Structure, Function, and Reproduction VIRUSES Structure, Function, and Reproduction

WHAT IS A VIRUS? A non-cellular particle made up of genetic material and a protein coat that can invade living cells. Viral structure is very simple. Nucleic acid (DNA or RNA) Surrounded by a protein coat (capsid) Viruses are much smaller than even the smallest bacteria 10-400 micrometers

VIRUS SHAPES Complex Cylindrical Polyhedral Tobacco Mosaic T4 Bacteriophage Influenza Virus RNA Capsid proteins Head Tail sheath DNA Tail fiber RNA Capsid Surface proteins Membrane envelope

VIRAL REPRODUCTION Viruses cannot reproduce on their own They must invade, or infect, a living host cell There are two different reproductive cycles: Lytic cycle Lysogenic cycle

LYTIC CYCLE Attachment to the host cell Penetration Injects DNA/RNA into host cell Take over of the host cell activity Construction of new virus parts (What are the new parts?) Assembly of new viruses Release of new viruses Viruses break out of (lyse) the host cell Host cell dies Ie. bacteriophage

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

LYSOGENIC CYCLE The virus does not reproduce and lyse its host cell… at least not right away. Attachment to the host cell Penetration Viral DNA is inserted into the host cell’s DNA Once inserted into the host cell’s DNA, the viral DNA is known as a prophage Every time the host cell divides, it makes more viral nucleic acid Does not harm the host cell May live in the host cell for many generations Eventually, the DNA of the prophage will become active Prophage removes itself from host DNA Enters into the lytic cycle

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

Beneficial and Harmful Aspects VIRUSES Beneficial and Harmful Aspects

BENEFICIAL ASPECTS The presence of a prophage can sometimes be beneficial Can block the entry of other viruses into the cell Can add useful DNA to the host cells DNA Viruses are extremely valuable in genetic research Gene therapy Introduce corrected genes into a victim of a genetic defect (ie. Cystic fibrosis) It’s possible that modified viruses may one day be used as medical tools Genetic engineering put gene that codes for big, red tomato plant in inactive virus and inject virus into plant

BENEFICIAL ASPECTS continued Viruses can be used to make vaccines Substances that contain the weakened or inactive disease-causing virus When injected into the body, vaccines provide immunity to the disease They can only provide protection if they are used before an infection begins

HARMFUL ASPECTS Viruses are parasites Causes disease Depend entirely on the host cell for reproduction and energy supply They harm the host cell Causes disease Only a small number of viruses are capable of causing diseases in humans However, these pathogens (disease producing agents) are responsible for a large amount of human suffering Can infect: Plants (causing destruction of crops) Animals (including humans; influenza viruses mutate rapidly and can spread from animals to people) Bacteria (even the very smallest of single-celled organisms are susceptible to viral infection)

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

HOW ARE VIRUSES TRANSMITTED? Air Chicken pox, measles, flu Contaminated food or water Hepatitis Mosquitoes Yellow fever, West Nile Sexual contact Herpes, AIDS

VIRAL SPECIFICITY Viruses have very specific host requirements This is seen at the attachment step during viral reproduction How does a virus recognize and attach to a host cell? Viral capsid or envelope fits into a receptor on the surface of the host cell like a lock and key If the lock and key don’t fit, then the virus cannot enter the cell

ARE VIRUSES ALIVE? NO!!! They do not have cellular characteristics They are not made of cells They cannot reproduce on their own When did viruses evolve? Probably evolved from living cells since they cannot “survive” without a living cell