Viruses

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.

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.

Some Virus StructuresT4 Bacteriophage

Tobacco MosaicVirus Influenza

VirusHead

Tail sheath

DNA

Tail fiber

RNACapsid

Surfaceproteins Membrane

envelope

RNA

Capsidproteins

Magnification: 82,000X Magnification: 200,000X Magnification: 1,000,000X

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.

Lytic Infection

• Host cell lysed and destroyed – kills cells quickly!– Ex. Cold, flu viruses

• Virus takes over cell and uses cell to make copies of itself

The Lytic Cycle

Bacteriophage enzyme lyses thebacterium’s cell wall, releasingnew bacteriophage particles thatcan attack other cells.

Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles

Bacteriophage takes over bacterium’s metabolism, causing synthesis of new bacteriophage proteins and nucleic acids

Bacteriophage injects DNA into bacterium

Bacteriophage attaches to bacterium’s cell wall

Bacteriophage

Bacteriophage DNA

Bacteriophage protein

Bacteriophage protein coat

Bacteriophage DNA

Bacterialchromosome

Lytic Cycle

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. Can lie dormant for years!

The Lysogenic Cycle

Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles

Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can attack other cells

Bacteriophage DNA inserts itself into bacterial chromosome

Bacteriophage DNA (prophage) may replicate with bacterium for many generations

Bacteriophage DNA (prophage) can exit the bacterial chromosome

Bacteriophage DNA forms a circle

Bacteriophage DNA

Bacterialchromosome

Bacteriophage injects DNA into bacterium

Prophage

Lytic Cycle

Lysogenic Cycle

Lytic vs. Lysogenic

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

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.

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

Common Diseases Caused by Viruses

Oncogenic viruses

Retroviruses

Adenoviruses

Herpesviruses

Poxviruses

DNA

RNA

DNA

DNA

DNA

cancer

cancer, AIDS

respiratory infections

chickenpox

smallpox

Type of Virus Nucleic Acid Disease

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.

Similarities Between Bacterial and Viral Infections

• Modes of transmission similar– Through the air, in water, in food, in body

fluids, animal bites/scratches

• Both cause diseases and can be deadly

• Both have ways of being prevented– Vaccines for both and good

hygiene/sanitation prevents disease

Differences Between Bacterial and Viral Infections

• Antibiotics only treat BACTERIAL infections! They do NOT work on viral infections.

• Viral infections are treated by the body’s immune system

• Bacteria are living cells who poison or “eat” our own cells.

• Viral cells inject genetic material into our own cells and use our own cells against our body