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:1. Lytic Cycle 2. Lysogenic Cycle

Lytic Cycle1. 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.

2. Synthesis virus DNA (or RNA) hijacks host cell and programs it to replicate viral components (nucleic acids, enzymes, capsid proteins, etc)

3. Assembly all components of virus (nucleic acids, enzymes, proteins) come together to create new virus particles

4. Release new virus particles released from the infected cell to find other cells. Host cell dies (lysis).

Lysogenic Cyle1. 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.

2. Reproduction Viral DNA is replicated along with host DNA during normal mitosis processes(*Remember: mitosis is how cells reproduce*)

3. Cytokinesis daughter cellsof 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 VirusesDNA 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.