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Transcript
VIRUSES
Chapter 9.8 and 9.9
Biology 391
•
What are they
–
–
•
•
Details & Examples
Life Cycles
–
–
–
–
•
Structure & Classification
Discovery
Lytic
Lysogenic
Evolution
Prions and Viroids
Impacts & Applications
Viruses – What are they?
•
Could they be the beginnings of life?
– (Sec 17.5)
– Are viruses even alive?
•
•
Life = “self-sustained chemical system that is
capable of undergoing Darwinian, or
biological evolution” - NASA
Biological Evolution includes
1. Self-reproduction
2. Mutation that can be inherited
3. Natural selection
Defined & Classified
• Non-living
– what characteristics are they missing?
• Genetic material surrounded by a protein coat
• Require a host cell to reproduce
• Categorized by genetic material
– Single stranded or double stranded DNA
– Single stranded or double stranded RNA
• Categorized by shape of protein coat (CAPSID)
– Enveloped, Helix, Isohedron (polyhedron)
More classification
• Retroviruses
– Have RNA genomes & contain special enzymes
that transcribe RNA to DNA within host
– Reverse Transcriptase
– Ex: HIV
• Bacteriophages
– Viruses that attack bacteria
– Have DNA
Discovery…
Tobacco Mosaic Virus
• 1883 – 1935
– The idea of a virus the crystallization of
TMV
• Experiment:
– Something wrong
with the plants –
tested for bacteria by
straining homogenate
– Filtrate still infectious
–
http://www.cellsalive.com/howbig.htm
Viruses are Host Specific
• In multicellular organisms viruses specialize in
attacking particular cell types
• Cold viruses attack membranes of respiratory tract
• Measles viruses infect the skin
• Rabies viruses attack nerve cells
• Some viruses linked to cancer (e.g. T-cell leukemia,
liver cancer, cervical cancer)
• Herpes virus attacks mucous membranes of mouth
and lips (causing cold sores)
– Other herpes virus type causes genital sores
• HIV virus attacks specific white blood cell type,
causing AIDS
(a) Envelope
(lipid bilayer)
Spikes
Core
Proteins
HIV
(b)
Herpes Viruses
Viral RNA in
protein coat
Glycoproteins
Chapter 19
Protein
Coat
9
Reverse
Transcriptase
Swine Flu?
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Influenza.html
Viral Replication (2 methods)
• LYTIC
– Virulent, immediate, quickly kills
• LYSOGENIC
– Long-term and may lie dormant
– Similar to lytic (eventually) but integrates into
genome
• Viruses are parasites.
- Any good parasite does not kill it’s host. Why?
-
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html
The Lytic Infection
1. ATTACHMENT – virus connects with cell surface
2. INSERTION – virus injects its genome (DNA or RNA)
into host cell
3. REPLICATION – the viral genome is replicated
– The DNA of the host cell is inactivated, and the viral DNA
takes over making viral proteins and viral nucleic acid.
– The viruses are assembled (nucleic acid inside protein
coat)
4. BURST - The cell then bursts open (lyses) and the
newly formed virus particles are released, free to
infect other cells.
The Lysogenic Infection
1. ATTACHMENT – virus connects with cell surface
2. INSERTION – virus injects its genome (DNA or RNA) into
host cell
3. INTEGRATION – the viral DNA inserts itself into host
cell’s DNA
4. REPLICATION – the viral genome is replicated
– The DNA of the host cell is still replicated and the life of the cell
may continue as normal, replicating the virus too.
– The virus can stay dormant for years
5. BURST – at some point, the virus becomes virulent.
- The cell then bursts open (lyses) and the newly
formed virus particles are released, free to infect other
cells.
