Viruses

Virus – non-cellular particle made up of genetic material and protein. Reproduce only by infecting living cells. –Viruses share some, but not all, characteristics of living organisms. Viruses possess genetic material in the form of nucleic acids wrapped in a protein coat, are not cellular, and cannot reproduce on their own.

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

Infection – Lytic Cycle 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.

The Lytic Cycle Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can 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 Bacterial chromosome Lytic Cycle

Infection – Lysogenic Cycle 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)

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 Bacterial chromosome Bacteriophage injects DNA into bacterium Prophage Lytic Cycle Lysogenic Cycle

Lytic vs. Lysogenic

Plant Viruses –Viruses that infect plants can stunt growth and diminish plant yields. –Most known plant viruses have RNA rather than DNA as their genetic material. –Viral plant diseases have no cure and are best prevented by producing plants that resist viral infection.

Animal Viruses –Viruses that infect animals cells are a common cause of disease and may have RNA or DNA genomes.

Retroviruses Important group of viruses AIDS, contain RNA, once inside the cell, the RNA produces DNA (using the enzyme reverse transcriptase) 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

Viroids and Prions –Two classes of pathogens are smaller than viruses. 1. Viroids are small, circular RNA molecules that infect plants. 2.Prions are misfolded proteins that somehow convert normal proteins to the misfolded prion version, leading to disease. –Prions are responsible for neurodegenerative diseases including mad cow disease

Emerging Viruses –Emerging viruses are viruses that have suddenly come to the attention of science. Examples are H1N1 and avian flu.

Avian Flu –Avian flu infects birds, infected 18 people in Hong Kong in 1997, and since has spread to Europe and Africa, infecting over 400 people and killing more than 240 of them. If avian flu mutates to a form that can easily spread between people, the potential for a major human outbreak is significant. –Over 100 million birds have either died from the disease or been killed to prevent the spread of infection.

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 cancer cancer, AIDS respiratory infections chickenpox smallpox Type of VirusNucleic AcidDisease

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.