Virus Structure and Method of Invasion understanding-viruses-video.htm
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Transcript of Virus Structure and Method of Invasion understanding-viruses-video.htm
• http://videos.howstuffworks.com/tlc/29852-understanding-viruses-video.htm
• Viruses called bacteriophages– Can infect and set in motion a genetic
takeover of bacteria, such as Escherichia coli
Figure 18.10.5 m
• Bacteriophages, also called phages– Have the most complex capsids
found among viruses
Figure 18.4d
80 225 nm
50 nm(d) Bacteriophage T4
DNA
Head
Tail fiber
Tail sheath
• Recall that bacteria are prokaryotes– With cells much smaller and more
simply organized than those of eukaryotes
• Viruses Are smaller and simpler still
Figure 18.20.25 m
Virus
Animalcell
Bacterium
Animal cell nucleus
• Obligate parasites --A virus has a genome but can reproduce only within a host cell
•Reproduction is their only true characteristic of being alive
• Specific to type of cells they target-poliomylelitis virus attacks nerve cells-hepatitis virus attacks liver cells
Viral Structure
• Not a cell!• Hijacks biochemical machinery of
host cell to carry out processes necessary to reproduce
• Obligate intracellular parasite
Viral Genomes
• Viral genomes may consist of– Double- or single-stranded DNA– Double- or single-stranded RNA
Figure 18.4a, b
18 250 mm 70–90 nm (diameter)
20 nm 50 nm(a) Tobacco mosaic virus (b) Adenoviruses
RNADNACapsomere
Glycoprotein
Capsomereof capsid
Capsids and Envelopes
• A capsid– Is the protein shell that encloses the viral genome
– Can have various structures
• Some viruses have envelopes– Which are membranous coverings
derived from the membrane of the host cell
Figure 18.4c
80–200 nm (diameter)
50 nm(c) Influenza viruses
RNA
Glycoprotein
Membranousenvelope
Capsid
Invasion of a cell by a virus
• Virus can lie dormant for many years until it comes into contact with a suitable host cell
• Binds with molecules on surface of host cell
• Herpes-whole virus enters cell
• Bacteriophage-viral DNA injected via hollow tail
Alteration of Cell Instruction
• Virus takes control of cell machinery• Depends on host for
-ATP-supply of free nucleotides
• Virus suppresses cell’s normal nucleic acid replication and protein synthesis
• Manufactures many identical copies of viral nucleic acid and protein coats
The Lytic Cycle
• The lytic cycle– Is a phage reproductive cycle that
culminates in the death of the host– Produces new phages and digests the
host’s cell wall, releasing the progeny viruses
• The lytic cycle of phage T4, a virulent phage
Phage assembly
Head Tails Tail fibersFigure 18.6
Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.
1Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed.
2
Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell.
3Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms.
4
Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles.
5
The Lysogenic Cycle
• The lysogenic cycle– Replicates the phage genome without
destroying the host
• Temperate phages– Are capable of using both the lytic and
lysogenic cycles of reproduction
• The lytic and lysogenic cycles of phage , a temperate phage
Many cell divisions produce a large population of bacteria infected with the prophage.
The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.
Phage DNA integrates into the bacterial chromosome,becoming a prophage.
New phage DNA and proteins are synthesized and assembled into phages.
Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.
Certain factorsdetermine whether
The phage attaches to ahost cell and injects its DNA.
Phage DNAcircularizes
The cell lyses, releasing phages.Lytic cycleis induced
Lysogenic cycleis entered
Lysogenic cycleLytic cycle
or Prophage
Bacterialchromosome
Phage
PhageDNA
Figure 18.7
Retrovirus
• Contains RNA• No DNA to transcribe into mRNA
• Reverse transcriptase injected into cell by virus with RNA-enzyme reverses normal transcription-Produces viral DNA from RNA-Virus uses DNA to replicate
Some video clips
• Bacteriophage entering a cell
• HIV replication
• http://www.youtube.com/watch?v=HhhRQ4t95OI
Emerging Viruses
• Emerging viruses– Are those that appear suddenly or suddenly
come to the attention of medical scientists
• Severe acute respiratory syndrome (SARS)– Recently appeared in China
Figure 18.11 A, B
(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.
