CHAPTER 19BACTERIA AND

VIRUSES

19.2 Viruses I. What is a Virus?A. Viruses are particles of nucleic acid,

proteins, and in some cases lipids. 1. Viruses can reproduce only by

infecting living cells. 2. All viruses have one thing in

common: they enter living cells and once inside, use the machinery of the infected cell to produce more viruses.

19.2 Viruses

B. A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.

1. The virus binds to a host and tricks the cell into allowing it inside.

19.2 Viruses 2. Once inside, the viral genes are

expressed and the cell is used to make more viruses.

3. Viruses are highly specific to the cells they infect.

C. capsid - a virus’s protein coat.

Viral Structure• Variety of shapes

*Helical-spiral *Rod- little sticks*Spherical- round*Polyhedral- multisided

Helical

Rod

Viral Structure

spherical

polyhedral

T4 BacteriophageTobacco Mosaic

Virus

Influenza Virus

Head

Tail sheath

DNA

Tail fiber

RNACapsid

Surfaceproteins Membrane

envelope

RNA

Capsidproteins

Figure 19-9 Virus Structures

• D. bacteriophages - viruses that infect bacteria.

19.2 Viruses

II. Viral Infection (two types)A. Lytic infection: a virus enters a cell,

makes copies of itself, and causes the cell to burst or lyse.

Figure 19-10 Lytic and Lysogenic Infections

19.2 Viruses

B. Lysogenic Infection: a virus integrates its DNA into the DNA of the host cell, and the viral genetic information replicates along with the host cell’s DNA.

19.2 Viruses

1. Lysogenic viruses do not lyse (kill) the cell right away.

2. Instead they may remain inactive for a period of time.

Figure 19-10 Lytic and Lysogenic Infections

• III. Retroviruses - contains RNA as their genetic information.

19.2 Viruses

A. Get their name because their genetic information is copied backwards. RNA DNA

B. Responsible for some types of cancers in humans and also AIDS.

19.2 Viruses

IV. Viruses and Living CellsA. Viruses are nonliving things.1. Viruses do not grow, metabolize, or

carry out homeostasis.

19.2 Viruses

2. Yet viruses, after infecting living cells, can reproduce, regulate gene expression, and evolve.

B. Because viruses are completely dependent upon living things, it is more likely that viruses evolved after living cells.

Figure 19-11 Viruses and Cells

19.3 Diseases Caused by Viruses

I. Bacteria and viruses are everywhere in nature but only a few cause diseases.

A. Pathogen = disease-causing agent.B. All viruses reproduce by infecting

cells and diseases result when infection causes harm to the host.

19.3 Diseases Caused by Viruses

.IV. Viral Disease in HumansA. Viruses produce disease by

disrupting the body’s normal equilibrium.

Oncogenic viruses

Retrovirus

Adenoviruses

Herpes viruses

Poxviruses

DNA

RNA

DNA

DNA

DNA

Cancer

Cancer, AIDS

Respiratory infections

Chickenpox

Smallpox

Type of Virus Nucleic Acid Disease

Common Diseases Caused by Viruses

19.3 Diseases Caused by Viruses

B. Viruses attack and destroy certain cells in the body causing symptoms of the disease.

C. Viral diseases cannot be treated with antibiotics.

D. The best way to protect against most viral diseases is by prevention, often by the use of vaccines before infection.