1. INVADERS

2. 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

3. Virus
nonliving particle that can infect both prokaryotic and eukaryotic cells

4. 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)
Helical – Tobacco Mosaic Virus (TMV), Rabies, measles
Icosohedreon – Adenovirus, Herpes Simplex, Chicken Pox, Polio
Spherical – Influenza Virus
3) Envelope – bilipid membrane that surrounds the capsid (only some viruses)
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

5. Function of Viruses 1) Cause Disease by infecting the host cell
2) Used in Genetic Research and biotechnology

6. 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.

7. 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, lollipop-shaped capsid and enveloped (chart pg485)

8. 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
e-coli being wiped out by T4 bacteriophages

9. How do Viruses Spread
Air
Water
Food
Bodily Fluids
Contact

10. 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
Transcription: DNA -> mRNA
Translation: mRNA -> proteins

11. 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)
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

12. Lytic Cycle – followed when virus is virulent (active)
ADSORPTION- virus particle attaches to a host cell.
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

13. The steps of the lysogenic cycle: 1) Viral nucleic acid enters cell
Lysogenic Cycle - followed when virus is in latent or temperate state
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.

14. Replication in DNA viruses
Two methods:
vDNA->mRNA –> make new viruses (in cytoplasm)
DNA – Provirus – (Gets incorporated into Host Cell’s DNA) – Makes new viruses

15. 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)

16. Replication in DNA Viruses
Method 2
Viral DNA is injected into the host cell
Viral DNA moves into the nucleus and becomes incorporated into the host cells DNA as a provirus
Virus takes over the cell and makes newe viruses as a provirus

17. Replication in RNA viruses
The vRNA serves right away as mRNA and then is translated into proteins (New Viruses)
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)

19. 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
Vaccination and vector control are the two most effective ways to control viral diseases

20. 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

21. 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 disease & kuru (degenerative nerve diseases of the brain)

22. 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