(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.
• Outbreaks of “new” viral diseases in humans– Are usually caused by existing viruses that expand
their host territory
Viral Diseases in Plants• More than 2,000 types of viral diseases of
plants are known• Common symptoms of viral infection include
– Spots on leaves and fruits, stunted growth, and damaged flowers or roots
Figure 18.12
• Plant viruses spread disease in two major modes– Horizontal transmission, entering through
damaged cell walls – Vertical transmission, inheriting the virus from
a parent
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 proteins that cause brain diseases in mammals
– Propagate by converting normal proteins into the prion version
Figure 18.13
Prion
Normalprotein
Originalprion
Newprion
Many prions
• Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria
• Bacteria allow researchers– To investigate molecular genetics in the
simplest true organisms
The Bacterial Genome and Its Replication
• The bacterial chromosome– Is usually a circular DNA molecule with few
associated proteins
• In addition to the chromosome– Many bacteria have plasmids, smaller circular
DNA molecules that can replicate independently of the bacterial chromosome
Operons: The Basic Concept
• In bacteria, genes are often clustered into operons, composed of– An operator, an “on-off” switch– A promoter– Genes for metabolic enzymes
• Bacterial cells divide by binary fission– Which is preceded by replication of the bacterial
chromosomeReplicationfork
Origin of replication
Termination of replication
Figure 18.14
Mutation and Genetic Recombination as Sources of
Genetic Variation• Since bacteria can reproduce rapidly– New mutations can quickly increase a
population’s genetic diversity
• The trp operon: regulated synthesis of repressible enzymes
Figure 18.21a
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.
Genes of operon
Inactiverepressor
Protein
Operator
Polypeptides that make upenzymes for tryptophan synthesis
Promoter
Regulatorygene
RNA polymerase
Start codon Stop codon
Promoter
trp operon
5
3mRNA 5
trpDtrpE trpC trpB trpAtrpRDNA
mRNA
E D C B A
DNA
mRNA
Protein
Tryptophan(corepressor)
Active repressor
No RNA made
Tryptophan present, repressor active, operon off. As tryptophanaccumulates, it inhibits its own production by activating the repressor protein.
(b)
Figure 18.21b
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
• In a repressible operon– Binding of a specific repressor protein to the
operator shuts off transcription
• In an inducible operon– Binding of an inducer to an innately inactive
repressor inactivates the repressor and turns on transcription
• The lac operon: regulated synthesis of inducible enzymes
Figure 18.22a
DNA
mRNA
ProteinActiverepressor
RNApolymerase
NoRNAmade
lacZlacl
Regulatorygene
Operator
Promoter
Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.
(a)
5
3
mRNA 5'
DNA
mRNA
Protein
Allolactose(inducer)
Inactiverepressor
lacl lacz lacY lacA
RNApolymerase
Permease Transacetylase-Galactosidase
5
3
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
mRNA 5
lac operon
Figure 18.22b
• Inducible enzymes– Usually function in catabolic pathways
• Repressible enzymes– Usually function in anabolic pathways
• Regulation of both the trp and lac operons– Involves the negative control of genes, because
the operons are switched off by the active form of the repressor protein
Positive Gene Regulation
• Some operons are also subject to positive control– Via a stimulatory activator protein, such as
catabolite activator protein (CAP)
Promoter
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.
(a)
CAP-binding site OperatorRNApolymerasecan bindand transcribe
InactiveCAP
ActiveCAPcAMP
DNA
Inactive lacrepressor
lacl lacZ
Figure 18.23a
• In E. coli, when glucose, a preferred food source, is scarce– The lac operon is activated by the binding of a
regulatory protein, catabolite activator protein (CAP)
• When glucose levels in an E. coli cell increase– CAP detaches from the lac operon, turning it off
Figure 18.23b(b)Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.
When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
Inactive lacrepressor
InactiveCAP
DNA
RNApolymerasecan’t bind
Operator
lacl lacZ
CAP-binding site
Promoter