Viruses pp. 104 to 107

Viruses vs. Bacteria: What’s the difference? Bacteria  unicellular organisms Virus  protein-covered genetic material (DNA) that has NO CELLS, but invades other living cells True or false? “Viruses are living things.” FALSE!!!

What are Viruses? Not a living organism! Complicated assembly of molecules: Proteins (protective) DNA or RNA Lipids Carbohydrates No cells or organelles Dormant outside a living cell (can’t do anything, including reproduce) Once it invades a living cell, it can reproduce

Viral Diversity Viral specificity: viruses can only invade certain host cells (based on DNA, proteins, shape) Example of viruses: tobacco mosaic virus (tobacco plant), rabies (mammals), swine flu (pigs & humans), HIV (immune system) Ex. bacteriophage viruses can only infect bacteria

Viral Diversity: Hosts: Plants Animals Animals Bacteria

Viral Reproduction All viruses enter host cells, but some are not pathogenic (infectious) – they do not kill their host or make them “sick” There are two processes viruses can use to make more copies of themselves: Lytic Cycle Lysogenic Cycle

Lytic Cycle Attachment (entrance)  virus chemically recognizes host cell and attaches to it. Whole virus or only DNA enters cell cytoplasm. It integrates itself into host cell DNA. Synthesis  virus DNA (or RNA) hijacks host cell and programs it to replicate viral components (nucleic acids, enzymes, capsid proteins, etc) Assembly  all components of virus (nucleic acids, enzymes, proteins) come together to create new virus particles Release  new virus particles released from the infected cell to find other cells. Host cell dies (lysis).

Lysogenic Cyle Attachment (entrance)  virus chemically recognizes host cell and attaches to it. Whole virus or only DNA enters cell cytoplasm. It integrates itself into host cell DNA. Reproduction  Viral DNA is replicated along with host DNA during normal mitosis processes (*Remember: mitosis is how cells reproduce*) Cytokinesis daughter cells of mitosis have virus DNA integrated with their own Virus will often be in a dormant state (lysogeny) until it becomes activated by a stimulant (eg. Change in pH, temperature, food supply, etc) –” triggers the lytic cycle.

Lytic Vs. Lysogenic Cycle Lytic cycle: take as little as 25 -40 mins produces up to 300 new virus particles, but kills host cell Lysogenic cycle: host is not killed many generations of virus can be produced without harm to the host Virus is dormant until stimulant triggers activation of virus. It will then switch over to the lytic cycle.

Examples of Viruses DNA viruses Chicken pox Mono Small pox RNA viruses Rabies Measles Mumps The Flu AIDS Common cold Some cancers Symptoms appear as cells are destroyed (lytic cycle) Most viruses hard to treat since antibiotics (inhibits growth of microorganisms) cannot kill them. Vaccines: Vaccines “train” our bodies to defend against viruses. By injecting a small amount of virus (dead or weakened) into our bodies, our immune system learns how to fight it off by building antibodies against it. It builds up an army of WBC’s & antibodies to kill the living virus immediately before it can reproduce. In the future, exposure to that same virus won’t wreak havoc on our bodies and make us sick because antibodies will already be available to fight it off. This is also why some people feel a little sick after getting their annual flu shot.