Evolution: Reproduction & Survival
• Viruses evolve very quickly
– Faulty genome reproduction enzymes = lots of
mistakes
– Lots of copies made per cycle compounds mistakes
– Multiple strains come about very quickly
• Survival Mechanisms
– Are not killed by antibiotics (designed for
prokaryote membranes & metabolism)
– Upon lysing, pick up surface membranes
• Evade host immune destruction in future
Viroids & Prions
• VIROIDS
– Short RNA segments capable of self replication inside
a host
– Cause many plant diseases (degraded by most
animals)
• PRIONS
– Small protein particles that cause disease
– Chronic Wasting disease, CJD (Creutzfeldt-Jakob
Disease), Scrapie in sheep, Bovine spongiform
encephalopathy (BSE or “Mad Cow Disease”)
• These diseases create holes in brain tissue
9.9 Impact of Viruses
• Viruses are a serious threat to cellular life
– Antibiotics are useless against viruses
– Modern technologies allow viruses to spread at a
greater rate
– Vaccines are best defense
– Anti-viral drugs interfere with viral protein production
and/or specificity proteins
– Some diseases caused by viruses: chicken pox, small
pox, common cold, flu, ebola, mumps, measles, rabies
and AIDS
• Usefulness of Viruses
– Delivering DNA in Cloning Experiments
– Delivering gene segments in Gene therapy
– Attacking Cancer cells
Gene Therapy (p410)
• Treating the genetic defect itself rather than treating
the symptoms of a disease
• Genetically engineer viruses using recombinant DNA
technology (insert desired genes into viral genome)
• GERM-LINE THERAPY
– Change genetic sequence of gametes
• SOMATIC CELL THERAPY
– Target the cells that express the gene
– Ex: insulin gene in pancreas; growth hormone in pituitary
• Trouble: directing to correct cell
Controlling area of integration
Other Applications
• Because viruses are host specific & specialized to
attack specific host cell, we can use them too
• Bacteriophages can treat bacterial diseases
– Rise in bacterial antibiotic resistance makes standard
drugs less effective
– Bacteriophages specifically target host bacteria
– Bacteriophages are harmless to human body cells
Chapter 19
22
Recap
• How are viruses defined?
• Why do viruses evolve so quickly?
• Why might viruses be useful in fighting
cancers?
• Are viruses alive? Why or why not?
• What is the difference between viruses,
viroids and prions?
• What is the difference between lytic and
lysogenic infections?
Extra Slides
How Viruses Replicate: HIV
1a. Virus attaches to receptor
1b. Core disintegrates; viral
RNA enters the cytoplasm
cytoplasm
nucleus
vRNA
3b. DNA transcribed
into mRNA & viral
RNA, which move
to cytoplasm
DNA
mRNA
2. Viral reverse transcriptase
makes
DNA using viral RNA
Chapter 19
25
3a. DNA enters nucleus &
chromosomes
How Viruses Replicate: HIV
6. Viruses bud from
plasma membrane
5. Viral proteins & RNA
assembled
Chapter 19
4. Viral proteins made
26
using mRNA
How Viruses Replicate: Herpes
2a. Viral envelope merges
with nuclear membrane
2b. Protein coat
disintegrates;
viral DNA
copied & enters
nucleus
nucleus
envelope
coat
DNA
DNA
mRNA
(cytoplasm)
1. Virus enters cell
by endocytosis
Chapter 19
3. Viral DNA transcribed to mRNA,
which
27 moves to cytoplasm
How Viruses Replicate: Herpes
envelope
coat
5. New viruses
assembled & bud
from nucleus, get
envelope from inner
nuclear membrane
nucleus
DNA
DNA
mRNA
(cytoplasm)
4. MRNA makes
proteins, which
enter nucleus
Chapter 19
28
6. Newly formed
viruses leave the
cell by exocytosis
Influenza A structure
• ~100 nm in diameter
• sheathed in a lipid bilayer (derived from the plasma membrane of its host)
• Studded in the lipid bilayer are two integral membrane proteins
– ~ 500 molecules of hemagglutinin ("H") and
– ~100 molecules of neuraminidase ("N")
• Within the lipid bilayer are
– ~3000 molecules of matrix protein
– 8 pieces of RNA
• Each of the 8 RNA molecules is associated with many copies of a
nucleoprotein
• several molecules of the three subunits of its RNA polymerase
• some "non-structural" protein molecules of uncertain function