19-2 Viruses- virus means “poison”

What is a Virus?• non-living – no metabolism and

can’t reproduce on their own• DNA or RNA core surrounded

by a protein coat• use a living cell’s internal

structures to reproduce themselves

• Capsid – protein coat - often with proteins on it that help it invade a host cell – often highly specific

• once inside the host the viral genes get the host cell to make viral proteins and DNA/RNA

- bacteriophage – a virus that attacks bacteria

Viral Infection1. Lytic Infection

- virus enters a cell; has copies of itself made; and causes cell to burst and release more viruses

2. Lysogenic Infection- virus enters a cell and has its DNA integrated into the DNA of the host cell- as cell replicates its DNA it also replicates the viral DNA- prophage – viral DNA embedded in host DNA

Lytic Pathway

Lysis of host cell is induced; infectious particles escape.

Tail fibers and other parts are added to coats.

Viral protein molecules are assembled into coats; DNA is packaged inside.

Viral DNA directs host machinery to produce viral proteins and viral DNA.

Virus particles bind to wall of suitable host. Viral genetic material enters cell cytoplasm.

Stepped ArtFig. 21.20 Page 358

Lysogenic Pathway

Viral DNA usually becomes integrated into the bacterial chromosome.

Prior to prokaryotic fission, the chromosome and integrated viral DNA are replicated.

After binary fission, each daughter cell will have recombinant DNA.

Viral DNA is excised from chromosome and cell enters lytic pathway.

Stepped ArtFig 21.20 (2)Page 358

- may stay inactive for a long time - eventually something causes the DNA of the prophage to be activated and it enters the Lytic cycle and destroys the host cell

Retroviruses• Viruses which have RNA as genetic material• When they infect a cell the cell first makes

DNA from RNA template (reverse transcription)

• DNA inserts into cell’s DNA and can stay dormant

• Can cause some cancers, HIV causes AIDS, colds

(Copy Figure 19-11)

19-3 Diseases Caused by Bacteria and Viruses

While most bacteria and viruses are not harmful a few are pathogens (disease causing)

- Disease results when a virus or bacteria interfere with the host’s ability to get enough nutrients or energy

- Louis Pasteur developed the Germ Theory of Disease when he proved that bacteria can cause disease

Bacteria cause disease by directly breaking down cells for food or by releasing toxins that travel throughout the body and interfere with normal activity in the host.

- Vaccines are used to stimulate the immune system by exposing it to a dead or weakened pathogen.

- Antibiotics are used against bacterial infections that the immune system doesn’t beat.

- Bacteria can be controlled by sterilization with heat; disinfectants; and correct food processing and handling.

Lyme Disease

Viral Disease

- Viruses produce disease by disrupting the body’s normal equilibrium

- Most viruses are very specific in what cells they attack – polio only attacks motor neurons

- Viral infections cannot be treated with antibiotics.