virus es big idea 3: living systems store, retrieve, transmit, and respond to info essential to life...
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Viruses
Big Idea 3: Living systems store, retrieve, transmit, and respond to info essential to life processes.
Essential Knowledge
• 3C3: Viral replication results in genetic variation, and viral infection can introduce genetic variation into
the hosts.
Structure of Viruses• Viruses are not cells
small infectious particles consisting of nucleic acid in a protein coat or a membranous
envelope
Viral Genomes
Made of either:
– Double- or single-stranded DNA, or
– Double- or single-stranded RNA
RNA Viruses
Need Reverse Transcriptase: enzyme that makes double stranded DNA to match w/ host cell
Lack replication error-checking mechanisms higher rates of mutation (HIV!)
Herpes Virus-DNA
Ebola Virus-RNA
Capsids and Envelopes• A protein shell that encloses viral genome
• Built from protein subunits called capsomeres
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
• Highly efficient allow for rapid evolution and acquisition of new phenotypes!
Viral Replication
• AKA: phages, viruses that infect bacteria
• Have a long capsid head that encloses DNA
• A protein tail attaches phage to host and injects DNA inside
Bacteriophage
Fig. 19-1
0.5 µm
Viruses Need Hosts!• Intracellular
parasites that have a host
range (a limited number of host cells that it can
infect)
Viral Reproductive Cycles
Virus VC
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
Reproductive Cycles of Phages
• 1st Rep Cycle: lytic cycle Death of host cell• Virulent phage: virus that only reproduces via lytic cycle
Fig. 19-5-1
Attachment1
Fig. 19-5-2
Entry of phageDNA anddegradation ofhost DNA
Attachment1
2
Fig. 19-5-3
Synthesis of viralgenomes andproteins
Entry of phageDNA anddegradation ofhost DNA
Attachment1
2
3
Fig. 19-5-4
Phage assembly
Assembly
Synthesis of viralgenomes andproteins
Entry of phageDNA anddegradation ofhost DNA
Attachment1
2
4
Head Tail Tail fibers
3
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
• 2nd reprod. Cycle: latent/lysogenic cycle
• No destroying host cell viral DNA is mixed into host cell’s chromosome prophage
• Gives host cell properties like increased pathogenicity in bacteria
Reproductive Cycles of Phages
• Phages that use both lytic and lysogenic cycles
• Env. signal can trigger switch
Temperate Phages
Lambda Phage Attacks E. Coli
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.
Evolution of Viruses
• Maybe plasmids?? (circular DNA in bacteria and yeasts) and transposons,
small mobile DNA segments
• Mimivirus (double-stranded DNA) is 2nd largest virus discovered
• Virus evolved before or after cells?
Megavirus (2 strand DNA) Oct. 2011, largest virus
• Harmless derivatives of pathogenic microbes that stimulate immune system to mount
defenses against actual pathogen
• Viral infections cannot be treated by antibiotics
Vaccines
With your partner, make a list of vaccines you have
received!
• Epidemic: New cases of a disease appear in a local area aka outbreak
• If epidemic spreads to another locale pandemic
Epidemics vs Pandemics
Plant Viruses
Most have an RNA genome– Horizontal transmission, entering through
damaged cell walls – Vertical transmission, inheriting the virus from a
parent
Viroids and Prions: Simple
• Viroids: circular RNA molecules that infect plants and disrupt growth
• Prions: slow-acting, virtually indestructible infectious proteins that cause brain diseases in
mammals
Ex: Prion Mad Cow Disease
Ex: Prion Creutzfeldt Jakob Syndrome
Understanding Viruses Moviehttp://www.schooltube.com/video/360cafa902fb60e55f61/Understanding-Viruses