1. Viruses

2. Viruses
Virus – non-cellular particle made up of genetic material and protein. Reproduce only by infecting living cells.
TMV – tobacco mosaic virus – first heavily studied virus
Dmitri Ivanovski – 1892, cause of TMV – juice extracted from infected plants
Martinus Beijerinck – 1897, named TMV particle viruses
Wendell Stanley – 1935, purified TMV into a crystal – viruses are not living things.

3. Virus Anatomy Many shapes possible
Typical virus composed of a core of either DNA or RNA (not both), surrounded by an outer protein coat, or capsid. Capsid includes proteins that enable the virus to enter a host cell.

4. Some Virus Structures Tobacco Mosaic T4 Bacteriophage Virus
Influenza Virus
RNA
Capsid
proteins
Head
Tail sheath
DNA
Tail fiber
RNA
Capsid
Surface
proteins
Membrane
envelope
Magnification: 82,000X
Magnification: 200,000X
Magnification: 1,000,000X

5. Viral Infection
*Viruses attach with tail fibers and inject genetic material into host cell. Most viruses are highly specific to the cells they infect. Generally, they are species specific, but they may infect larger groups of organisms (rabies and mammals).
- Bacteriophages – viruses which invade bacteria.

6. Lytic Infection Host cell lysed and destroyed – kills cells quickly!
Ex. Cold, flu viruses
1.) Infection – virus attaches to host cell’s wall and injects genetic material.
2.) Growth – viral DNA “hijacks” cell replication and uses those mechanisms to reproduce its own DNA.
3.) Replication – viral DNA makes thousands of copies of itself. All of this can take place in 25 minutes in some bacteria. Eventually, the bacterial cell wall bursts and releases newborn viruses to colonize other cells.

7. The Lytic Cycle Lytic Cycle Bacteriophage protein coat
Bacteriophage DNA
Bacterial
chromosome
Bacteriophage attaches to bacterium’s cell wall
Bacteriophage enzyme lyses the
bacterium’s cell wall, releasing
new bacteriophage particles that
can attack other cells.
Lytic Cycle
Bacteriophage injects DNA into bacterium
Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles
Bacteriophage
Bacteriophage DNA
Bacteriophage protein
Bacteriophage takes over bacterium’s metabolism, causing synthesis of new bacteriophage proteins and nucleic acids

8. Lysogenic Infection
Does not lyse host cell right away – lies in wait and doesn’t kill cell quickly
Ex. Herpes (cold sores), HIV viruses
Virus embeds its DNA into DNA of host cell and is replicated along with the host cell’s DNA.
1.) Viral DNA enters host cell but does not reproduce right away – it inserts itself into the host’s DNA (prophage).
2.) The virus may stay in the host’s DNA for many generations, each time the host reproduces, it also reproduces the virus.
3.) Eventually, the viral DNA is activated and the growth, replication, and lysing of the host cell occurs. (radiation, chemicals, extreme temperatures, other infections, etc can trigger the virus to switch into Lytic Cycle)

9. The Lysogenic Cycle Lytic Cycle Lysogenic Cycle Bacterial chromosome
Bacteriophage DNA
Bacteriophage injects DNA into bacterium
Bacteriophage DNA (prophage) can exit the bacterial chromosome
Lytic Cycle
Lysogenic Cycle
Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can attack other cells
Bacteriophage DNA forms a circle
Bacteriophage DNA (prophage) may replicate with bacterium for many generations
Prophage
Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles
Bacteriophage DNA inserts itself into bacterial chromosome

10. Lytic vs. Lysogenic

11. Retroviruses Important group of viruses
AIDS, contain RNA, once inside the cell, the RNA produces DNA which acts as a prophage and binds with the cell’s DNA.
Virus may remain inactive for many years
“Retro” means backward, RNA to DNA, may cause some forms of cancer and AIDS

12. Prions Protein Infectious Particles 1972, Stanley Prusiner
No DNA or RNA, just proteins
Cause scrapie in sheep, mad cow disease

13. Are Viruses Alive? Viruses are known as parasites
Viruses are not cells and cannot reproduce independently. But, when infect living cells, can make copies of themselves, regulate gene expression, and even evolve.

14. Diseases Caused by Viruses
Pathogens – disease producing agents
Viruses – smallpox, polio, measles, AIDS, mumps, influenza, Yellow Fever, rabies, common cold.
No cure, the only way to be healed is for your own immune system to produce antibodies to resist the virus. Antibiotics don’t work for viruses!
Interferons – release by cells when they are attacked by a virus, might be a possible treatment.
Cancer – oncogenic viruses cause cancer. Not all viruses cause cancer, not all cancers are caused by virus.
Prevention is best protection – vaccinations – weak or killed virus

15. Common Diseases Caused by Viruses
Type of Virus
Nucleic Acid
Disease
Oncogenic viruses
Retroviruses
Adenoviruses
Herpesviruses
Poxviruses
DNA
RNA
cancer
cancer, AIDS
respiratory infections
chickenpox
smallpox

16. Diseases Caused by Bacteria
Bacteria – diphtheria, tuberculosis, typhoid fever, tetanus, Hansen Disease, syphilis, cholera, bubonic plague
Bacterial illnesses are caused by toxins produced as bacteria grow
Bacterial disease can be treated with antibiotics and prevented with vaccines
Bacteria are becoming resistant to antibiotics.