Post on 27-Dec-2015
Virus characteristics
• Not alive
• Obligate intracellular parasites. Infect cells-turn into a virus factory.
• Computer virus analogy
Virus Characteristics
• Contain only protein and nucleic acid (either DNA or RNA)
• Nucleic acid can be SS or DS-makes up virus core
• Core surrounded by a protein capsid. Shape of capsid gives the virus its characteristic shape. Some virus have envelope (animal virus only-produced by budding)
• Some virus have enzymes.
Virus characteristics
• Virus are host specific
• A. animal virus
• B. plant virus
• C. bacterial virus (phage)
• Sometimes a virus can mutate so it can infect a new host (avian flu)
LE 18-11
Young ballet students in HongKong wear face masks toprotect themselves from thevirus causing SARS.
The SARS-causing agent is acoronarvirus like this one(colorized TEM), so named forthe “corona” of glyco-proteinspikes protruding form theenvelope.
Virus Life cycle
Simplified virus life cycle (lytic). Virulent virus. What kind of cell can be infected by a virus (any-plant, animal, bacteria) Typical cycle-20-30 minutes.
• A. Attachment (specific-received e-mail)• B. Entry (don’t open that e-mail)• C. Synthesis• D. Assembly (spontaneous)• E. Release
LE 18-5
DNAVIRUS
Capsid
HOST CELL
Viral DNA
Replication
Entry into cell anduncoating of DNA
Transcription
Viral DNA
mRNA
Capsidproteins
Self-assembly ofnew virus particlesand their exit from cell
LE 18-8
RNA
ER
Capsid
HOST CELL
Viral genome (RNA)
mRNA
Capsidproteins
Envelope (withglycoproteins)
Glyco-proteins Copy of
genome (RNA)
Capsid and viral genomeenter cell
New virus
Template
LE 18-10
HOST CELL
ReversetranscriptionViral RNA
RNA-DNAhybrid
DNA
NUCLEUS
ChromosomalDNA
Provirus
RNA genomefor thenext viralgeneration
mRNA
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIVMembrane ofwhite blood cell
Treatment of viral diseases
Infectious cycle becomes an uncontrolled chain reaction. How can you deal with it?
A. Antibiotics? NoB. Immune system. Can’t respond quick
enough first time you see virus (immunization). Colds, flu.
C. Antiviral drugs-AIDSD. Restriction enzymes-if you’re a bacteria.
Why do virus infections cause disease symptoms?
• Damage to tissue-cell destruction (some cells regenerate (epithelium of upper respiratory tract), some can’t (nerves-polio)
• Cause infected cells to produce toxins (infected cell becomes transformed)
• Viral cancer-oncogenes. Hep B-liver cancer; Herpes-Burkitt’s lymphoma; papovavirus-cervical cancer
• Immune system-inflammation, lymphokines
Emerging virus
• Evolve and infect individuals who are only resistant to ancestral virus (influenza)
• Spread from species to species. Hanta virus-deer mice to humans.
• Spread from small populations to larger. AIDS
• Environmental disturbances can increase emergence. Roads into tropical rainforest, global warming, airplanes)
Lysogenic life cycle (temperate virus)
Characteristics of the Lysogenic life cycle
• Steps Provirus inserts into infected cell’s genome. Repressor gene usually represses viral gene expression. Induction.
• Differences in lytic and lysogenic life cycle
LE 18-7
Phage
Phage DNA
The phage attaches to ahost cell and injects its DNA.
Phage DNAcircularizes
Bacterial chromosome
Lytic cycle
The cell lyses, releasing phages.Lytic cycleis induced
or Lysogenic cycleis entered
Certain factorsdetermine whether
Lysogenic cycle
Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.
The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.
Prophage
Many cell divisionsproduce a large population of bacteria infected withthe prophage.
Daughter cellwith prophage
Phage DNA integrates into thebacterial chromosomes, becoming aprophage.
New phage DNA and proteins aresynthesized and assembled into phages.
Consequences of Lysogeny
• none-(viral genetic info lies dormant)
• Transformation-makes cell cancerous (viral gene is an oncogene)
• Induction-M.S.? Herpes-
Prions
• Infectious proteins
• Proteins are misfolded-cause other proteins to be misfolded
• Misfolded proteins impair cellular function
• “Mad cow” disease
Origin of Virus?
There are three main hypotheses regarding the origins of viruses:
a. The progressive, or escape, hypothesis states that viruses arose from genetic elements that gained the ability to move between cells;
b.The regressive, or reduction, hypothesis asserts that viruses are remnants of cellular organisms;
c. The virus-first hypothesis states that viruses coevolved with their current cellular hosts.
Progressive hypothesis
Genetic material moved from cell to cell via injured cell surfaces?
Evolution of capsid proteins may have facilitated infection of undamaged cells.
Candidates for sources of viral genomes?
a. plasmids?
b. Transposons? (Mobile genetic elements)
Mirror image rna sequences
Often associated with rna virus
AAAAAAUUUUUXXXXXAAAAAAUUUUUU
RNA folds back on itself
DS RNA recognized by a system of enzymes and proteins (the COP)
The COP then destroys an DS m-rna with these mirror image sequences
RNai
Fig. 18-13
miRNA-proteincomplex(a) Primary miRNA transcript
Translation blocked
Hydrogenbond
(b) Generation and function of miRNAs
Hairpin miRNA
miRNA
Dicer
3
mRNA degraded
5
RNAi Video
• http://www.nature.com/nrg/multimedia/rnai/animation/index.html