1. Chapter 24
Viruses

2. Viral Structure & Replication
Discovery of Viruses
Virus: nonliving particle made up of nucleic acid and a protein coat or nucleic acid and a lipoprotein coat
Cause many diseases in living organisms
Useful tools for genetic research

3. Viral Structure & Replication
Discovery of Viruses
Late 1800s
Living or nonliving?
1935, Wendell Stanley crystallized the tobacco mosaic virus (TMV)
Concluded: infective agent that could be crystallized–––>not made of cells

4. Viral Structure & Replication
Characteristics of Viruses
Nonliving b/c:
No cytoplasm or organelles
No metabolism or homeostasis
Do not grow by dividing in two
Need other cells to reproduce

5. Viral Structure & Replication
Viral Size & Structure
Very small
Shape is the result of genome or protein coat
Helix Shape: capsid, protein coat, forms the helix
TMV, Rabies, Measles
Icosahedron: 20 triangular faces & 12 corners
Adenovirus, herpes simplex, chickenpox, polio
Spherical: circular
Influenza
Envelopes: bilipid membrane that surrounds membrane
Helps new viruses to recognize host cells

6. Viral Structure & Replication
Classification of Viruses
RNA or DNA
Genome single-stranded/double-stranded & linear OR circular
Nature of capsid & presence or absence of an envelope

7. Viral Structure & Replication
Replication in DNA Viruses
Inserts DNA into host cell’s chromosome
Provirus
Transcribe mRNA  Translate viral proteins
Use cell’s enzymes to make new viral DNA
Assembled to make new viruses
Replication in RNA Viruses
Some: enter host cell & serve directly as mRNA
Others: Transcribed & is a template for the synthesis of mRNA & more copies of viral genome

8. Viral Structure & Replication
Replication in RNA Viruses, continued
Retroviruses: use reverse transcriptase, which uses RNA as a template to make viral DNA, which then creates proteins that become part of the new viruses

9. Viral Structure & Replication
Replication in Viruses that Infect Prokaryotes
Lytic Cycle
Invades a host cell, produces new viruses, and ruptures (lyses) the host cell when releasing newly formed viruses
Virulent: viruses that reproduce only by the lytic cycle
Destroy the cells that they infect
Ex: T phages

10. Viral Structure & Replication
Replication in Viruses that Infect Prokaryotes
Lysogenic Cycle
Hide in their host cell for days, months, or years
Called Temperate Viruses

11. Viral Structure & Replication
Viruses: Tools for Biotechnology
Replace large pieces of the DNA of a phage with DNA of a particular interest, such as the human gene for cystic fibrosis
Insert recombinant DNA into empty phage heads and allow them to infect bacteria
Bacteria produces millions of copies
Invaluable tools for medical research

12. Viral Structure & Replication
The Origin of Viruses
Evolved from early cells
Naked pieces of nucleic acid that could travel from one cell to another
Entered cells through damaged membranes
Genes evolved to allow for protective protein coats and recognition proteins to attack healthy cells
Example: Influenza virus and HIV

13. Viral Diseases Vectors of Viral Disease What is a vector?
Intermediate host that transfers a pathogen or a parasite to another organism
Include: humans, animals, mosquitoes, ticks, and fleas
West Nile Virus:
Mosquito---bird(WNV)---human, the virus can spread to human

14. Viral Diseases Human Viral Diseases Can affect: Brain Liver Heart
Lungs
Skin

15. Viral Diseases Viruses & Cancer What is cancer???
Cells divide at an uncontrolled rate and form a tumor that invades surrounding tissue
Viruses contain viral oncogenes:
genes that cause cancer by blocking the normal controls on cell reproduction
Viral DNA inserts itself into a host’s chromosome near a proto-oncogene --- controls cell growth
Converted to a oncogene
HPV: cervical cancer
Hep. B: liver cancer

16. Viral Diseases Emerging Viral Diseases
Illnesses caused by new or reappearing infectious agents that typically exist in animal populations-- often in isolated habitats--and can infect humans who interact w/ these animals
Ebola: hemorrhagic fever
Hantavirus: caused outbreak of pneumonia
SARS: avian flu

17. Viral Diseases Vaccinations
Solution with a harmless version of a virus, bacterium, or toxin
Causes an immune response  body forms an antibody
Inactivated virus: not able to replicate in a host
Attenuated virus: weakened form of the virus that cannot cause disease
Attenuated viruses provide greater protection from disease

18. Viral Diseases Vector Control Drug Therapy Mosquito-control programs
Rabies Vaccinations
Drug Therapy
Antiviral drugs interfere w/ viral nucleic acid synthesis
Small number of these drugs, WHY?

19. Viral Diseases Viroids Prions
Smallest know particles that can replicate
Made of RNA
Infect plants
Prions
Infectious protein particles that have no genome
Abnormal forms of a natural brain protein, that convert normal brain proteins into prions
Clump together inside of cells---death
BSE: cattle
Scrapie: sheep
CJD/kuru: humans

20. Review
CH 24: Viruses

21. Question #1 What are viruses made of? Enzymes and fats
Carbohydrates & ATP
Protein and nucleic acids
Mitochondria & lysosomes

22. Question #2 How do viroids differ from viruses
Viroids are larger in size
Viroids do not have a capsid
Viroids do not have nucleic acids
Viroids can cause disease in plants

23. Question #3
During which of the following processes does a phage kill its host?
Conjugation
Transcription
The lytic cycle
The lysogenic cycle

24. Question #4
Which of the following is one reason why viruses are not considered living organisms?
Viruses are able to grow
Viruses do not metabolize
Viruses can reproduce by splitting
Viruses are too small to be easily observed

25. Question #5 Which of the following does the diagram represent? A virus
A prion
A virod
A bacterium

26. Question #6 X To which of the following is label X pointing? Envelope
Nucleic acid
Protein coat
Cell membrane X

27. Question #7 Complete the following analogy: Skin : Person :: Capsid :
Virus
Insect
Fungi
Bacterium

28. Question #8
Which of these four illnesses/diseases are caused by a virus?
Malaria, West Nile, Yellow Fever, Lyme
HPV, HIV, Influenza, Chickenpox
Gang green, West Nile, Measles, Smallpox
Heart disease, Tuberculosis, DMV, AIDS

29. Question #9
Genes that cause cancer by blocking the normal controls of cell reproduction are called
Protease inhibitors
Viroids
Virulent
Oncogenes