDNA
Tailsheath
Tailfiber
50 nm50 nm50 nm20 nm
(a) Tobacco mosaic virus
(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
• Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell
• Each virus has a host range, a limited number of host cells that it can infect
Viral Reproduction
• Once a viral genome has entered a cell, the cell begins to manufacture viral proteins using the host cell’s materials (enzymes, ribosomes, tRNAs, amino acids, ATP, etc.)
Transcriptionand manufactureof capsid proteins
Self-assembly of new virus particles and their exit from the cell
Entry anduncoating
Fig. 19-4VIRUS1
2
3
DNA
Capsid
4
Replication
HOST CELL
Viral DNA
mRNA
Capsidproteins
Viral DNA
• Phages are the best understood of all viruses
• Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
The Lytic Cycle• The lytic cycle culminates in the death of
the host cell by producing new phages and digests the host’s cell wall, releasing the progeny viruses
• A phage that reproduces only by the lytic cycle is called a virulent phage
• Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA
Animation: Phage T4 Lytic CycleAnimation: Phage T4 Lytic Cycle
Fig. 19-5-5
Phage assembly
Head Tail Tail fibers
Assembly
Release
Synthesis of viralgenomes andproteins
Entry of phageDNA anddegradation ofhost DNA
Attachment1
2
4
5
3
The Lysogenic Cycle• The lysogenic cycle replicates the phage
genome without destroying the host• The viral DNA molecule is incorporated into
the host cell’s chromosome and is called a prophage.
• Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
• Viruses that can be lysogenic or lytic are called temperate phages.
Animation: Phage Lambda Lysogenic and Lytic CyclesAnimation: Phage Lambda Lysogenic and Lytic Cycles
Fig. 19-6
PhageDNA
Phage
The phage injects its DNA.
Bacterialchromosome
Phage DNAcircularizes.
Daughter cellwith prophage
Occasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.
Cell divisionsproducepopulation ofbacteria infectedwith the prophage.
The cell lyses, releasing phages.
Lytic cycle
Lytic cycleis induced or Lysogenic cycle
is entered
Lysogenic cycle
Prophage
The bacterium reproduces,copying the prophage andtransmitting it to daughter cells.
Phage DNA integrates intothe bacterial chromosome,becoming a prophage.
New phage DNA and proteinsare synthesized andassembled into phages.
Animal Viruses
• Classified as DNA or RNA viruses, single or double-stranded
• Many have envelopes with glycoproteins that are specific for receptors.
• The glycoproteins are made by the ER and added to the host cell’s membrane which envelopes the emerging viruses.
Fig. 19-7
Capsid
RNA
Envelope (withglycoproteins)
Capsid and viral genomeenter the cell
HOST CELL
Viral genome (RNA)
Template
mRNA
ER
Glyco-proteins
Capsidproteins Copy of
genome (RNA)
New virus
Table 19-1a
Table 19-1b
RNA Viruses
• The broadest variety of RNA genomes is found in viruses that infect animals
• Retroviruses use reverse transcriptase to copy their RNA genome into DNA (HIV is ex.)
• The viral DNA that is integrated into the host genome is called a provirus
• Unlike a prophage, a provirus remains a permanent resident of the host cell
Fig. 19-8aGlycoprotein
Reversetranscriptase HIV
RNA (twoidenticalstrands)
Capsid
Viral envelope
HOST CELL
Reversetranscriptase
Viral RNA
RNA-DNAhybrid
DNA
NUCLEUS
Provirus
ChromosomalDNA
RNA genomefor thenext viralgeneration
mRNA
New virus
Fig. 19-8b
HIVMembrane ofwhite blood cell
HIV entering a cell
0.25 µm
New HIV leaving a cell
Evolution of Viruses
• Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid
• Candidates for the source of viral genomes are plasmids and transposons, small mobile DNA segments
• Mimivirus, a double-stranded DNA virus, is the largest virus yet discovered
mimivirus
Which came first,the cell or the mimivirus?
The virus was first isolated in 1992 from amoebae growing in a water tower.
Viral diseases in animals
• Symptoms caused by - toxins - body’s defense mechanisms• Vaccines – weakened or derivatives of
viral particles capable of causing an immune response
• Antibiotics not effective• Some antiviral medications interfere
with viral nucleic acid synthesis
Where do new viruses come from?
• Mutations of existing viruses
• The dissemination of an existing virus to a more widespread population
• Or spread between species
• Epidemic – general outbreak of a disease
• Pandemic – global epidemic
Fig. 19-9
(a) The 1918 flu pandemic
(b) Influenza A H5N1 virus
(c) Vaccinating ducks
0.5 µm
Plant viruses
• More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots
• Most plant viruses have an RNA genome
• Plant viral disease can spread by vertical transmission from parent plant or by horizontal transmission from an external source.
Fig. 19-10
Viroids and Prions: The Simplest Infectious Agents
• Viroids are circular RNA molecules that infect plants and disrupt their growth
• Prions are slow-acting, virtually indestructible infectious misfolded proteins that cause brain diseases in mammals
• Prions propagate by converting normal proteins into the prion version
• Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions
Viroids in Plants
Fig. 19-11
Prion
Normalprotein
Originalprion
Newprion
Aggregatesof prions
Misfolding of proteins to form prions
Scrapie in sheep