Viruses Are they alive?

The word ‘virus’ comes from Latin meaning poison A virus is so tiny it cannot be seen with a light microscope, which makes it much smaller than a bacterium

Are they Alive? Viruses are not considered living by many biologists. Why not? Well…. Living things cannot crystallize, but viruses do Living things can reproduce on their own (asexual reproduction) or with members of their own species (sexual reproduction) Viruses cannot do either Living things grow and develop, viruses do not Living things obtain and use energy, viruses don’t Living things respond to their environment, viruses don’t

Structure Very simple Have a core of genetic material (DNA or RNA) Core is surrounded by a protein coat called a capsid Many different shapes – See page 479 – sketch a few on the back of your notes!

Tricking their way into a host cell Proteins bind to surface of host cell Host cell is ‘tricked’ into letting virus in Once virus is in, it takes over, kind of like a Trojan horse Viruses are very specific to host Plant viruses infect plant cells (i.e. tomato viruses only infect tomatoes) Animal viruses infect animal cells (i.e. pig viruses only infect pig species) Bacterial viruses infect bacterial cells (these viruses are called bacteriophages,or ‘phages’)

Changing Hosts Sometimes a virus suddenly starts to infect a species that is not it’s normal host This is bad for the new species as it has no defense against the virus The mechanism for this is being researched Examples: HIV (crossed from chimps to humans) Swine flu (crossed from pigs to humans) Avian flu (crossed from poultry to humans)

Infection Cyles Two processes: Lytic infection cycle Lysogenic cycle Fast Lysogenic cycle Slower (can be years)

Lytic Infection Cycle Steps Virus binds to host cell DNA is injected into host cell, remaining separate from host DNA Virus takes over host cell, causing new viral parts to be made New viruses assemble Viruses leave host cell Phages lyse (burst) bacterium cell wall and host dies Other viruses do not necessarily cause lysis

Lysogenic Infection Cycle Steps Virus binds to host cell DNA is injected into host cell DNA inserts itself into host DNA – called a prophage Every time host divides, more viral DNA is made Can stay in the lysogenic (prophage) stage for a long time Virus is inactive Certain factors can activate DNA of prophage (ultraviolet light, chemicals,…) Viral DNA separates from host Enters Lytic cycle, taking over host cell

Retroviruses Nasty little particles Contain RNA When retroviruses infect a cell, they make a DNA copy of their RNA called retroviruses, because they copy their genetic info backwards usually it’s DNA to RNA Can remain dormant for years Eventually become active, direct production of new viruses and cause death of host cell

HIV HIV = Human Immunodeficiency Virus This is a retrovirus It attacks the cells of the immune system The immune system can no longer work properly, allowing many other diseases to flourish HIV causes the disease called AIDS AIDS = Acquired Immunodeficiency Syndrome This is the disease, not the virus To sum it up: HIV is the name of the virus, AIDS is the name of the disease

Treating Viral Infections Very few drugs available (but there are some) Antibiotics don’t work (they’re for bacterial infections) Can usually just treat the symptoms Prevention is best ex: Vaccinations, hand washing