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INVADERS
Chapter 24: Viruses
Objectives:
• Summarize the discovery of viruses
• Describe why viruses are not considered
living organisms
• Describe the basic structure of viruses
• Compare the lytic and lysogenic cycles of
virus replication
• Summarize the origin of viruses
Virus
nonliving particle that can infect
both prokaryotic and eukaryotic
cells
Virus Structure-vary in shape and size
All viruses contain two main parts:
1)
Nucleic Acid (Genome) – Either DNA or RNA
2)
Capsid – protein coat or lipid protein coat (many different shapes)
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3)
Helical – Tobacco Mosaic Virus (TMV), Rabies, measles
Icosohedreon – Adenovirus, Herpes Simplex, Chicken Pox, Polio
Spherical – Influenza Virus
Envelope – bilipid membrane that surrounds the capsid (only some
viruses)
–
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Formed from nuclear membrane or cell membrane as it leave the host cell
Proteins in the envelope helps new viruses recognize host cell.
Examples: Influenza, chicken pox (varicola), HIV
Function of Viruses
1) Cause Disease by infecting the host cell
2) Used in Genetic Research and
biotechnology
Characteristics of Viruses
Cells are nonliving!
– Are not made of cells
– Do not have organelles or cytoplasm
– Can’t carry out metabolic processes such as
metabolism and homeostasis
– Do not grow through Cell Division
– Can’t reproduce outside their host cell
(Need host cells structures to reproduce)
Wendell Stanley was the first scientist to crystallize a virus. This is
evidence that viruses are not made of cells and are not alive.
Classification of Viruses
A. Genetic Material:
1) RNA or DNA
2) Single Stranded or Double Stranded
3) Linear or Circular Genetic Information
B. Capsid:
1)Shape
2)Presence or absence of an envelope
Example: SARS – RNA, Single Stranded, Linear, lollipopshaped capsid and enveloped (chart pg485)
Other ways viruses are classified:
-Different viruses infect bacteria, plant and animal hosts
Example: Bacteriophages – viruses that infect bacteria
-due to receptor site forming to specific proteins on cell wall or membrane
How do Viruses Spread
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•
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Air
Water
Food
Bodily Fluids
Contact
Obligate intracellular parasites
(Viruses)
Viruses replicate only by using the host cells
enzymes and organelles to make more
viruses. Protein synthesis is controlled by the viral
genome and the host cell becomes a virus making
factory.
Protein Synthesis review
1) Transcription: DNA -> mRNA
2) Translation: mRNA -> proteins
Lytic Cycle vs. Lysogenic Cycle Reproduction of
viruses
Lytic Cycle: Reproduction of Virulent Virus
•
Destroy the cell they infect
Stages:
1) Adsorption
2) Penetration
3) Replication
4) Maturation (Assembly)
5) Release
Lysogenic Cycle: Reproduction of Temperate Virus
•
Lays dormant in the host cell before it destroys it
•
When triggered will go into the lytic cycle
Lytic Cycle –
followed when virus is virulent (active)
1. ADSORPTION- virus particle attaches
to a host cell.
2. ENTRY
The particle injects its genetic
instructions (DNA or RNA) into the host
Injected genetic material ‘hi-jacks’ the
cell’s machinery and recruits the host’s
enzymes.
3. REPLICATION
Enzymes make parts for the new virus
particles
4. ASSEMBLY -new particles assemble the
parts into new viruses
5. RELEASE -Cell explodes (lyses) releasing
new viruses which search for a new
host cell
Lysogenic Cycle -
state
followed when virus is in latent or temperate
The steps of the lysogenic cycle:
1) Viral nucleic acid enters cell
2) Viral nucleic acid attaches to host cell nucleic acid,
creating a prophage
3) Host cell enzyme copies viral nucleic acid
4) Cell divides, and virus nucleic acid is in daughter
cells
5) At any moment when the virus is "triggered", the
viral nucleic acid detaches from the host cell's DNA
and enters stage 2 of the lytic cycle.
Common symptoms that appear to "trigger" the viral
DNA are hormones, high stress levels (adrenaline),
and free energy within the infected cell.
Replication in DNA viruses
Two methods:
1) vDNA->mRNA –> make new viruses (in
cytoplasm)
2) DNA – Provirus – (Gets incorporated into
Host Cell’s DNA) – Makes new viruses
Replication in DNA Viruses
Method 1
1. Virus enters the host cells cytoplasm and takes
control of the host cells protein synthesis pathway
2. Enzymes transcribe mRNA from the viral DNA
3. Host ribosomes translate Viral mRNA into viral
proteins and enzymes replicate the viral DNA
4. New viral particles are assembled (Capsids and
nucleic acids)
Replication in DNA Viruses
Method 2
1) Viral DNA is injected into the host cell
2) Viral DNA moves into the nucleus and
becomes incorporated into the host cells
DNA as a provirus
3) Virus takes over the cell and makes
newe viruses as a provirus
Replication in RNA viruses
RNA Viruses
RNA
1) The vRNA serves right away as mRNA and then is
translated into proteins (New Viruses)
2) Viral RNA is transcribed into mRNA and then is
translated into proteins and new viruses
Retro RNA Viruses
Contain Reverse Transcriptase which uses RNA
as a template to create DNA, which is then
inserted into the host cells DNA (ex:HIV)
VRNA - > VDNA - > VRNA - > Proteins (Viruses)
Viral Diseases
• Vector – intermediate host that transfers a pathogen
or parasite to another organism
– Ex: humans, mosquitos, ticks, fleas.
• Human viral diseases - chickenpox, shingles, viral
hepatitis, AIDS, etc…
• Protease inhibitors – drugs that block virus
reproduction
• Oncogenes – viral genes that cause cancer by
messing with cell division checkpoints
• Proto-oncogene – controls cell growth
Prevention and Treatment
Vaccine – weakened sample of virus or virus parts which triggers your body’s immune
system
Attenuated virus – weakened virus that cannot cause disease
Inactivated virus – unable to replicate in host
Natural immunity - antibodies are specialized proteins formed in B cells of immune
system
- antibodies block attachment sites of viruses
http://www.pbs.org/wgbh/nova/bioterror/vaccines.html
Disease causing particles even smaller than viruses
Viroids
– short single strand of RNA without a capsid
- interfere with cell processes and cause new viroids
to be made
- are found only in plants
Prions
- glycoprotein particle
- able to reproduce in mammalian cells
- Prion diseases: mad
cow disease; Creutzfeldt-Jakob
& kuru (degenerative nerve diseases of the brain)
disease
Genetic engineering
Genetic engineers use viruses to carry desirable genes from one cell to another
Improved agriculture – herbicide tolerant soybeans; rot resistant tomatoes; fast growing fish;
meatier chickens
Correcting genetic disorders - experimental; only done in animal with success; not allowed in
humans
Pest control – insert genes of plants that create chemical to resist insects into crop plants such as
corn
Manufacturing of medicine - human insulin &clotting factors 8 & 9
http://www.pbs.org/wgbh/harvest/engineer/transgen.html