the genetics of viruses i. background ii. structure iii. life cycles
TRANSCRIPT
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The Genetics of Viruses
I. Background
II. Structure
III. Life Cycles
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Viral infections, past & present
Viral infection have been one of the major infectious
challenges of the human species, for as far back as we
can tell.
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Polio
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Ebola Virus
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Herpes Virus
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Severe acute respiratory syndrome (SARS)
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.
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Let’s size them up…
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Compare the size of: eukaryotic cell, bacterial cell and a virus
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0.5 m
I. Background Everyone is at risk
for infection! Bacteriophages
infect bacteria only All eukaryotes
(animals, plants, fungi, protist) all vulnerable
Size 20nm-250nm
Ghost phage
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Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known.
Spots on leaves and fruits, stunted growth, and damaged flowers or roots
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Tobacco Mosaic Virus
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I. Background Virus Discovery:
Tobacco mosaic disease (1930’s)
Stunts growth produces the speckled coloration
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I. Background Viral Evolution proposal: Fragments of
cellular nucleic acid Reproduce within host cells only
(non-living) Obligate intracellular parasites Host range
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II. Structure of viruses
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II. Structure of Viruses
1. Nucleic acid Enclosed in a protein coat Genomes may be
ds/ss DNA ds/ss RNA
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18 250 mm 70–90 nm (diameter)
20 nm 50 nm(a) Tobacco mosaic virus (b) Adenoviruses
RNADNACapsomere
Glycoprotein
Capsomereof capsid
II. Structure of Viruses
2. Capsids Protein Various shapes &
structures Capsids are
produced by host
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Viral structure
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II. Structure of Viruses
3. Envelopes Found in many
animal viruses Glycoprotein and
lipids Derived by host “Spikes” fuse with
membrane or receptor mediated entry
80–200 nm (diameter)
50 nm(c) Influenza viruses
RNA
Glycoprotein
Membranousenvelope
Capsid
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Receptor mediated model
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Fusion Model
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Basic Infection: Just Genome and Capsid
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III. Life Cycles
Bacteria and Eukaryotic Models
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Figure 18.4d
80 225 nm
50 nm(d) Bacteriophage T4
DNA
Head
Tail fiber
Tail sheath
III. Life Cycles:
1. Bacteriophages complete two reproductive mechanisms:
lytic cycle lysogenic cycle
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III. Life Cycles A. Lytic Cycle:
Digests the host’s cell wall, releasing the progeny viruses
Kills host Virulent phage
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Phage assembly
Head Tails Tail fibers
Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.
1 Entry 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
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Life Cycles B. Lysogenic cycle
Incorporate viral DNA into bacteria genome (propahge)
Phage genome is replicated (for free!) w/o destroying the host
Temperate phages capable of using both cycles
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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
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III: Life Cycles
2. RNA viruses typically infect animals Retroviruses (HIV), use reverse
transcriptase to make cDNA Integrated into genome, provirus
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• The reproductive cycle of HIV, a retrovirus
Figure 18.10
mRNA
RNA genomefor the nextviral generation
Viral RNA
RNA-DNAhybrid
DNA
ChromosomalDNA
NUCLEUSProvirus
HOST CELL
Reverse transcriptase
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIV Membrane of white blood cell
The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.
1
Reverse transcriptasecatalyzes the synthesis of aDNA strand complementaryto the viral RNA.
2
Reverse transcriptasecatalyzes the synthesis ofa second DNA strandcomplementary to the first.
3
The double-stranded DNA is incorporatedas a provirus into the cell’s DNA.
4
Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.
5
The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).
6
Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.
7 Capsids areassembled aroundviral genomes and reverse transcriptase molecules.
8
New viruses budoff from the host cell.9
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III: Life Cycles Life after infection
Viruses may damage or kill cells Tissue damage Toxins that lead to disease symptoms
may be produced Asymptomatic
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Sources http://www.aapsj.org/ http://www.stanford.edu/group/virus/1999/jchow/
rep.html http://pathmicro.med.sc.edu/mhunt/RNA-HO.htm http://www.brooklyn.cuny.edu/bc/ahp/LAD/C5/
C5_Viruses.html http://www.biology.com Campbell Reece Mitchell. Biology, Prentice Hall
1999, 2001 Sherris. Medical Microbiology: An introduction to
infectious disease, Appleton and Lange,1990
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http://teachers.eastern.k12.nj.us/nabi/biology/index.html
Good links